Vegetable Notes 2026 Vol. 38:14

Sweet Corn

European corn borer (ECB) trap counts are still low for this time of year, and reports of damage in the field are rare. See the Sweet Corn Insect Management Field Scouting Guide for instructions on sequential scouting and thresholds.

Corn earworm trap captures continue to increase gradually as we get later in the season and corn begins to silk. Our sites reported an average of about 5 moths captured this week, corresponding to a recommended spray interval of 5 days, though several sites continue to report very low numbers that don't warrant sprays. 

Multiple Crops

Scarab beetles are out en masse and chewing on foliage of many different crops like cucurbits, basil, beans, peppers, greens, and more; not to mention berries and grapes. Asiatic garden beetles hang out in the soil and chew on plant roots, they are also abundant now and can cause reduced plant stand in severe cases. Insecticides may be needed to control adult beetles if numbers are high and damage is significant. The New England Vegetable Management Guide lists products for Japanese and/or Oriental Beetles in asparagus, basil, okra, and sweet corn. For controls in a crop where these beetles are rarely a pest and therefore not mentioned in the Guide, check the label of commonly used broad spectrum synthetic pyrethroids, carbamates, and neonicotinoids (as foliar spray). Organic options include neem products and pyrethrin.

Squash Bugs Out Now

Squash bugs are a perennial problem in cucurbit crops, causing leaf dieback, Anasa wilt, and physical damage to squash fruit. Nymphs are emerging now, so it’s a good time to start including them in your pest management plan or taking note of their presence and making a plan for next year.

Cultural Control

If possible, rotate cucurbit crops between fields as far apart as possible. Placing row covers over the young crop prevents adult access until blooming, when covers must be removed. Natural enemies of the squash bug include the tachinid fly Trichopoda pennipes which is a parasitoid that attacks nymphs and adults and can commonly be found in cucurbit fields at this time of year, and several wasps that parasitize eggs (Hymenoptera: Encyrtidae and Platygastridae). Squash bugs like sheltered hiding places, so keep field edges mowed and free of debris to reduce overwintering sites. Plastic and straw mulch and reduced tillage systems encourage higher populations, probably by providing good hiding places. In small plantings, boards can be used to attract adults seeking a protected hiding place; check in the morning or evening and spray with insecticide or capture and squish. Remove crop residues and/or till field immediately after harvest to kill adults before they move to field edges seeking shelter. Research has shown that squash bugs are highly attracted to Blue Hubbard squash, which can be used effectively as a trap crop planted earlier in the season along field edges. The trap crop must receive an insecticide application or be mechanically destroyed before eggs hatch.

Scouting and Chemical Control

Scout plants from seedling to vining/flowering stage to detect adults as well as eggs and nymphs. After flowering, thresholds are based on egg masses and young nymphs, but also note adults and large nymphs while scouting. Care must be taken to avoid harming pollinators when spraying flowering cucurbits as they are highly dependent on pollinating insects.

There are two key windows for control:

• Target adults on young plants (before flowering or vining). An insecticide application made when adults are colonizing plants in June can prevent subsequent egg and larval populations, but products with low efficacy may struggle to control adult squash bugs, which have a tough exoskeleton and are much less susceptible to insecticides than small nymphs. Coverage is easier at this time, and broad-spectrum pyrethroids (e.g. bifenthrin, lambda-cyhalothrin, permethrin) or carbaryl, which are reported to be effective on adults at this stage, can be used without risk to bees on the crop. OMRI-approved pyrethrin products are available (e.g. PyGanic). The threshold for targeting adults has been determined for watermelon crops at an average of 1 adult per plant (Dogramaci et al. 2006), but in more susceptible crops such as summer squash and zucchini, it might be appropriate to use a lower threshold (e.g. 1 adult per 2 or more plants). Aim for coverage of underside of leaves and stems where bugs hide. Systemic furrow, drip, or seed treatments and sprays for cucumber beetle at the seedling stage may also control colonizing squash bug adults.
• Target smaller nymphs on flowering plants. Scout for egg masses and note the first emergence of nymphs. The threshold is reached at an average of 1 egg mass per plant and when the first nymphs are seen. Good coverage of undersides of leaves is needed. For newly laid eggs and nymphs, consider a foliar application of acetamiprid (Assail 30 SG) which has moderate toxicity to bees (lower than other neonicotinoids). Adults and larger nymphs are more difficult to control, partly because they hide in the lower canopy and near the soil. An organic option for nymphs is a mixture of pyrethrin (a contact toxin) and azadirachtin (an insect growth regulator, derived from neem). This is gentler on bees than a high rate of pyrethrin alone and includes two modes of action. Insect growth regulators work to disrupt the molting process, so they are useful only on immature stages. Treat late in the day when the flowers are closed to reduce risk to bees. 

