Vegetable Notes 2024 Vol. 36:9

It rained this week, in some places quite heavily in a short period of time, making fields inaccessible. Crops that are already planted are starting to look lush—kale, collards, and cabbage plantings are filling in, potatoes are up and folks are hilling, and strawberries are ripening, with the first berries on shelves now! Sweet potatoes have been planted, and brassicas and greens continue to go in. Field tomatoes are taking off, so now trellising is a regular task, along with continual planting, harvesting, and weeding. We’re still in the early season of greens, greens, greens on farmstand shelves and wholesale lists, but summer squash is around the corner.

We have a full schedule of summer field walks and twilight meetings, with the first coming up in a few weeks on June 12 at Tangerini's Farm in Millis. We'll be hearing from farm owner Steve Chiarizio about the new tile drainage system and bioreactor that they installed at the farm last year. A representative from Alleghany Services, who installed the system, will also be on-hand to answer questions. And Maria Gannett, UMass Extension Weed Specialist, and Sue Scheufele, UMass Extension Vegetable IPM Specialist, will talk about sweet corn weed and insect pest management options. Register in advance so we can get a headcount for food ordering! See the Events section for more info, or click here to register.

Stemphylium Leaf Blight of Onion

Stemphylium leaf blight (SLB) of onion is caused by the fungus Stemphylium vesicarium. This disease has become increasingly problematic in onion growing regions of eastern North America over the past 15 years. We haven’t seen SLB in Massachusetts, although symptoms can look very similar to those of purple blotch or onion downy mildew, so it is possible that it is present but being misdiagnosed in the field. Downy mildew may be possible to definitively ID in the field but distinguishing between SLB and purple blotch requires a microscope.

Stemphylium produces small, elliptical, tan to brown or purplish lesions on onion foliage, sometimes with concentric rings. In severe cases, lesions may coalesce and defoliation may follow. Bulbs are not infected, but loss of photosynthetic capacity results in decreased bulb size and quality.

The fungus has a wide host range that includes ornamental plants. It has also been reported on alfalfa, asparagus, parsley, celery, pear, radish, soybean, and spinach. Recent research has shown that S. vesicarium may also infect several species of common weeds, sometimes without producing visible symptoms. Infected crop residue, volunteer onions, and weeds may be sources of inoculum in spring.

Host specificity of S. vesicarium strains is not well understood. One study from Spain demonstrated that isolates from garlic, onion, and asparagus could cause disease in all three hosts. Another study showed that isolates from parsley were pathogenic to carrot, celery, and pear, but not to leek, onion, spinach, or tomato. Recently, an outbreak of leaf spot in celery caused by S. vesicarium was observed following rotation away from onion in Michigan. S. vesicarium does not cause gray leaf spot (GLS) of tomatoes—that disease is caused by Stemphylium solani and S. lycopersici. Although the pathogen may be seedborne, seed is not considered to be a significant source of inoculum in conventional onion production in North America; in addition to the widespread use of fungicide treated seed, most domestic onion seed is produced in arid areas of the continent where SLB is uncommon.

A successful management program for SLB includes controlling weeds in and around onion fields. Control insect pests, especially onion thrips, which may increase disease incidence and severity. Rotate with non-host crops. No resistant onion cultivars are currently available. Disease forecasting models established for other similar foliar diseases have so far not been able to reliably predict S. vesicarium infection periods.

Several fungicides are labeled for use against Stemphylium leaf blight on onions. For a full listing, see the onion, scallion, and shallot disease control section of the New England Vegetable Management Guide. Research conducted in Ontario and New York onion fields found that a significant number of S. vesicarium isolates collected were insensitive to azoxystrobin (e.g. Quadris) , boscalid (e.g. Endura), cyprodinil (e.g. Switch, Inspire Super), and/or pyrimethanil (e.g. Scala, Luna Tranquility).

--Written by Angie Madieras, UMass Plant Diagnostic Lab

Colorado Potato Beetle Management

Colorado potato beetle (CPB) adults are likely just emerging from their overwintering sites and will soon begin to lay eggs on the undersides of leaves. This pest can quickly skeletonize potato and eggplant leaves and does significant damage to these crops every season. It is critical to control CPB when larvae are small, as larval damage is more severe than that of adults, with larger larvae causing 85% of the feeding damage. Adults are also much more tolerant to pesticides due to their hard outer wing case and the fact that they consume relatively little leaf material that may be treated with insecticide. Good control of CPB in June will not only protect vulnerable crops now, but will also reduce the number of beetles in the next generation that will overwinter to feed on next year’s crops.

Life Cycle

In the Northeast, CPB feeds on solanaceous crops and weeds including potato, eggplant, tomato (primarily seedlings), horsenettle, and Eastern black nightshade. CPB overwinters in the adult stage, generally in soil (up to 12 inches deep) in the woods and brushy borders next to host crops, though some burrow into soil right in the field. In late spring, the beetles emerge to search for host plants. Adult CPB are fairly slow and clumsy; they can fly when temperatures are warm enough, but more often they walk into fields from overwintering sites. Heavy feeding may occur on edges of non-rotated fields. If beetles do not find host plants via walking, they will fly in search of food. Once they reach a host plant, adults feed, mate, and lay clusters of yellow-orange eggs on the undersides of leaves. One female can lay up to 300 eggs in her lifetime. Eggs hatch in 7 to 10 days, depending on temperature. The larvae are reddish-orange and grub-like with rows of black spots along the sides of their abdomen. They, along with the feeding damage they cause, are easily seen on leaves. Larvae go through four molts before they pupate—each stage of the larva is called an “instar”. In the first instar, the larvae are about the same size as the eggs and in the second instar they are about ⅛” long. Mature, fourth instar larvae are hump-backed and plump, and reach ⅝” before they drop to the soil and pupate. Adults emerge from pupae after 10 to 14 days, leaving round exit holes at the soil surface. In southern New England, there is a second generation of eggs, larvae, and adults in late July, while there is only one generation in northern New England. Beetles fly or walk out of fields in August, seeking overwintering sites at field edges.

