Pest: Stemphylium Gray Leaf Spot of Tomato

Gray leaf spot can be caused by several fungal species in the Stemphylium genus. Stemphylium solani and S. lycopersici are most common in North America, but S. botrysum also causes the disease. Gray leaf spot is typically considered to be most prevalent in humid tropical and subtropical regions, however, the disease was first seen on high tunnel tomatoes on farms in Rhode Island and Connecticut around 2019, and Maine farmers began to share photos of similar symptoms a few years later.

The arrival of gray leaf spot in Maine is likely an aspect of the new climate reality that growers will continue to need to adapt to. Though it likely can’t be tested scientifically, it seems probable that recent summers’ prolonged bouts of higher heat and higher relative humidity — which can also cause elevated temperatures to persist overnight to a greater extent — have allowed this disease to expand the range in which it infects tomato plants and spreads. While we’ve been having summers with these conditions more frequently, the disease can likely only spread so far in a given year, and we may now be seeing overwintering populations that have gradually established themselves further and further north over many seasons.

Pest/disease identification and lifecycle:

Stemphylium infections can start from nearby infected plants; can be seedborne; or, more likely in its gradual spread into Maine, can start from infected crop debris from prior years. Fungal spores produced on living or dead plant tissues spread via splashing rain or irrigation water, or go airborne and travel on the wind in favorable conditions. The disease survives and infects most successfully in relatively hot and wet weather conditions: requiring liquid water or simply high humidity to infect leaf surfaces, and developing most successfully at 77 degrees Fahrenheit. Though petioles and stems can become infected, the disease is mostly noticed on leaves.

Because of the manner in which gray leaf spot presents itself, it may be helpful to think of it as “acting like early blight but infecting like botrytis.” One fortunate difference is that gray leaf spot does not affect tomato fruit the way that early blight or botrytis can. However, that’s likely to be cold comfort if your plant loses all of its leaves. Initial symptoms show up as small dark brown specks, which turn lighter — eventually turning tan to gray in their centers as they enlarge. The centers of these lesions will commonly crack as they begin to dry out. As the disease progresses, the entire leaf may turn gray, dry, and crispy. Early leaf symptoms of Stemphylium gray leaf spot can be easily confused for bacterial leaf spot or Septoria leaf spot (see Fig. 1). The disease development and symptoms of Stemphylium seem, however, to also have many similarities with Alternaria linariae, the disease which causes “early blight” (not to be confused with the more devastating “late blight” caused by Phytophthora infestans and responsible for the great potato famine). 

Both gray leaf spot and early blight overwinter on infected crop debris in the soil — spread via splashing water, or from windborne spores from active infections, and tend to progress up an infected plant from the lower leaves towards the upper leaves. The big difference is that early blight requires liquid water to successfully infect leaves, while gray leaf spot can do the same simply with high humidity. Because of this, Maine farmers’ first interactions with this disease were noticing symptoms that looked like early blight but showed up in covered production (i.e., greenhouses or high tunnels) the way that botrytis might — but early blight typically does not. The disease has also been found on tomato plants grown outdoors in Maine.

Damage and crops affected:

The defoliation, described above, is the primary damage caused by this disease, reducing the plant’s photosynthetic potential and also diminishing the affected leaves’ ability to shade fruit and protect it from sun scalding (see Fig. 2).

The three species of Stemphylium responsible for causing gray leaf spot on tomatoes are capable of infecting other solanaceous crops, like peppers and tobacco, and solanaceous weeds, however, the disease has only been identified on tomato plants in Maine thus far. These species have also been reported to infect cucumbers, lettuce, garlic, cotton, gladiolus, and blue lupine. Because Stemphylium gray leaf spot has only emerged as a potentially serious pest of tomatoes in Maine, so far, there is a possibility that only one of the species is present here, and that it is limited to solanaceous crops.

Management options:

The best strategies for management of gray leaf spot are likely to be multifaceted for many growers.

Cultural:

As with many tomato foliar diseases, any growing practices that facilitate rapid drying, and reduce trapped air around plants will be helpful in limiting initial infection and subsequent spread (e.g., greater plant spacing, and sucker, stem, and leaf pruning). Similarly, other common best practices for organic methods of disease management are also important. Because the disease overwinters on infected crop debris, it will be important to practice good sanitation and crop rotation. Remove or destroy infected tomato crop debris, or at least incorporate it into the soil to speed its decomposition before tomatoes are grown again in the same area. When conditions are still favorable for the disease (i.e., prolonged high humidity or leaf wetness during warmer periods) it will likely still be able to infect susceptible plants, unless a preventative fungicide has been applied.

Luckily, genetic resistance to gray leaf spot can be found in tomatoes that contain a single gene (Sm), which has been traditionally bred from a wild tomato relative into many modern varieties. This means that anyone willing to grow these varieties likely won’t see gray leaf spot infect their crop in the first place. It’s advisable to grow one or more resistant varieties for at least an “insurance portion” of your overall tomato plants, especially if you have already struggled with gray leaf spot showing up in your growing area. Cornell Cooperative Extension maintains a spreadsheet of crop varieties with disease resistances noted, available here.

Organic pesticides (as a last resort):

A copper-based fungicide is likely to be the most effective organic control for home and commercial growers (Cueva is one of many commercial options, and is sold by Bonide for home use under the brand Captain Jack’s Copper Fungicide). Commercial growers may also see some improved control with the addition of Oso (polyoxin D zinc) to their management schedule. 

Please note: This information is for educational purposes. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control, and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

Authors

Caleb Goossen

The post Stemphylium Gray Leaf Spot of Tomato appeared first on Maine Organic Farmers and Gardeners.

Pest: Aphids

Aphids are perhaps the soft-bodied insect pest most well-known to both farmers and gardeners. There are many different species of aphids, but green peach (Myzus persicae), melon (Aphis gossypii), potato (Macrosiphum euphorbiae), and foxglove (Aulacorthum solani) are the most common aphid species of concern in the Northeast. 

Some aphid species may have greater affinities for specific crops than others, but these common aphids are generalists that happily infest many different crops. Damage potential from aphids is usually greatest in warm, protected growing spaces like tunnels/greenhouses or under row cover.

Aphids feed by stabbing hollow, sharply pointed mouthparts into plants to suck out their juices. While this direct removal of nutrients can have a negative impact on growth of infested plants — particularly when colony populations have exploded — a greater damage can come from the plant viruses that aphids can transmit. They also cause distorted growth where they’ve been feeding and cosmetic concerns from their waste. 

Aphids extract more sugary plant juices than they can actually use and excrete a waste that is still very sugary, referred to as “honeydew.” The sticky and shiny honeydew falls to foliage and other surfaces below the aphids and can be unsightly enough on its own, but it also feeds sooty mold, which leaves plants looking dirty and can grow thick enough to reduce their photosynthetic potential. Aphids also molt their exoskeletons multiple times as they grow rapidly, and these discarded white skin casts are sometimes the most visible sign of their infestation — and may be concerning to customers of seedlings/nursery crops. 

Pest/disease identification and lifecycle:

While aphids are often a shade of green, their coloration can vary quite a lot — even within the same species — and they are sometimes shades of orange, pink, red, or black. Some aphid species are larger than others, ranging from 1.5 to 3.5 millimeters (1/16 to 1/8 inch) in length, but they also vary in size by age.

In many situations it is not important to identify exactly which species of aphid(s) are infesting your crops, but it can be the critical difference between failure and success when managing them with biological control options like parasitoid wasps. In that case, it’s necessary to determine if you have the smaller species (green peach or melon aphids) or the typically larger species (potato or foxglove aphids). The two most commonly purchased biocontrol species of parasitoid wasps have a preference for larger or smaller aphids.

While I recommend that every grower at least have a hand lens to help them make their own identifications while scouting crops, aphid species are difficult to tell apart and oftentimes the fastest or best way to know which species you have is to take a very detailed photo or three, and send them to an expert for identification assistance (e.g., extension or certain biocontrol supply companies). There are many affordable clip-on lenses to help you take magnified photos with a smartphone, and even some tiny stick-on lenses that can be reused many times if well cared for (Blips is a brand that I’m familiar with). 

Some of the best identifying features of aphid species include their overall size, the positioning of the “tubercles” that support their antennae, and the size of their “cornicles” (two tubes protruding from their rear). Of the species listed here, all but the melon aphid typically have the appearance of an indentation on the front of their head, in between their antennae. The most memorable description of which I’ve heard is that it looks like they ran face first into a tiny two-by-four. Melon aphids will also always have dark cornicles, regardless of the color of their body. Green peach aphids’ cornicles will mostly match the color of their body but have black tips at their ends. Foxglove aphids may also have black cornicle tips but with a distinguishing feature of distinctive dark spots on their bodies at the base of their cornicles. Foxglove aphids also tend to have darker coloration at their leg joints and at the many joints of their particularly long antennae. Potato aphids’ bodies may look a bit more segmented than the other species and have a dark stripe running down the middle of their backs.