Take note of re-entry and pre-harvest intervals of materials used on summer squash and zucchini that are being harvested frequently. 

For more information on rates and products for squash bug control, see the Cucumber, Muskmelon, and Watermelon insect section and the Pumpkin, Squash, and Gourds insect section of the New England Vegetable Management Guide.

--UMass Vegetable Program

Bacterial Diseases of Vegetable Crops

We are seeing lots of bacterial diseases popping up after the hot then wet weather a few weeks ago. Bacterial pathogens thrive in hot, humid weather, so this is not a surprise.

Plant pathogenic bacteria can generally be divided into two categories: leaf spots and blights, and vascular pathogens.

Bacterial leaf spot and blight diseases are most often caused by species of Pseudomonas and Xanthomonas and tend to begin as small water-soaked spots that eventually turn brown. A yellow halo may or may not be present around the spots. In severe cases, spots can coalesce and destroy entire leaves. Because bacterial infection of the leaf tissue is limited by leaf veins, bacterial leaf spots are often angular in shape; however, this may not always be the case, and non-bacterial pathogens such as the downy mildews can also cause angular leaf spots.

These are the most common bacterial leaf spot diseases in vegetables:

• Bacterial leaf spot of tomato and pepper (Xanthomonas campestris pv. vesicatora) causes small, green-yellow spots on foliage that coalesce into large brown streaks. Spots do not have concentric rings or a halo. On tomato fruit, green fruit can become infected, and raised, brown, scab-like spots will develop.
• Bacterial speck of tomato (Pseudomonas syringae pv. tomato) lesions look very similar to those of bacterial leaf spot but will eventually develop a yellow halo. Spots on fruit are dark but not raised, and the surrounding tissue is often more intensely green than unaffected areas. As with bacterial leaf spot, only green fruit is susceptible to infection.
• Angular leaf spot of cucurbits (Pseudomonas syringae pv. lachrymans) affects all cucurbit crops, but cucumbers are most commonly affected. The pathogen is seed-borne so it often appears in the spring on cucurbit transplants. Lesions are initially small and round but become trapped by leaf veins and become angular. The centers of the lesions often crack and drop out, producing a “shot-hole” appearance. Under moist conditions, a milky white exudate containing bacterial cells may ooze out of the lesions on the lower leaf surface. 

Click on images to enlarge and see descriptions. 

Some bacterial pathogens are capable of entering the host’s vascular system, where their proliferation blocks water-conducting vessels, causing a symptom known as vascular wilt. Plants suffering from vascular wilt will show symptoms of wilting even when soil moisture is adequate. Wilt may occur on only one side of affected hosts or may involve the entire plant.

Three common vascular diseases in vegetable crops are:

• Black rot of brassicas (Xanthomonas campestris); characteristic V-shaped lesions with brown centers and yellow margins form at the leaf margins when bacteria enter through hydathodes. Veins within lesions turn black. When bacteria enter through wounds, irregular lesions and localized vascular discoloration typically occur. 
• Bacterial canker of tomato (Clavibacter michiganensis); typically causes marginal necrosis of leaves. Cankers and vascular discoloration are visible in stems. Adventitious roots are often abundant. Fruit may develop small white lesions with brown centers. Peppers are also susceptible though are not as commonly affected as tomato.
• Bacterial wilt of cucurbits (Erwinia tracheiphila); cucumbers and melons are most susceptible to this disease while symptoms in squash and pumpkin are typically milder. Cucumbers and melons develop wilting that may or may not disappear when temperatures drop in the evening. Eventually, leaves turn yellow or brown and plants collapse. Bacterial strands may be observed when the lower stem is cut and the two ends are gently pulled away from each other. Striped cucumber beetles spread the pathogen from plant to plant. 

Management of bacterial diseases can be challenging, both in the greenhouse and in the field. They are best controlled by an integrated pest management (IPM) plan including elements such as crop sanitation, cultivar selection, and preventative application of bactericides. It is also helpful to understand a few things about bacterial biology.

