Vegetable Notes 2026 Vol. 38:17

Nightshades

Tomato hornworm has been spotted in MA this week. Parasitism by a braconid wasp is common; if you see the oblong, white cocoons of the parasitic wasp covering a hornworm, leave it be or move it off of the foliage so that the parasitoids can complete their lifecycle without the caterpillar inflicting more feeding damage on your crop. Chemical control is not usually warranted in field tomatoes but may be worth it in high tunnel crops. Targeted Bt materials (e.g., XenTari, Dipel, Javelin) are effective against hornworms and will not affect non-target (non-caterpillar) insects. Target small larvae, as large larvae will have to eat a lot of treated foliage in order to receive a lethal dose. 

Sweet Corn

Bird damage was observed in sweet corn this week. The most common bird pests of sweet corn ears are red-winged blackbirds, starlings, and crows. Red-winged blackbirds tend to be the most common in MA. This species nests in hayfields, marshes, and ditches and often causes the most damage in fields near those sites. May through July, their most common food source is insects, but their diet shifts to grain and weed seeds at this time of year. Often, multiple repellant devices need to be implemented to prevent the birds from becoming habituated to any one device. Repellant devices include scare balloons, moving devices like “balloon guys” or bird kites, laser scarecrows, sound cannons, shellcrackers, distress or raptor calls, and frequency deterrents like sonic nets. Netting will effectively exclude birds but is quite an investment at-scale. For more information see Preventing Bird Damage to Sweet Corn.

Corn earworm (CEW) trap counts have spiked dramatically, indicating that storms moving up from the south this week and last have carried with them high populations of CEW moths (Table 2). A tighter spray interval of 4 days is warranted across most of the state, while growers in some locations where over 91 moths were captured this week are recommended to use a 3-day interval based on existing thresholds. 

Fall armyworm (FAW) damage is continuing to be seen in fields. FAW feeds primarily in whorl-stage corn, initially causing windowpane scars on the leaves and wet pellets of frass. As the caterpillar grows, it causes very obvious, raggedy feeding damage and produces lots of sawdust-like frass. FAW caterpillars have a dark head capsule with a distinct inverted Y shape. Scout fields and use a 15% infestation threshold. Save diamides (e.g., Besiege, Coragen, IRAC Group 28) for silk sprays targeting corn earworm. More frequent sprays targeting the spike in corn earworm numbers can also reduce fall armyworm pressure. 

Sap beetles are causing damage now in sweet corn. These are small black beetles with or without orange spots. The adults and the small, thin, white larvae both feed in corn kernels. Sap beetle damage is most common on farms that also grow fruiting crops and have nearby cull piles, and in varieties with exposed tips. For more information, see the article in this issue.

Foliar Diseases of Onion

Onions that survived the high onion maggot pressure this spring are sizing up and some have been harvested already. This article gives an overview of the most common foliar fungal diseases of onion in Massachusetts: Botrytis leaf blight, purple blotch, downy mildew, and the new kid on the block, Stemphylium leaf blight. These diseases are similar in that they are favored by warm, humid temperatures, they overwinter in crop residues and cull piles, and their spores are spread by wind and splashing water. Although these diseases occur on onion leaves, they can indirectly affect yields by reducing bulb sizes and directly lead to losses by causing bulb rots in storage. If you plan on using fungicides to control these diseases, catching the diseases early is crucial. If you don’t plan on spraying, identifying infected vs. clean varieties or fields can help you decide what to sell quickly and what to store through the winter.

Botrytis leaf blight (Botrytis squamosa) causes small, white lesions, 1–5 mm long, surrounded by green-white halos. As the disease progresses, lesions become sunken and tan and may split down the middle. Symptoms often begin in older leaves. Botrytis overwinters in onion cull piles, on crop debris, or as sclerotia (small, dense masses of fungal tissue with a dark rind) in the soil. In the spring, sclerotia produce spores that are spread by wind and cause infection in onion crops if foliage is wet for long enough and temperatures are warm. Botrytis leaf blight, along with downy mildew, is not favored by hot weather—the pathogen thrives in cool to warm temperatures and shuts down when it gets too hot. Severely affected fields will appear blighted, with most leaves dead and desiccated. Losses in yield occur because of smaller bulb size resulting from premature leaf senescence. The Botrytis species that causes leaf blight in onion is different from the one that causes Botrytis neck rot in alliums and gray mold in many crops.