Management options:

Scouting: 

Scouting is ideally performed early and often enough in greenhouse settings to catch aphid infestations before they cause more apparent damage. It can be more difficult to catch infestations early in outdoor settings, but they also tend to be less consequential to overall production, and are frequently kept in check by wild predator and parasitoid populations. Scouting is most important when transitioning new plants to protected growing situations where aphids can multiply rapidly and be protected from predators and parasitoids, e.g., when quarantining incoming plant material like nursery starts before bringing them into a greenhouse; when planting transplants out to a bed that will be covered with row cover; or when checking prior crops and weeds in a tunnel that will be turned over to a new crop — both in the fall before winter crops and in the spring before seedling production or summer crops.

Aphids are usually found on stems, buds, and the underside of leaves on tender new growth, where they can feed more easily. Magnification is essential when you desire to identify aphids by species (see the sections on identification and biocontrol options). Careful investigation is important to catch and quantify early infestations. However, it may be easiest and most efficient to first scan larger areas of plants for yellowing or distorted upper leaves, and/or lower leaves with a shiny look (from honeydew), and/or a collection of white skin casts. These cast skins are often an easily noticed early sign of an infestation.

Yellow sticky cards can be used to monitor aphid presence but should not be expected to provide an infestation warning early enough to manage aphids most effectively. Remember that colonies will typically only begin producing winged adults after they have begun to be overcrowded. (However, yellow sticky traps can be beneficial in letting you know that you have a colony nearby producing winged adults, if you didn’t know that already!)

While scouting for aphids, you should also be looking for signs, or the presence, of predators and parasitoids. Some insecticides for aphid control could also be damaging to their natural enemies and should be avoided if aphid populations aren’t too high and predator and parasitoid populations are present. Predatory larvae of several species (green lacewings, lady beetles, Aphidoletes midges) can often be fairly easily identified among aphids when present and are worth learning in addition to their better known adult forms. Presence of parasitoid wasps can be verified by finding aphid “mummies” — the typically brown, rounded body of an aphid that a parasitoid wasp has laid an egg into, sometimes also with a circular exit hole showing where the newly pupated wasp offspring has emerged. Ants can also be an indicator of an aphid infestation, particularly in garden settings: They will often “farm” aphids, moving them around to other plants and protecting them in exchange for being able to feed on the honeydew they produce.

Cultural:

In all situations, good sanitation will help to reduce chances for aphids to jump to your next crop. This includes removing/burying crop debris that may be harboring overwintering eggs, and controlling weeds that may be hosting aphid colonies that could jump to your crop — this is particularly important in a tunnel that’s to be planted to winter greens, or when a winter greens tunnel is to be terminated and turned over to spring or summer production. A two-week “dead period” with no plants may be helpful to starve out any existing aphid colonies, particularly in the late winter when tunnels are mostly closed up and winged adult aphids are less likely to find their way in from elsewhere.

Because of their frequency as prey to many different species, aphids in outdoor settings are essentially the potato chip of the insect world, and their populations are typically kept in check by wild predators and parasitoids. Management considerations are focused on fostering and maintaining a diversity of habitat to support these beneficial insects, and sometimes knocking back an aphid infestation temporarily, while waiting for wild predators and parasitoids to build up enough numbers to keep populations in check. Keep in mind that aphid reproduction can be much faster than that of predators and parasitoids, and this difference is typically most noticeable earlier in the season, especially if aphids have had a chance to multiply in a warmer protected space. A powerful blast of water is frequently sufficient to physically remove enough aphids from plants to set their population back temporarily. Doing this once or twice in June may be the only intervention required in a small garden, or even on a farm if a diversity of beneficial insects are present and supported. Having a diversity of plants helps to provide beneficial insect habitat and maximize bloom period for an extended supply of pollen and nectar. Examples include dedicated habitat strips, interplanting crops with species like alyssum, and something as simple as letting cilantro and dill go to flower after primary leaf harvest is done. 

Avoid overfertilization with nitrogen, as it can result in rapid growth of succulent plant tissue, which tends to be weaker and more easily fed upon. Aphids have greater growth and reproductive success rates on plants receiving higher levels of nitrogen. 

Biocontrol Options:

In protected growing environments like high tunnels and heated greenhouses, purchased biocontrol species can be very effective. In outdoor field settings, it typically does not make financial sense to buy biocontrol species, except possibly if releasing within a row-covered planting. This is primarily because of the high mobility of the adult forms of these species and the frequently pre-existing but often undervalued presence of wild predatory and parasitoid species. The difference in value of plants per square foot of growing area in the field versus under cover is also a key factor.  

Parasitoid wasps are among the most effective during warmer months and when an area is heated, provided that they are introduced preventatively or at the very first sign of aphids. Many predatory species will also assist in reducing aphid populations but may not have the same control potential as parasitoids. In all cases, the biocontrol species’ ideal temperature and daylength conditions must be considered, as some will not be effective in winter months.

For best results with parasitoid wasps, aphid species should be identified. The wasp species Aphelinus abdominalis and Aphidius ervi will attack and lay eggs into larger potato and foxglove aphids, while Aphidius colemani will attack and lay eggs into the smaller green peach and melon aphids (see the identification section). An alternative approach to identifying the aphid species present, or to deal with the presence of both a larger and a smaller species, is to order a blend of parasitoid wasp species and leave it to them to decide which aphids they’ll be hunting down.

Because parasitoid species take a longer time to build up a population capable of controlling an aphid outbreak, one method that can make the purchase of A. colemani most effective and worthwhile is the use of a “banker plant” system, typically oats, grown in successions to maintain a population of cereal aphids (which do not feed on most common greenhouse crops). The banker plants are grown within some sort of insect exclusion, which is sequentially removed to provide hosts for your population of parasitoid wasps to lay eggs in, keeping them reproducing and providing a continued presence.

The most common predatory species can often be purchased from commercial sources fairly easily: green lacewing (Chrysoperla carnea), convergent lady beetle (Hippodamia convergens), and Aphidoletes midges (Aphidoletes aphidimyza). While Aphidoletes midges have evolved for their larvae to specifically prey on aphids, and could potentially provide a similar level of control to parasitoid wasps, green lacewings and lady beetles are both generalists and may also feed on thrips and whitefly. Lady beetles may also feed on scale and mealybugs as well. In all three species, their larvae do the most feeding on aphids, though adult lady beetles will also feed on them.

Some important considerations when shopping for predatory species include the time of year (in regards to both temperature and daylength); how enclosed they will be (are tunnel sides open?); and will they find enough food to keep going (both the aphids, which their larvae require, and flowers, which the adults may require for pollen and nectar). While there may be other lab-reared lady beetle species available at times, the most common species available for sale is the convergent lady beetle, which are frequently wild collected, posing a potential ethical concern as well as a chance of them arriving to you infested with their own wild parasitoid or disease species.

In general, lady beetles are the only species that will remain active in the late fall and winter months, but they are also quick to leave if they do not find abundant food and are able to escape. As winter transitions to spring, and tunnels and greenhouses are warmer, Aphidius parasitoid wasp species have the greatest potential efficacy if brought in prior to aphid colonies blowing up in population. This period of efficacy can be extended with repeated releases or the use of a banker plant system. In protected growing spaces in the summer months, Aphidoletes midges may be the best biocontrol response to an aphid flare-up, but green lacewings can help to keep low populations in check and will also feed on other common pests.

Organic pesticides (as a last resort):

In all cases, pesticide use should be considered carefully in regards to potential interference with predator and parasitoid species, whether wild or purchased. Oftentimes the best use case for pesticides against aphids is as the first punch in a one-two punch approach: first knocking back a population that may have been discovered too late for truly effective management with biocontrol species alone. Check with biocontrol suppliers regarding pesticide compatibility with the species you are purchasing, particularly if you have already deployed some.

Being soft-bodied, aphids are more vulnerable to some lower-risk pesticide options than other pests, and it may be best to start with a soap product like M-Pede or an oil product like stylet oil. 

Other partially effective options — but potentially more consequential to beneficial insects — include pyrethrin (e.g., PyGanic) and azadirachtin (neem-based products, e.g., Aza-Direct, AzaGuard, etc.). These are probably best reserved for severe infestations, and scheduled a day or two prior to arrival of biocontrol species, to allow for their breakdown. Without following up with biocontrol species, these materials may make the situation worse, killing slower-to-rebound beneficial insects, while only temporarily hindering the rapidly reproducing aphids.