Most plant pathogenic bacteria are capable of growing at a wide range of temperatures, but they tend to thrive at slightly warmer temperatures than many fungi, and each has an optimum range in which disease development is most likely to occur. For instance, Pseudomonas syringae pv. syringae, which causes a leaf spot on solanaceous crops, is most active at 61-75°F, while the optimum temperature range for Xanthomonas species is 77-86°F. Although disease development may slow or even cease at temperatures well outside the optimum, infections may simply remain quiescent until environmental conditions once again become conducive to disease. 

While bacteria may sometimes be present in aerosols and can be carried on the wind, they are generally not as easily moved about by air currents as fungal spores are. Bacterial transmission between plants is most commonly facilitated by splashing water (especially wind-driven rain and overhead irrigation) and human activity. Insects may also carry bacteria on their bodies or in their saliva. Unlike many fungi, bacteria cannot penetrate host tissue directly, but instead must enter through small wounds or natural openings such as stomata and hydathodes. Plant pathogenic bacteria require the presence of free moisture for several hours in order to cause infection, and so prolonged periods of high humidity and leaf wetness are highly conducive to disease development. Cultural management practices aimed at lowering humidity and leaf wetness duration are therefore critical for disease prevention. In the greenhouse, these practices include heating and ventilating and increasing horizontal air flow by using fans. In the field, use drip irrigation instead of overhead, and plant in areas of full sun, adequate soil drainage, and good air circulation. Proper plant spacing and weed management should be employed in both scenarios.

Start with clean seeds and transplants. A good IPM program involves starting with clean seeds and transplants. Like some fungal diseases, bacterial diseases can also be seed-borne. Hot water treatment can significantly reduce the number of bacterial cells present in seed. The UMass Extension Vegetable Program offers a hot water seed treatment service; click here for more information. If pots or stakes must be re-used, they should be sanitized. Do not save seeds from infected plants. Grow resistant varieties when available. Some bacterial pathogens (e.g. Xanthomonas campestris pv. vesicatoria, which causes leaf spot of peppers) are comprised of several races, so keep in mind that resistant cultivars may not be resistant to all races. Current information on disease resistant vegetable cultivars may be found at vegetablemdonline.ppath.cornell.edu/Tables/TableList.htm. 

Field sanitation. Most plant pathogenic bacteria do not survive more than a month or two on their own in the soil; however, they can survive for much longer inside of infected plant debris in the soil and may also persist in perennial weeds. Sanitation is therefore another important cultural practice for disease prevention and management. Remove infected plants and plant debris from the greenhouse or field. It is also advisable to remove healthy looking plants adjacent to symptomatic ones. In the field, plow deeply at the end of the season to bury remaining plant debris and speed its breakdown. In no-till systems, remove crop debris as thoroughly as possible and dispose of it off-site. Rotate away from host crops for at least two years. In addition to decreasing relative humidity, good weed management also removes alternative host plants. 

Chemical control. Some fungicides containing copper, copper plus mancozeb, and phosphorus acids also have bactericidal activity and are labeled for use on vegetables. Organic products based on botanical oils, Bacillus species, and other ingredients may also be helpful. Trade name of products with those active ingredients can be found in Table 26 of the New England Vegetable Management Guide. Keep in mind, however, that the efficacy of these products is limited and there is no substitute for good crop management practices. 

Tips for maximizing spray efficacy include the following:

• Obtain an accurate diagnosis. With the exception of those mentioned above, most fungicides have no effect on bacteria. Knowing that a disease is caused by a bacterium and not a fungus enables the grower to select an effective product to apply, potentially saving time, money, and unnecessary applications of pesticides.
• Be aware that none of these products can cure a plant that is already infected, but they can help prevent healthy plants from becoming infected; therefore, they are best used as protectants.
• Thorough coverage is imperative.
• Remove symptomatic plants and plant debris from the greenhouse or field as thoroughly as possible before spraying. 
• Don’t apply bactericides when plants are wet or use an air blast sprayer. The force of the air blast sprayer can spread drops of moisture containing bacteria among plants and at the same time cause small wounds through which infection can occur. 
• Do not rely on copper alone for bacterial disease management. Copper-resistant strains of some plant pathogenic bacteria have been identified. 
• Experimental evidence suggests that greater efficacy may be achieved when a plant defense activator like acibenzolar-s-methyl (e.g., Actigard) or extract of Reynoutria (e.g., Regalia) is included in the spray program.

--Written by Angela Madeiras, UMass Plant Disease Diagnostic Lab