Purple blotch (Alternaria porri) causes boat-shaped (i.e., a birds-eye-view of a canoe) lesions with distinctive purple-brown sporulation. Lesions are initially water-soaked with white centers; as the pathogen expands within the leaf, the lesion may develop concentric rings. Leaves with large spots turn yellow and blow over in wind. Purple blotch and Stemphylium leaf spot (described below) both continue to infect at high temperatures (into the 90s), making them the prominent mid-summer diseases of onion where present. Almost no infection occurs below 55°F. Like other species of Alternaria, the purple blotch pathogen overwinters in infected crop debris and can be carried on seed. Spores are produced when temperatures are warm (optimum temperatures are 77°F to 81°F) and humidity is high, and are spread by wind and splashing rain or irrigation water. Older leaves and leaves with heavy thrips damage are most susceptible, as purple blotch can easily infect dead leaf tissue and move from there to healthy tissue. Bulbs may become infected at harvest through the neck or wounds; bulb decay from purple blotch shows first as a watery rot around the neck causing yellowish to wine-red discoloration. As it moves through the scales, tissue dries to a papery texture.

Downy mildew (Peronospora destructor) causes 1- to 2-inch long, pale-green, oval-to-round spots that expand and coalesce, girdling leaves and causing them to collapse. As the pathogen progresses, it produces fuzzy gray sporulation that becomes purple. Spores are produced at night and disseminated by wind the following day. Symptoms are first seen on older leaves when dew is present. Downy mildew and purple blotch often occur together. Similar to purple blotch and Botrytis leaf blight, downy mildew infection can limit bulb growth and cause bulb rots. Infection occurs when leaves are wet and usually when temperatures are below 75°F. Like Botrytis leaf blight and purple blotch , downy mildew shuts down at high temperatures. The pathogen overwinters in fall-planted, overwintering onions or volunteer onion plants as oospores (thick-walled survival spores), or as mycelium in stored bulbs, seed, or crop residue. Infected bulbs become soft and shriveled, some sprouting prematurely. The pathogen directly infects roots of new plants in the spring, which become systemically infected and produce spores that spread to other plantings. Downy mildew of onion affects only alliums and is caused by a different pathogen than the downy mildews of other vegetable crops.

Stemphylium leaf spot (Stemphylium vesicarium) is a major disease of onions in the large-scale onion producing areas of NY and MI and is becoming more prevalent in areas where onion production is on a smaller scale, including MA. Initially in reports from NY, Stemphylium was found only infecting dead onion leaf tissue, but over time it was found infecting healthy tissue as well. It commonly infects plants following infection by another disease or damage from onion thrips feeding, first infecting the dead tissue and then moving to healthy tissue. Spots are water-soaked and light-yellow to brown. They elongate and can coalesce into large lesions and develop concentric rings. Spots can be purple and produce dark brown sporulation—they are virtually impossible to distinguish from purple blotch spots without a microscope. In areas where Stemphylium leaf spot is widespread, populations of the pathogen have shown decreased sensitivity to fungicides in FRAC Groups 7, 9, and 11, and signs of resistance development to Group 3 are starting to appear. The species of Stemphylium that causes leaf spot on onions is a different species than the one that infects tomato. 

Cultural Control

There are a number of cultural practices you can employ to help manage all four of these foliar pathogens. 

• Destroy cull piles, where the pathogens can overwinter on residues and volunteer plants.

• Practice a 3-year crop rotation out of alliums.

• Till under crop residues promptly after harvest and bury residues deeply.

• Use drip irrigation where possible. If using overhead irrigation, irrigate in the morning on sunny days to minimize the amount of time that foliage is wet.

• Control weeds to maximize air circulation and reduce leaf wetness times.

• Provide proper nutrients. Over-application of nitrogen can cause plants to produce unnecessarily lush foliage, increasing humidity in the canopy. Senescent leaves due to nutrient deficiencies will be more susceptible to pathogens. 

• Control onion thrips, as feeding damage can make plants more susceptible to disease. Scout crops beginning in early May and treat at a threshold of 1-3 thrips/leaf (organic growers should use the lower threshold). For insecticide recommendations see our Onion Thrips article in the June 25, 2026 issue of Veg Notes.

Chemical Control

Lists of labeled fungicides for onion disease control are available in the New England Vegetable Management Guide, here. Cornell Cooperative Extension also provides in-depth fungicide program recommendations for onions, but resistance to certain FRAC groups may be more widespread in NY than in other states because onions are grown on a much larger scale in NY than in other New England states. Those recommendations from Cornell, updated for 2026, are available here, and organic fungicide recommendations are here.