Aphids are also vulnerable to bio-insecticides, like the Beauveria bassiana entomopathogenic fungi products, but those can also infect beneficial insect species. The best use case for these products is likely in fall and winter high tunnel production when most biocontrol insect species are not active, and when days are short and humidity levels tend to be higher, giving the fungus a better chance to successfully infect the pests before drying out. Good spray contact with the pests is needed for best efficacy, as well as repeated applications. However, thorough coverage may be difficult to achieve in densely grown winter greens.  

Please note: This information is for educational purposes. Any reference to commercial products, trade, or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control, and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

Authors

Written by Caleb Goossen

The post Aphids appeared first on Maine Organic Farmers and Gardeners.

Disease(s): Common Foliar Diseases of Alliums

This fact sheet discusses botrytis leaf spot (Botrytis squamosa), purple blotch (Alternaria porri), stemphylium leaf blight (Stemphylium vesicarium) and downy mildew of onion (Peronospora destructor). These are the most commonly occurring foliar diseases of alliums in the Northeast, and many of their disease dynamics are closely linked; if one of these diseases is present, it’s common to find three or four of them at the same time. Organic management of these diseases is similar, but it’s important to know the differences between the diseases to help inform the timing of your management choices. 

Pest/disease identification and lifecycle:

Botrytis leaf spot is caused by the pathogen Botrytis squamosa. Botrytis neck rot, a common storage disease of onions, can be caused by any one of three other species of Botrytis (B. cinerea, B. porri or B. allii). B. squamosa overwinters in crop debris and in the soil as sclerotia, hardened black masses of mycelia that are adapted to withstand extreme conditions, and can survive several months. As conditions warm in the spring, those sclerotia form conidia, an airborne spore that serves as inoculum for the disease in the new growing season when wind carries it to new plantings. These spores can also form on leaves of resprouting infected bulbs in cull piles or compost piles that haven’t reached killing temperatures. Botrytis spores require long periods of leaf wetness or high humidity and temperatures between 55 and 75 F to germinate and cause disease. Unlike Alternaria and Stemphylium, B. squamosa is able to infect healthy leaf tissue relatively easily. Spores of B. squamosa can infect by forming an appressorium — a specialized fungal structure that can penetrate the leaf surface — or hyphae can directly enter the leaf through microscopic pores in leaf surfaces called stomata. 

Allium downy mildew is caused by Peronospora destructor, which is a fungus-like organism in the class Oomycota. Downy mildew also produces airborne conidia during cool (52-72 F) and wet (at least 95% humidity) weather conditions. These cool and wet conditions are also the most favorable to these spores’ successful germination and infection of otherwise healthy leaves. A spore can infect a wet leaf in only two to three hours in conditions below 61 F, but requires five hours of leaf wetness between 61 and 68 F. The pathogen cycle, from initial infection to sporulation, takes between one and two weeks depending upon conditions, allowing the disease to spread quickly between and within fields under the right conditions. 

Purple blotch is caused by Alternaria porri, while stemphylium leaf blight is caused by Stemphylium vesicarium, two fungal organisms in the same family. Because these two pathogens cause very similar diseases, and because their management is largely the same, especially in organic growing, you may see them both referred to as a single disease: purple blotch. Purple blotch and stemphylium leaf blight both infect plants through injuries in the leaf surface caused by mechanical damage, extreme weather, thrips, or prior infection such as botrytis leaf spot. Both pathogens overwinter in crop debris, and in nearby weed debris (especially wild alliums) releasing airborne spores called conidia in the spring to be carried by wind. Alternaria disease progression is favored by hot weather (77-85 F), while stemphylium leaf blight is favored by warm weather (72-77 F). It can take as few as five to six days for new lesions from either disease to begin producing spores, depending on conditions. These airborne spores from new lesions can then spread, causing secondary infections throughout the season. 

Damage and crops affected:

These four diseases are all specialists of alliums, meaning that they can infect nearly any allium including onions, garlic, shallots, chives and leeks, along with many wild allium species, which can act as reservoirs for the diseases. Importantly, botrytis leaf spot is often the initial entry point for downy mildew, stemphylium leaf blight and purple blotch, as it can create injuries in the plant and weaken the plant such that its defenses are diminished. Even in minor infections, all of these diseases can impact sales of leeks and chives because the lesions occur on the salable part of the plant. Minor infections can also stunt plant growth and bulb size in onions, garlic and shallots. Any of these initial infections can also create opportunities for secondary infection both in the field and in storage. In severe infections, any of these pathogens can progress to total collapse of the plant before harvest. Beyond creating opportunity for secondary infections in the bulb, downy mildew can directly infect the bulb in systemic infections. Purple blotch and stemphylium, in severe instances, can also attack and girdle seed stems in allium seed crops. 

Management options:

Scouting: 

Especially during weather conditions that promote the successful infection and spread of these diseases, it’s advisable to scout for symptoms to inform timing of management choices. In dry weather, with good cultural management, these diseases are fairly limited. In cool, wet weather (65-77 F), botrytis leaf spot and downy mildew are more likely to emerge, and stemphylium leaf blight may follow. Purple blotch is more common in warmer wet weather (77-85 F). 

Early symptoms of botrytis leaf blight are approximately ¼-inch oval-shaped silvery halos, which are easier to see when the leaf is wet (fig. 1). Necrotic spots will eventually form within the halos. Often, these spots can be confused with thrips damage — however, thrips damage does not present with the telltale halos. In rare instances, botrytis leaf spot can become so severe that spots grow larger, tan, and ultimately coalesce.

In fields, downy mildew often first appears as circles of yellowed plants measuring a few feet in diameter (fig. 2). On individual infected leaves, pale spots develop, then begin to turn a brown or purple hue. When plants are wet, or conditions are humid, fuzzy gray-white growths are visible, emerging overnight and often “burning off” in clear sunny conditions. This sporulation gives this disease its name — downy mildew (fig. 4). In systemic infections, leaves first become paler green, then yellow to brown, and then die back entirely, and plants are often stunted and distorted. Violet lesions may appear, but the disease can be differentiated from purple blotch by this fuzzy mycelial growth, systemic yellowing of the plant, and by spongy, shriveled or watery bulbs.

In both stemphylium and purple blotch, lesions often begin as brown, oval-shaped “thumbprint” spots, which grow larger with time and cause necrotic streaks along the affected leaf (fig. 5-6). These lesions eventually form spores, which disperse to neighboring leaves and plants. In purple blotch, as spores develop, the lesions typically turn purple, and in stemphylium leaf blight, they typically turn dark brown to black. However, differentiating between the two in the field is often nearly impossible, even for experts. 

Cultural:

There are no varieties of allium that are resistant to botrytis leaf blight, downy mildew, purple blotch or stemphylium. 

Use at least a three- or four-year rotation with all allium crops. In order to make your rotations most effective, consider whether it would help to also manage wild alliums in nearby areas, which can harbor these diseases in “off” years.” As with all disease management, plant and manage your crop to promote rapid drying. 

Sanitation is key! While spores from these diseases can be dispersed by implements and insects, they are mostly spread by wind. That means any crop debris or culls from previous years left exposed anywhere on your farm or property can be a source of inoculum for another year of disease. Ensure that all allium crop debris is incorporated into the soil or deeply buried under compost as quickly as possible after harvest, and definitely well in advance of the next allium crop being planted. Culled onions should be deeply buried or disposed of. Culls can be composted, but home compost piles are often too small to reach the temperatures required to kill any conidia without very attentive management. Overwintering onions can act as a “green bridge” to carry allium diseases through the winter into the next season, and should be undertaken cautiously.

Managing thrips and preventing botrytis leaf spot early in the season can be great protectants against following infections of stemphylium leaf blight and purple blotch, by limiting the injury sites through which those fungi can enter. It’s also important to avoid over-applying nitrogen fertilizers in allium plantings, especially later in the season. Higher nitrogen levels in the plants, especially later in the season, is associated with more severe disease presentation in stemphylium leaf blight and purple blotch.  

Organic pesticides (as a last resort):

Several biofungicide products may help to reduce infection when applied preventatively. These products work either by colonizing aboveground plant surfaces, effectively outcompeting disease spores before they can infect the plant, and/or by causing the plant to induce its systemic resistance. In other words, they can cue the plants to strengthen their defense mechanisms against pathogens. Products containing the bacteria Bacillus amyloliquefaciens, such as Double Nickel and Amplitude, as well as products containing Bacillus subtilis such as Serenade Opti, are labeled for control against Botrytis for both commercial growers and home gardeners. To decide when it may be appropriate to spray based on weather conditions, keep an eye on NEWA, which has models for botrytis leaf blight, downy mildew and purple blotch. 

Please note: This information is for educational purposes. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control, and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

Authors

Written by Mariam Taleb and Caleb Goossen

Source material

Steentjes, Maikel B. F., Scholten, Olga E., and Jan A. L. van Kan. March 2021. “Peeling the Onion: Towards a Better Understanding of Botrytis Diseases of Onion.” Phytopathology. DOI: 10.1094/PHYTO-06-20-0258-IA

Utah State Extension. “Purple Blotch, Stemphylium Leaf Blight” Utah Vegetable Guide. https://extension.usu.edu/vegetableguide/onion/purple-blotch-stemphylium-leaf-blight

Pscheidt, J.W., and Ocamb, C.M. March, 2023. “Onion (Allium cepa) Botrytis Leaf Blight.”  Pacific Northwest Plant Disease Management Handbook. https://pnwhandbooks.org/plantdisease/host-disease/onion-allium-cepa-botrytis-leaf-blight

Swett, C.L. Aergerter, B.J., Turini, T.A. and A.I. Putman. Feb 2019. “Downy Mildew” UC IPM Pest Management Guidelines: Onion and Garlic. https://ipm.ucanr.edu/agriculture/onion-and-garlic/downy-mildew/

The post Foliar Diseases of Alliums appeared first on Maine Organic Farmers and Gardeners.

Pest: White mold (Sclerotinia sclerotiorum)

Pest/disease lifecycle, most common damage symptoms and crops affected:

White mold (Sclerotinia sclerotiorum) is a widespread disease of over 300 species of plants. It is only a sporadic problem in “normal” and dry growing seasons; however, white mold can be very damaging in wet years and, if it goes unrecognized or is not well-managed, it can continue to be problematic in following seasons, regardless of weather. Understanding the disease organism’s lifecycle is critical to successfully managing it.

White mold infections can occur in most, if not all, vegetable crops, but damage is most commonly noted on lettuces, green beans, cucurbits, brassicas, carrots, potatoes and tomatoes, especially when grown in higher-humidity environments like high tunnels and greenhouses (fig. 1-3). A related species, Sclerotinia minor, causes “lettuce drop” in lettuce, and organic management considerations are similar.

White mold overwinters as sclerotia — dark, hardened masses of mycelium, said to resemble rat droppings (fig. 4), which are very tough and can survive in the soil for five to eight years. In the spring and early summer, overwintered sclerotia in the top 2-4 inches of soil are triggered to germinate in moist soil as temperatures reach the 50s (F). Sclerotia will not germinate, however, unless they have been stratified (undergone a prolonged moist cold period in temperatures below 40 F). 

Germinated sclerotia in the top 4 inches of soil produce mycelium, which may infect nearby plants directly. This is the primary pathway of initial infections in the earlier part of the season. When soil remains cool and very wet, sclerotia also produce a mushroom-like fruiting body at the soil surface, called an apothecia (fig. 5), which releases thousands of airborne spores, called ascospores. Apothecia are most likely to be formed after the plant canopy closes, creating humid conditions that favor spore survival. The majority of apothecia grow from sclerotia in the top 1 inch of soil. If these spores land on a plant under appropriate environmental conditions, they can infect the plant. 

Airborne spores require injured or dead tissue as a food source to initiate infection of healthy plant tissue. This can be as minimal as a fallen flower petal, or a leaf that has dead leaf margins. Successful germination of airborne spores is favored by relatively long periods of wetness (16 to 72 hours) and moderate temperatures (68 to 77 F). These conditions produce the most rapid development of disease, and white mold infection severity is greatly reduced in very hot weather. Because of these requirements, white mold infections often start on crop debris, such as fallen flower petals, that have landed in spots that also tend to collect moisture, such as leaf axils (where leaf meets stem) or stem joints (fig. 1).

Initial infection may show up as a small, circular, water-soaked lesion, but it typically grows in size rapidly. Dead infected plant tissue may take on a dry, bleached papery look (called “timber rot” in solanaceous crops like tomatoes, fig. 1, 4), and may create a lot of white fuzzy mycelial growth in wet and humid conditions (from which the disease’s common name derives, fig. 2, 3). 

Importantly, an infected plant does not produce airborne spores that can infect other plants. An infection may spread from one plant to a neighboring plant in humid conditions via mycelia, if they are in direct contact with each other, but this is rare. The primary infection pathway is via direct infection from germinating sclerotia, and infection from airborne spores is secondary. 

In most years, white mold infection severity is thought to be most impacted from cool and wet spring weather — with all sclerotia in the top few inches of the soil assumed to have germinated due to favorable conditions. In summers where wet and cool conditions persist or reappear, white mold apothecia may continue to emerge from sclerotia buried deeper in the soil that have been brought up higher in the soil profile through cultivation or tillage, starting another cycle of infection from airborne spores. Sclerotia that formed in one growing season will remain dormant for that entire growing season, however, until they have undergone stratification. 

Management:

It is advisable to reduce sclerotia, or inhibit their ability to send up apothecia, in high-value greenhouse situations where high humidity from closed-up conditions in the spring may present ideal conditions for white mold to take hold. The disease is not uncommon, however, and there will likely always be sclerotia present in the areas surrounding your growing space if not within as well. In wet years when airborne spores are more likely to be produced and are more likely to successfully infect plants, this common, but typically minor, disease may suddenly become more of a concern. Limiting sclerotia dispersal to your soil is valuable to help reduce recurrences in following years, particularly from direct infection, which is less reliant on very wet conditions than infection from airborne spores. 

Scouting:

Identifying and disposing of infected plants is the most important element of long-term management of white mold. Oftentimes white mold is not noticed until infections have progressed to the point that leaves or entire plants are dying back, at which point sclerotia may have already been created, and had a greater chance of dropping to the soil. Scouting is easily incorporated into pruning and trellising of greenhouse tomatoes if you familiarize yourself with white mold symptoms, but may require dedicated effort in other crops.

“Timber rot” symptoms (fig. 1, 4) are frequently associated with white mold, and the initial infected tissue is often dried out. The white fluffy mycelial growth that gives the disease its common name is often described as “cottony” (fig. 2, 3), unlike white mycelial growth from other disease organisms, like Phytophthora species, which is sometimes described as “yeasty” and typically presents with wetter decay including  very large “water-soaked” lesions. White mold is primarily distinguished from botrytis infections (gray mold) by the color of the mycelium. White mold produces relatively large, hard black sclerotia which typically form inside the stem of an infected plant, though they are sometimes visible on the surface of plants, like when it shows up as “soft cottony rot” of carrots (fig. 2). 

In most years and most circumstances in the Northeast, it is likely not worth attempting to scout for white mold apothecia (fig. 5). Scouting for apothecia in instances of concern, however, may inform timing of management decisions to reduce infection potential of airborne spores, i.e., in very wet and cool springs, or in areas where a known history of white mold suggests a high number of sclerotia are present in the soil. Apothecia are most likely to emerge in the shaded and humid environment underneath plant canopies when soil has warmed up slightly (50-68 F) and remained very wet. White mold apothecia are fleshy, small, light tan-brown to light orange in color, and vaguely trumpet shaped. They could be easily confused for non-pathogenic bird’s nest fungi, however those fungal fruiting bodies can be distinguished from apothecia by the “eggs” within their “nest” (fig. 6). Immature bird’s nest fungi may still be “closed” at the top. To confirm their identity, squeeze a fruiting body between your fingers — the “eggs” will squeeze out.

Cultural options:

Protect this year’s plants: Manage plant spacing and greenhouse environmental controls to maximize airflow when possible, but especially in infective conditions (see above) and take other steps to reduce leaf wetness and facilitate rapid drying, such as regular pruning (when appropriate for the crop), avoiding overhead irrigation, scheduling irrigation carefully, etc. Overfertilization with nitrogen can favor excessive canopy density, which in turn can provide ideal humidity conditions for white mold apothecia and ascospore production.

Protect future years’ plants: Crop rotation is of limited usefulness in combating white mold because of this disease’s wide host range, but it does not infect onions, grasses or cereal grains. 