--Written by Genevieve Higgins

Sap Beetles in Sweet Corn

We have received several reports of high sap beetle pressure so far this summer, which is not surprising as sap beetle infestations tend to be worse in hot, dry years. Sap beetles, also known as picnic beetles, are often considered secondary pests of sweet corn, usually associated with damage caused by caterpillars. However, on some farms and in some years, they are a regular and troublesome pest in early sweet corn plantings even where caterpillars have been non-existent. Early sweet corn varieties tend to have poor tip cover, allowing sap beetle adults to lay eggs near the tip, where tiny larvae burrow into the kernels and make the ears unmarketable. Sap beetles can also be pests of strawberry and other fruits, so they tend to be more of a recurring problem on farms that grow both fruit and corn. The beetles are attracted to sweet, decaying plant material, particularly fruit. Growers with sweet corn plantings that are close to peach or apple orchards, where overripe dropped fruit can attract adult beetles, are vulnerable to invasions into corn and should look out for this pest when scouting. 

Life Cycle and Damage

Sap beetles overwinter as adults, often in the woods near previous feeding sites. Early sweet corn silk is an attractive early-season feeding and egg-laying site, especially when fruits and other hosts are rare. There are several generations per year. The most common sap beetles in corn are the dusky sap beetle (Carpophilus lugubris), which is 3.5–4.5 mm long and all black, and the four-spotted sap beetle (also known as picnic beetle, Glischrochilus quadrisignatus) which is 5–6 mm long and black with four irregular yellow spots. The most common species in strawberries is the strawberry sap beetle (Stelidota geminate).

Adults are first noticeable around the time that tassels and silk appear. Males produce an aggregation pheromone that attracts other beetles, both male and female. Adults move to corn at full tassel to feed on pollen, building up as corn matures and silk turns brown. There are 2 to 4 generations per year with peak infestations in July (larvae) and late July and August (adults). 

Beetles may invade corn borer tunnels or areas with other insect or bird damage, but they can also be found in corn that is free of caterpillar damage. They lay eggs in silks and the tip of ears. Eggs are milky white and resemble tiny grains of rice. The larvae are small, pinkish-white or creamy colored grubs about ¼-inch long. They may hollow out kernels of the upper half of the ear, making ears unmarketable. Adults may also hide between the layers of the husk. The problem can easily be overlooked until harvest, when adults show up in harvest bins and larvae are found in the ears. Full-grown larvae drop to the ground and pupate in the soil. 

Cultural controls are essential to managing sap beetles:

• Grow varieties with long, tight tip cover. Ears with exposed tips, especially super sweet and Bt varieties, are more susceptible to infestation. Some varieties with good tip cover are: Argent, Avalon, Awesome, Bon Jour, Cuppa-Joe, Precious Gem, Profit, and Providence.
• Use clean cultivation. Do not leave infested blocks standing. Mow aggressively to chop ears as soon as the block is finished. Deep plowing may be necessary after harvest for severe infestations; bury ears at least 4” deep.
• Control birds and ear-infesting caterpillars. See our articles Preventing Bird Damage in Sweet Corn, Identifying Caterpillars in Sweet Corn, and Corn Earworm Management for more information.
• Eliminate or deeply bury any cull piles or other areas with decaying vegetables or fruit, including infested ears.

Monitoring and Chemical Control. The most effective time to spray for sap beetles is in early silk, so scout blocks at full tassel and early silk to determine if beetles are present. Unfortunately, there are no specific thresholds based on scouting. Insecticides may be warranted in fields with a previous history of at least 10% ear damage. Research in Maryland showed that ear infestation begins just after silk emerges and that 1 or 2 applications made 3 and 6–7 days after silking is more effective than later or more applications. Later sprays did not improve control. Insecticides will reduce the number of damaged kernels and ears but will not completely control heavy infestations. Sap beetle adults and larvae are not susceptible to the Bt toxin that is present in Bt corn. Efficacy trials have shown that carbaryl (e.g. Sevin), lambda-cyhalothrin (e.g. Warrior II), bifenthrin (e.g. Bifenture), and methomyl (e.g. Lannate) are more effective than most other insecticides. However, carbaryl cannot be used during the early silk period while corn is shedding pollen and does not allow for hand harvesting after use. Most of the materials listed in the New England Vegetable Management Guide for use against sap beetles are highly toxic to bees, except for the neonicotinoid acetamiprid (Assail 30SG), which has a moderate bee toxicity rating. When spraying for sap beetle, consult the label and spray in such a way as to protect bees. 

--UMass Extension Vegetable Program