Remove and dispose of infected plants as soon as possible, but take care to prevent sclerotia from dropping to the soil whenever feasible. Steps can be taken to reduce survival of sclerotia and inhibit the production of apothecia in areas where it is suspected they entered the soil previously, particularly high-value production areas such as a greenhouse. Soil steaming or solarization (using clear plastic to trap heat in soil) may be effective in killing sclerotia, which are not expected to survive exposure to temperatures above 160 F for 15 to 20 minutes. Critical to this approach is verifying that the necessary temperature has been maintained for a long enough time across the entire soil depth where sclerotia are. In the Northeast, solarization may not be able to produce temperatures necessary to kill sclerotia very far into the soil profile, particularly in the wet and rainy years that white mold tends to be most problematic. If white mold is a new problem for you, and the soil wasn’t disturbed after infected plants were removed, you may only need to heat the top inch of soil. If sclerotia are already known to be present, or newly formed sclerotia have already been tilled into the soil profile, you may need to heat the soil at least to the depth of your tillage. At a bare minimum, treat to a depth of 4 inches — at which point germinating sclerotia are not anticipated to reach the soil surface to release spores, unless brought closer to the surface later on. 

In either case, soil steaming and solarization should be implemented before the application of any of the biocontrols listed below, because these methods would also kill any biocontrol organisms.  

Another strategy taken by some growers is to either bury sclerotia through inversion tillage (moldboard plowing; not rototilling, which would leave sclerotia spread throughout the soil depth profile) and/or add compost, soil, etc., on top of the area to ensure that sclerotia are too deep for apothecia to reach the soil surface (i.e., at least 4 inches). However, this approach requires careful planning to ensure that future tillage does not bring sclerotia back into the upper 4 inches of the soil for at least eight years.

Organic pesticides (as a last resort):

For home and commercial growers (some products mentioned may not be available to both home and commercial growers):

Several biofungicide products may help to reduce infection from airborne spores. Examples include, but are not limited to, Double Nickel, a bacteria-based fungicide, and BotryStop WP, which consists of the fungus Ulocladium oudemansii. These types of products are intended to work by pre-colonizing aboveground plant surfaces before disease spores can infect, and/or by inducing systemic resistance in plants (i.e., causing them to strengthen their pre-existing defense mechanisms), and as such need to be applied preventatively to have the best chances of being effective. 

For commercial growers:

White mold sclerotia are parasitized by the fungus Coniothyrium minitans. This fungus is commercially available as a biofungicide product called Contans. Contans needs to be applied before or after the heat of summer and three to four months prior to planting, and therefore is typically applied via soil incorporation in the early fall. Unfortunately, Contans is frequently only found in package sizes that are larger (and more expensive) than make sense for most farms in the Northeast. 

Another group of biofungicide products, such as BIO-TAM and Rootshield, consists of Trichoderma fungus species, which work primarily by colonizing plant roots and protecting them from infection from other fungi, though they may have antagonistic activity against white mold in the soil as well.

Though both Contans and BIO-TAM may reduce apothecia production, and therefore the number of airborne spores present, neither would protect above ground portions of plants from any airborne spores that do reach plants.

Please note: This information is for educational purposes. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control, and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

Authors

Written by Caleb Goossen and Mariam Taleb

Source material

Higgins, G. 2016. “White Mold.” UMass Extension Vegetable Program. https://ag.umass.edu/vegetable/fact-sheets/white-mold

Kleczewski, Nathan. 2016. “Sclerotinia vs ‘Birds Nest Fungi.’” University of Delaware Cooperative Extension. https://sites.udel.edu/weeklycropupdate/?p=9523

Sideman, Eric. 2001. “Tomatoes.” MOFGA. https://www.mofga.org/resources/tomatoes/tomatoes/

“White mold on tomatoes.” Cornell College of Agriculture and Life Sciences. https://blogs.cornell.edu/livegpath/gallery/tomato/white-mold-on-tomatoes/

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Pest: Slugs and Snails

Pest/disease identification and lifecycle, most common damage symptoms and crops affected:

Land slugs and snails are closely related mollusks, known together as “gastropods,” the primary difference between them being that snails have external spiraling shells. There are over 90 terrestrial gastropods found in Maine. The gray garden slug is most common and most destructive on the East Coast (fig. 1). Leopard slugs are more common on Maine’s coast and islands, though they are becoming more common inland over time. The tawny garden slug is less common, but still an important agricultural pest (fig. 2). Interestingly, all three of these key pest species are native to southern Europe. 

Snails and slugs thrive in damp, shaded, cool environments and are most active on wet and overcast days, or at night. In Maine, most slugs and snails overwinter as eggs and as adults, emerging at 32-40 F. All slugs and snails are hermaphrodites, so mating can happen between any two individuals of the same species. The eggs of gray garden slugs can take up to 100 days to hatch in cooler weather, but only 10 days in late May to early June (fig. 3). In damp, warm weather, gray garden slugs can complete their life cycle in less than three months and lay approximately 100 eggs.

Slugs and snails feed on a wide variety of plants and crops including: seedlings, leafy greens, ripening fruits that are close to the ground such as strawberries and tomatoes, and mushrooms. Of particular interest to slugs are any plants or plant parts that are especially tender, but they usually don’t do too much damage on more mature and fibrous growth. Slugs and snails feed by scraping leaves, fruits and flowers with their rasp-like tongues, leaving tell-tale irregular holes with smooth edges (fig. 4). 

Management options:

Slugs and snails differ from many other pests in that many of the most immediately effective management tactics happen to be approved for use in organic systems!

Scouting:

Slugs and snails use a muscular “foot” to glide over the ground, leaving shiny slime trails behind them. These silvery trails are an excellent sign of recent slug or snail presence (fig. 5). Signs of feeding damage are also important to look for, though those signs can be confused with other pests (fig. 4). Slug eggs are another sign that slugs are nearby (fig. 3). In warm, wet years, it’s particularly important to keep an eye out for signs of slug damage.

Cultural options:

The first step, as with many pests and diseases, is good sanitation. Because slugs and snails require moisture, it’s important to clear out any damp, dark habitats near your crops by removing boards, rocks, logs and dense growth that slugs may hide or lay eggs under. Where possible, improving drainage to allow soil to dry out between rain events can also reduce the number of eggs that successfully hatch. 

There are two primary strategies to deal with slugs and snails — creating barriers to keep them out of a crop, and managing the slugs and snails that are already in your fields and gardens.

So long as barriers at perimeters of crops are complete and well-maintained, they can be an effective choice, especially for smaller fields and gardens. 

Copper foil, mesh or tape wrapped around garden beds, etc., will stop slugs and snails from crossing because the copper oxidizes when it comes in contact with their slime, which gives them a slight shock. When copper is tarnished, the oxidation reaction has already happened, so it can’t affect the slugs. When copper barriers tarnish, wipe them down with vinegar to regain efficacy. Any copper barrier should be at least 2 inches wide, as slugs and snails will be more likely to cross a narrower barrier.  

Diatomaceous earth has also been shown to be an effective barrier, especially when laid liberally (e.g., in at least 1-inch thick bands). However, diatomaceous earth becomes less effective when wet, and works best in settings protected from rain, or should be reapplied regularly. Diatomaceous earth, while smooth to the touch to us, works because the particles have microscopic sharp edges, and the powder can absorb protective oils from slug skin, causing them to dry out. Both of those properties are impaired after wetting, even if only from repeated heavy dews. 

Simply maintaining a bare cultivated perimeter around your garden may also deter slugs and snails from entering. Unfortunately, slugs and snails are most problematic when it’s been rainy, limiting the efficacy of any barrier approach in particularly wet years. With any barrier approach, maintenance is critical to ensure a complete perimeter exists, as new plant growth, blowing leaves, etc., can create safe passage for slugs and snails over a barrier.

To manage slugs and snails that are already inside of fields and garden beds, many home growers report success with trapping and drowning slugs. To trap slugs, dig a small hole away from your field and place a cup in it so that it rests just above the soil. Fill the cup with beer (choose inexpensive light lagers — a slug research lab at Penn State recommends Keystone Light as consistently proving to be a slug favorite in their testing), or with water, a pinch of baker’s yeast and a drop of dish soap. Check the traps every morning, and replace them as needed. Commercial farmers should consider whether any remedy not registered for use in commercial settings is allowed under Board of Pesticide Control regulations. 

Pesticides approved for use in certified organic production (as a last resort):

Another option to control slugs and snails already in fields and gardens is to use iron phosphate bait products, such as Sluggo, which have been shown to be as or more effective as conventional pesticides. Iron phosphate based baits work by stopping slugs and snails from feeding, so do not be surprised to find slugs and snails present for two to three days after application, though they should stop causing new feeding damage after consuming the baits. Slug and snail baits are most effective while employing many of the options above in order to achieve full control and limit reintroduction of other slugs and snails from surrounding areas. 

Please note: This information is for educational purposes. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control, and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

Authors

Written by Mariam Taleb and Caleb Goossen

Source material

Dill, James F.  and Clay A. Kirby. 2020. “Pest Management Fact Sheet #5036.” University of Maine Cooperative Extension. https://extension.umaine.edu/ipm/ipddl/publications/5036e/

Willen, Cheryl A. and Mary Louse Flint. 2018. “Slugs and Snails” University of California Statewide Integrated Pest Management Program. https://ipm.ucanr.edu/PMG/PESTNOTES/pn7427.html

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Pest: Two-spotted spider mites (Tetryanchus urticae)

Pest/disease identification and lifecycle, most common damage symptoms and crops affected:

Two-spotted spider mites are the most common mite pest of crops in the Northeast. Because they thrive in hot and dry conditions, they are commonly found in greenhouses and high tunnels. Spider mites can feed on many crops but are very attracted to eggplant, tomatoes, cucumbers, beans and cannabis. Feeding usually occurs on the underside of leaves and on tender young tissue at growth points. Feeding damage can give leaves a mottled/speckled appearance or a dull bronzed look (Fig. 1), and damage on emerging leaves at growth points can result in distorted leaves as they enlarge. When spider mite numbers are high enough they can produce a fine webbing to protect themselves, which is the origin of their common name (Fig. 1, 2). The mites themselves are tiny, only about one-fiftieth of an inch long. With good vision, individuals can be spotted with the naked eye, but it’s much easier to spot them with some magnification, such as a hand lens. 

The two-spotted spider mite lifecycle can vary depending on temperature. Spider mites can live for three to four weeks, but in warmer temperatures, they can complete their lifecycle in just seven to fourteen days. Each female can lay up to 100 eggs, meaning that, especially in warmer weather, spider mite populations can grow very quickly. 

Management options:

Scouting:

One difficulty in managing spider mites is that they can often go unnoticed until feeding damage symptoms on the plant are obvious, at which point the populations are often already quite large. As mentioned above, it is helpful to have a hand lens to scout proactively for spider mites. Look under younger leaves of the plant species preferred by spider mites for small yellowish mites with the characteristic two black spots on their backs (Fig. 3). Some growers choose to interplant a few individual bush bean plants among high tunnel crops to monitor spider mite populations, as beans are very attractive to them.

Cultural options:

Because spider mites do best in hot, dry and dusty conditions, frequent rains or overhead irrigation can help to suppress their numbers somewhat. However, overhead irrigation as a management strategy needs to be weighed against plant disease risks from wet foliage, and may not provide satisfactory control. 

Biocontrol options:

The most common biocontrols for spider mites are the predatory mites Phytoseiulus persimilis and Neoseiulus californicus (formerly called Amblyseius californicus). These two predatory mite species live very differently, so it is important to know a little bit about them in order to select the best choice for your situation. P. persimilis reproduce fairly quickly and are voracious predators of spider mites, but they cannot survive without prey to eat. N. californicus are slower to reproduce, but because they also feed on pollen and other pests such as thrips, they can go for longer without prey so they are much better suited as a preventative control. N. californicus are also better than P. persimilis at tolerating cooler temperatures (between 50 F and 70 F) and lower humidity levels that might be found in spring greenhouses. For these reasons, growers typically use N. californicus when they first spot spider mites, or ideally before they see any as “insurance.” Growers typically use P. persimilis when spider mite populations are large or rapidly growing, or as a preventative measure at times in the summer when spider mites have shown up in the past. If spider mites get ahead of you, and you are choosing to both spray a pesticide and to order a predatory mite species for biocontrol, time your spray to allow pesticide residue to break down before the predatory mites arrive, as any pesticide material that will be effective against spider mites will also impact predatory mite species.

Much of the expense of using biological controls like these predatory mites is due to shipping costs, because mites are shipped alive and require rapid shipping. Growers may be able to save greatly on shipping if they coordinate group orders of species like N. californicus as part of a proactive spring greenhouse pest management plan.

Pesticides approved for use in certified organic production (as a last resort):

Spider mites can be impacted by some organic pesticide products (such as soaps, neem oil, horticultural oils, pyrethrin, etc.), but the most effective management approach is typically to introduce biological control organisms before spider mite populations get out of control. This is because many pesticides that work against spider mites will also reduce the populations of predators that feed on them. Predatory mite species (introduced or already present) typically reproduce more slowly than spider mites, which can lead to spider mite populations rebounding to eventually become even worse after spraying. 

Please note: This information is for educational purposes. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control, and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

Author

Written by Mariam Taleb and Caleb Goossen

Source material

“New England Vegetable Management Guide.” https://nevegetable.org/crops/insect-control-8  

Schausberger, Peter, Shuichi Yano and Yukie Sato. 2021. “Cooperative Behaviors in Group-Living Spider Mites.” Frontiers in Ecology and Evolution, Vol. 9. https://www.frontiersin.org/articles/10.3389/fevo.2021.745036/full 

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Pest: Mexican bean beetle (Epilachna varivestis)

Pest/disease identification and lifecycle, most common damage symptoms and crops affected:

If Mexican bean beetles have historically been a problem on your farm or in your garden, you will very likely see them again this year. They may be pests on snap beans, dry beans, soybeans and lima beans. While they are not a pest on every farm, some farms report repeated significant damage from these pests and have to take action to prevent crop loss if populations have built up to damaging levels. Using biological control can reduce, or supplant, the need for insecticides.

Mexican bean beetle adults are coppery brown with black spots. They look very much like large ladybeetles and are in fact closely related — but unlike lady beetles they feed on leaves, not other insects. Adults lay yellow-orange egg masses on the underside of bean leaves. These hatch into bright yellow, spiny oval larvae, which feed, molt several times as they grow, and pupate on the underside of leaves. Feeding damage from adults and larvae can reduce yield and injure bean pods if numbers are high. There are several generations per season, often with increasing populations in each generation.

Management options:

Biocontrol options:

Pediobius foveolatus is a commercially available biological control agent for Mexican bean beetle control and has a good track record in the mid-Atlantic states and among New England growers who have tried it. (Pediobius is pronounced “pee-dee-OH-bee-us.”) It is mass-reared and sold by the New Jersey Department of Agriculture and is also available from other beneficial insect suppliers. This small (1-3 millimeter), non-stinging parasitic wasp lays its eggs in Mexican bean beetle larvae. Wasp larvae feed inside the bean beetle larva, kill it, and pupate inside it, forming a brownish case or “mummy.” About 25 adult wasps emerge from one mummy. Control continues and in fact gets better as the season progresses and successive generations of the wasp emerge and search out new bean beetle larvae. 

Planning two to three releases at 7-10 day intervals will help ensure good timing. and coverage on several plantings. This makes biocontrol  well suited to our succession-planted snap bean crops. After a release in the first plants, it is advisable to leave that planting intact for a while, until the next generation of wasps has emerged from their mummies. As with any biological control, make releases as soon as the pest is present — not after it has built up to damaging numbers. The New Jersey Department of Agriculture Beneficial Insect Rearing Laboratory recommends two releases, two weeks in a row, coinciding with the beginning of Mexican bean beetle egg hatch. Wasps will lay their eggs in larvae of any size, but it is best to target newly-hatched young larvae. This will give control before damage has been done. Thus, timing is important. Watch for eggs and time the shipment for the first hatch of eggs into larvae. If in doubt about the timing of the hatch, release as soon as you see the eggs — if you wait for the larvae you may be playing catch-up. The release rate should be at least 2,000 adult wasps per field for less than an acre, or 3,000 per acre for fields of one acre or more. 

Pediobius wasps are shipped as adults or “mummies” (pupae inside dead larvae) from which adults will emerge. Order adults if you already have Mexican bean beetle larvae in the field. Ship for overnight delivery. Instructions for handling and release will come with the wasps. Wasps reproduce in the field and will still be around when the second generation of bean beetles hatch. Thus, it should not be necessary to make more than two releases. Like beans, Pediobius wasps are killed by frost.

Plan ahead by contacting a supplier to inform them of your expected release dates and acreage. Contact information for the New Jersey source is: Wayne Hudson, 609-203-9782, wayne.hudson@ag.state.nj.us, NJDA, Phillip Alampi Insect Lab, West Trenton, New Jersey 08628. https://nj.gov/agriculture/divisions/pi/prog/beneficialinsect.html. You’ll also get advice on how to use the wasps from this office.

Pediobius is also available from:

ARBICO organics, Arizona, 800-827-2847. Order online; orders ship on Wednesdays only, minimum seven day processing.

IPM Labs, New York, 315-497-2063. Contact to check availability.

Beneficial Insectary Inc., California, 800-477-3715.

Pesticides approved for use in certified organic production (as a last resort):

Moderate control can be achieved with spinosad (Entrust SC for commercial use, or Monterey Garden Insect Spray, Natural Guard Spinosad or Bonide Captain Jack’s Deadbug Brew for residential use). Mixtures of pyrethrin and azadirachtin (Azera, for example, or a tank mix of PyGanic EC 5.0 and Neemix) can also be used for commercial applications. (For home gardens, there is Azera Gardening, or standalone pyrethrin and azadirachtin products can be mixed, like PyGanic Gardening and Bonide Neem Oil, for example.) 

Please note: This information is for educational purposes. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control, and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

Source material attribution: Reprinted and modified by Eric Sideman and Caleb Goossen, from the University of Massachusetts Vegetable Notes Newsletter written by Ruth Hazzard and A. Brown.

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Pest: Tomato Brown Rugose Fruit Virus (ToBRFV)

Disease identification: Tomato brown rugose fruit virus, otherwise referred to as ToBRFV, is a highly virulent and aggressive plant virus that can cause serious infections on tomato and pepper species. ToBRFV behaves very similarly to other tobamoviruses such as ToMV (tomato mosaic virus) and TMV (tobacco mosaic virus). However, its main distinction from other tobamoviruses is its ability to overcome genetic resistance in tomatoes and cause severe fruit symptoms in otherwise genetically resistant varieties of tomatoes.

History: Tomato brown rugose fruit virus was first identified in 2015 on tomatoes grown in Jordan, where it was then traced back to its first occurrence in Israel in 2014. The virus has been confirmed in the United States, Mexico, Germany, Italy, Saudi Arabia, Israel, Jordan and Turkey. After the discovery of ToBRFV in Jordan and Israel, the virus spread rapidly due to the nature of seed production, the distribution chain, and ToBRFV’s seed transmissibility.

Image from: Zhang, S., Griffiths, J.S., Marchand, G., Bernards, M.A. and Wang, A. (2022)

Host plants: 

Tomatoes and peppers are the only natural hosts of ToBRFV.

Most common damage symptoms:

Fruit: Symptoms include blotching, pale color, necrotic brown spots, or otherwise totally aborted fruits.

Image from: American Seed Trade Association

Calyx: Vein browning and drying out. 

Image from: American Seed Trade Association

Leaves: Symptoms include distortion of the leaf blade, wrinkling and bubbling, mosaic, shoestring (long narrow leaf blades) and fern leaf (small leaflets). In young tomato seedlings, symptoms include mild to severe mosaic on leaves with dark green bulges, narrowness and deformation. In young pepper seedlings, symptoms include stunted growth and small yellow to brown rugose dots (having a rough, ridged or wrinkled surface) followed by necrotic blotches on fruits.

Image from: American Seed Trade Association

Note: The symptoms of this virus can vary by the variety of tomato or pepper infected with ToBRFV and in some cases infected plants may be asymptomatic.

Management options:

Known resistance genes for other types of tobamoviruses will not protect plants against ToBRFV. Instead, preventative crop management and exceptional sanitation practices are the only known measures of preventing and mitigating the spread of ToBRFV. It is important to note that the virus is very stable and survives for long periods of time in infected debris, soil, or other contaminated surfaces.

In tomatoes and peppers, mechanical transmission of the virus occurs very easily — touching and manipulation of infected plants can easily spread the virus to otherwise healthy plants. The virus is also known to spread via infected fruit, root-to-root contact, and seeds. It has been found that tomato seeds extracted from ToBRFV-infected fruits are 100% contaminated and can transmit the virus.

Preventative measures of healthy crops: 

• Only enter the crop or greenhouses with clean clothes/shoes.
• Use protective clothing that will stay in the greenhouse after use. 
• Follow good hygiene routines such as washing hands with soap/disinfectants before and after handling plants.  
• Sanitize all tools that come into contact with plants by using 10% bleach solution.
• Thoroughly clean and disinfect any surface areas at the end of every growing season. 
• Get transplants or seeds tested by diagnostic labs for the presence of infected stock.

Measures to reduce spread from infected crops: 

• Carefully remove symptomatic plant tissue and destroy it via burning/incineration in a manner to avoid airborne transmission. Do not lay infected plant material out in fields and do not compost. 
• Quarantine the greenhouse of infected stock and always use protective equipment (disposable coverall and gloves) when handling any suspected or confirmed to be infected plants.
• Wash all clothes in hot water and soap before using them again. 
• Avoid moving from infected greenhouses to uninfected houses/fields. 
• At the end of cultivation make sure all materials, tools and the greenhouse are cleaned and disinfected thoroughly.  
• Fields where confirmed infected crops have been grown should be removed from tomato and pepper growing for the foreseeable future — it is not yet known how long this virus can remain infective in a field environment, but it is expected to be a long time.

Sources

Written by Dr. Alicyn Smart, Associate Extension Professor and Ruby Bonilla

Goldy, Ronald. “Tomato brown rugose fruit virus (ToBRFV): A new concern for tomato and pepper producers,” Michigan State University, November 25, 2019, https://www.canr.msu.edu/news/tobrfv-a-new-concern-for-tomato-and-pepper-producers 

“Q&A ON THE NEW TOBAMOVIRUS: TOMATO BROWN RUGOSE FRUIT VIRUS (ToBRFV),” American Seed Trade Association, (n.d.) https://www.betterseed.org/wp-content/uploads/ToBRFV-QA.pdf 

Zhang, Shaokang, Jonathan S. Griffiths, Genevieve Marchand,  Mark A. Bernards, and Aiming Wang. “Tomato brown rugose fruit virus: An emerging and rapidly spreading plant RNA virus that threatens tomato production worldwide.” Molecular Plant Pathology, 23 (May 2022): 1262–1277. https://doi.org/10.1111/mpp.13229 

Chanda, Bidasha, Md Shamimuzzaman, Andrea Gilliard, and Kai-Shu Ling.“Effectiveness of disinfectants against the spread of tobamoviruses: Tomato brown rugose fruit virus and Cucumber green mottle mosaic virus.” Virol J 18 (2021): 7. https://doi.org/10.1186/s12985-020-01479-8 

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Pest: Allium Leafminer (Phytomyza gymnostoma)

First detected in the United States in Pennsylvania in 2015, Allium leafminer (Phytomyza gymnostoma) is a potential threat to all crops in the allium genus. Since its first detection, allium leafminer has rapidly spread throughout the Northeastern United States. Every growing season seems to bring sightings at new locations, so it is likely more a question of “when” than “if” this pest will show up near you. 

Pest Identification and Lifecycle:

Allium leafminer is an invasive pest of allium crops (onions, scallions, leeks, etc.) Adult allium leafminers are small gray/black flies (less than one-eighth inch) with an orange/yellow spot on the tops of their heads, and yellow “knees” (Fig. 1). Their eggs are small, white, and slightly curved and require magnification to see. The allium leafminer larvae (maggots) are more visible, growing to be about a third of an inch long, and white to yellowish in color. Pupae are red to dark brown and about one-eighth of an inch long.

Allium leafminers have two generations in a year. Adults emerge in late winter through early spring (March and April) after overwintering as pupae in host crop debris, or the soil immediately surrounding it. After mating, adults seek out host plants to lay eggs into. These first-generation eggs hatch out as larvae, which “mine” the inner leaf surfaces, moving downward towards the leaf base, or bulb. They then pupate in that plant tissue (Fig. 2), or in the soil surrounding it. This is the life stage in which they remain dormant through the summer, emerging as adults in late summer or early autumn (typically in early September). Those adults quickly begin to lay a second generation of eggs into the leaves of various alliums, whose larvae will once again mine the leaves of these plants. This second generation of allium leafminer larvae become the next overwintering generation, pupating inside crop tissues, or in the soil immediately surrounding them.

Damage and crops affected: 

The second generation of allium leafminer is often the most damaging to crops, both because the population is typically much larger than in the first generation, and because the presence of larvae, their feeding damage, or pupae can greatly impact the salability of crops that are present at that time. Allium leafminer can infest any species in the genus Allium, including onion, garlic, shallot, green onion, chives, and ornamental and wild alliums. Initial damage symptoms are caused by the adult flies as they leave distinctive white circles caused by their ovipositors (egg-laying appendages) puncturing the leaf surface (Fig. 2). While this ovipositing is how they lay eggs under the surface of the leaf, this damage also allows adult flies to easily feed on plant sap and not all oviposition marks will contain eggs. After larvae hatch from the eggs and begin feeding within the leaf, classic leaf mining damage appears as white tunnels and galleries spreading under the outer surface of the leaf, and larvae often move downwards towards the leaf base or into the bulb. In addition to impacts on crop photosynthetic capacity, and potentially significant cosmetic injury, severe infestation may provide enough larval feeding to cause distorted growth or even plant death. Both the oviposition punctures and subsequent leaf mining wounds (Fig. 3) create potential entry points for fungal and bacterial pathogens, which can be as, or more, damaging as the leafminers. Direct damage to bulb crops is less of a concern from this pest.

Allium leafminer damage symptoms are identical from both spring and fall generations, however, the crop species and growth stages present at those times can result in different risk scenarios. Adult flies are attracted to larger leaves to lay eggs into. Because of this, spring egg laying by the overwintered generation is more likely to occur on plants that typically have larger leaves at this time; perennial species like chives or ornamental alliums, wild species like ramps, and overwintered annuals like garlic or overwintered onions. Recently transplanted, or direct seeded onions, leeks, scallions, etc., may still have leaves small enough to be less attractive to the adult flies. When the first-generation adults begin to fly and seek egg-laying host plants, most garlic and onion crops will have already been harvested, and many ornamental or wild species will have gone dormant for the year. With this reduction in presence of host species, any remaining allium leaves will be at a greater risk from what may be an even larger second generation. Leeks, scallions, chives, and any other allium with healthy green leaves are at the greatest risk at this time.

Management Options

Scouting

Inspect any bought-in allium plants and bulbs for signs of infestation — particularly the presence of pupae — before planting them. However, the adult flies are mobile, and while not long-distance fliers, it appears that the pest is spreading quite competently on its own, without purchase of infested planting stock as a necessary means for its arrival.

To monitor for allium leafminers, scout for their telltale oviposition/feeding damage — repeated punctures in allium leaves. This typically occurs starting in late March through April in the Northeast. Following that, larval leaf mining may be visible, though take note that in tubular-leaved species like onions, scallions, and chives leaf mining under the leaf surface could perhaps be confused with “windowpane” feeding damage (Fig. 4) from leek moth caterpillars (the presence of that invasive pest may be just as concerning, or even more so!). While adult allium leafminer flies can be found on yellow sticky cards, the damage they cause to allium leaves is likely to be more easily noticed by the untrained eye.

Though overall damage is typically worst from the second generation of this pest in fall crops, especially leeks, overwintered onions, perennial chives, and early scallions may provide ideal springtime scouting sites to observe the arrival of this new invasive pest. There is evidence that allium leafminers prefer chives to scallions, onions, and leeks. Recent observations have reduced confidence in the reliability of this, however, early research had indicated that overwintered adult allium leafminer flies emerge when growing degree day (base 1 C, or 33.8 F) accumulation reaches 350. Around that time, adult allium leafminers emerge, mate, and begin ovipositing — the most obvious sign of their presence — and may continue to do so for several weeks following.

You can quickly see the growing degree day (GDD) accumulation in your area with maps provided by the Maine Climate Office, including one calculated on a base 33.8 F which is equivalent to 1 C, this page of GDD maps for Maine. You can also find the closest weather station to you at The Network for Environment and Weather Applications (NEWA)  to view current GDD accumulationin your area, as well as the expected accumulation in the coming six days.GDD accumulation is calculated starting at the beginning of the calendar year (January 1) and you may need to input the current date as the “end date” of the calculation. Be sure that the calculator is set to a base temperature of 1 C. 

Cultural

Crop rotation combined with row cover or exclusion netting is typically the most effective organic method to reliably achieve the greatest crop protection from flying pests. For this strategy to work, attention must be paid to the location where the prior season’s second generation of allium leafminer pupae are most likely to have overwintered. Pupae will be found in living or dead crop tissue, or the soil immediately surrounding it. This could include wild or planted perennial allium locations, a field where annual alliums were grown in the prior year, or even a cull pile where infested crop debris was not destroyed or sufficiently buried. To exclude adult allium leafminers from laying eggs in new plantings, they must be covered before the emergence of adults from the overwintered generation. Covering fall plantings to prevent first-generation adults from laying eggs for the second generation may also be effective if done before late August when adults may begin to emerge. Fall emergence and egg laying is thought to occur for a more prolonged period of time than in the spring.

Careful timing of planting and harvesting susceptible crops, combined with exclusion is likely to provide satisfactory control for most growers that are of a small enough scale to easily cover plantings. Reflective plastic mulch may also reduce damage somewhat. Removal and destruction, deep tillage, or deep burial of crop debris will help to reduce the amount of overwintering pupae emerging the following spring.

Biocontrol

There are no known commercially available effective biocontrols at this time. Shortly after allium leafminer was first detected in Pennsylvania, researchers there identified some very low rates of parasitism from two species of wild parasitoid wasps (Halticoptera circulus and Chrysocharis oscinidis). More parasitoid species are known to attack allium leafminer in its native Europe, and reports from there are of its greatest damage potential being in the first few years after it shows up in an area as a new pest, suggesting that natural enemies native to the continent likely played a significant role in keeping its population to levels of lesser concern. There may be some hope of native or introduced predator and parasitoid species controlling populations of this pest in the future, but as of now that should not be counted on to occur for some time, if at all.

Pesticides

Field trials performed by Cornell Cooperative Extension have found that three applications, spaced seven to ten days apart, of two spray mixes, to be the most impactful for controlling the most damaging fall flight of the allium leafminer, especially as the adults have a prolonged emergence window. The most effective spray program they identified was to apply, two weeks after the fall flight begins, a mixture containing a pyrethrin (e.g., PyGanic) and an azadirachtin product (e.g., AzaGuard, or many other options). Be aware that this mixture loses efficacy drastically when the final mixture pH is neutral or above. Ideally the water used should have a pH between 5.5 and 6.5, and it is best to avoid adjuvants that may raise pH, such as M-Pede. A more appropriate adjuvant for this tank mixture would be OROBOOST at a 0.25% concentration (by volume). That was followed by the first of two applications of a spinosad product (e.g., Entrust) at a rate of 6 oz./acre, mixed with a 1-5% solution (by volume) of M-Pede, which improved contact and therefore efficacy of the spinosad product.

Please note: This information is for educational purposes. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control, and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

Sources

Written by Caleb Goossen.

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Condition: Waxy Breakdown (an abiotic disorder)

Pest/disease identification and lifecycle, most common damage symptoms and crops affected:

Waxy breakdown of garlic is a physiological condition that sometimes affects garlic, though typically only rarely in Maine. Development of the condition is not fully understood but often associated with very high temperatures and/or sunscald damage around harvest time, or during curing. Low oxygen and poor ventilation during curing and storage may also contribute to the condition. Excessive respiration rates after harvest may be involved in the development of the condition.

Symptoms begin as light yellowish discoloration in the flesh of the clove, often taking on a “water soaked” appearance that may be slightly more transparent than the surrounding healthy tissue. As the condition progresses the entire clove turns a deep yellow color, and exhibits the waxy, soft texture that gives the condition its name. Eventually cloves may turn an amber brown color and become noticeably reduced in size. 

Waxy breakdown affects cloves on an individual basis — though many or all cloves in the same bulb may be impacted. Because it only affects the flesh of the clove and not the protective layers around it, waxy breakdown may not be easily spotted until skins are removed from cloves, or the condition progresses enough that cloves begin to shrivel, leaving void space underneath the bulb’s surrounding wrapper layers. 

Management options:

Because this condition may be affected by excessive respiration rates the primary management approach is to follow best practices for rapid curing and drying of plants that are already headed towards dormancy, while minimizing risks of excessive heat and sun damage.

• Stop irrigating garlic well before harvest, and attempt to harvest in dry conditions when possible.
• Aim to provide the best ventilation possible in your curing environment.
• Avoid temperatures above 90 F.
• Provide some shading from direct sunlight.

Hoophouses can be great for curing garlic, if a shade cloth is used and sides and ends are open as often as possible. In a hoophouse, make sure to keep garlic elevated, particularly if black ground cloth is in place, as that may get very hot on sunny days. Barns/sheds can also work well provided that very good airflow is provided. In both scenarios, keeping garlic elevated allows for better airflow and ventilation.

Author

Written by Caleb Goossen

Sources

Cornell College of Agriculture and Life Sciences. 2022. “Waxy breakdown on garlic.” Accessed August 25, 2022. https://blogs.cornell.edu/livegpath/gallery/garlic/waxy-breakdown-on-garlic/

Oregon State University Extension Service. 2022. “Waxy breakdown of garlic. February 7, 2022. https://bpp.oregonstate.edu/sites/agscid7/files/bpp/attachments/waxy-breakdown-of-garlic-feb-7-mlp.pdf

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