Monitoring & Thresholds

The scouting procedure below was established for potato, but a similar process can be used in eggplant. Spray thresholds for both crops are listed in Table 1. There are no established thresholds for CPB in tomato, as it is not a preferred crop.

Scout weekly. Scout for beetles on 30 to 50 plants (or individual stalks when plants are taller than 18”). One recommended procedure is to walk the field in a V-shaped pattern and stop at 10 sites across the field. Randomize your selection of sites using a set number of paces (e.g. stop every 10 to 30 paces, depending on field size). At each location, select 3 to 5 plants or stalks, depending on the size of the plants. Alternatively, select plants or stalks individually at random across the field. Count adults, large larvae (greater than half-grown), and small larvae (less than half-grown) separately. A spray is warranted if any of the following thresholds are met:

If you are getting near the spray threshold, pay extra attention and scout again in 3-4 days. The New England Vegetable Management Guide recommends this tighter scouting schedule if numbers are above 15 adults, 75 small larvae, or 30 large larvae per 50 plants/stalks. 

Use these scouting sheets to help keep track of the CPB populations: Potato, Eggplant, Tomato. These can be used for several different insects and diseases in each crop.

Controls & Prevention:

• Rotation. The single most important tactic for CPB management is to rotate potatoes, eggplants, and tomatoes to a field that is at least 200 yards from the previous year’s fields. Barriers such as roads, rivers, woodlands, and fields with other crops are helpful, because CPB adults are such slow and clumsy movers. This single practice delays and reduces colonization by adults, and therefore reduces the number of eggs and larvae in the field later.
• Crop health. Starting with healthy seed and maintaining good crop nutrition helps plants grow well and withstand feeding injury.
• Physical barriers such as straw mulch, trap crops, and trench traps may delay and reduce infestation.
• Straw mulch. It has been well documented that when potatoes or eggplants are mulched with straw, fewer CPB adults will settle on the plants and fewer eggs will be laid. This can be accomplished on larger plantings by planting into a rye cover crop, mowing down the rye, then pushing the rye straw over the plants after they emerge. For smaller plots, straw may be carried in.
• Perimeter trap cropping. Potato trap crops may be planted earlier than the main crop to attract beetles before the main crop emerges or planted between overwintering sites and this season’s crop. Flame, vacuum, or spray the border crop before beetles move into the main crop. Another approach is to plant 3 to 5 rows of potatoes treated with a systemic insecticide in a perimeter around the field; this treated border will kill up to 80% of the colonizing beetles. Planting main potato crops later than normal may also cause beetles to leave the field before potatoes emerge, resulting in lower beetle numbers.
• Trenches. This is a common recommendation, but we’ve never actually seen it in practice! The recommendation is to install plastic-lined trench traps next to overwintering sites at least 1 week before adults emerge, with trenches 1 - 2’ deep and 6 - 24” wide at the top. They can be U- or V-shaped with side walls sloping at angles between 65° and 90°. Beetles walking from field borders fall into the trench and cannot fly out. Let us know if you’ve ever tried it and if you’ve run into any pitfalls (maybe literally!).
• Flaming. Flame weeders can be used to kill colonizing adult beetles in potatoes when the crop is less than 5” tall. Move rapidly using a tractor-mounted or hand-held flamer. The goal is to scorch beetles, not burn them to death— injury to antennae and legs renders them unable to orient and climb plants, while burning them to death will result in excessive plant damage. At this early stage, healthy emerging potatoes have sufficient reserves to regrow foliage and establish well.
• Biological control. Naturally occurring predators and parasites of CPB help suppress populations and prevent crop injury. Natural enemies that attack CPB eggs or larvae include twelve-spotted lady beetle (Coleomegilla maculata), spined soldier bug (Podisus maculiventris), a ground beetle (Lebia grandis), and a parasitic tachinid fly (Myiopharus doryphorae). The fungus Beauvaria bassiana (e.g. Mycotrol) has been shown to suppress beetle populations, though it does not provide immediate control. Using selective rather than broad-spectrum insecticides can help to conserve these natural enemies. Be aware that lady beetle egg masses look very similar to CPB egg masses, but lady beetle eggs are more yellow and slightly smaller (~1mm) than CPB eggs (~1.7 to 1.8mm), which are more orange.
• Hand removal. For smaller plantings and early infestations, walking through the crop to squish egg masses or drop adults into buckets of soapy water can be very effective to delay the build-up of damaging populations.

Chemical Control

Scout to determine if a damaging population is present. Labeled conventional products include pyrethroids, neonicotinoids, novaluron (e.g. Rimon), cyromazine (e.g. Trigard), and diamides (e.g. Verimark, Exirel). In recent years, we have observed resistance to both neonicotinoids (e.g. Assail, Belay, Venom, Actara, Cruiser, Platinum) and synthetic spinosads (e.g. Radiant) in New England. When using products that control only larvae or only small larvae, scout for eggs, note egg hatch, and apply controls before larvae reach third instar to avoid the worst feeding injury. For materials that control all stages, you may wait and scout for adults and larvae to determine the need to apply insecticides.

For organically managed fields, the selection of insecticides is limited to fewer active ingredients including spinosad (Entrust), azadirachtin (Azatin), pyrethrin (Pyganic), and Beauvaria bassiana (Mycotrol O, Botanigard), which can be tank-mixed and/or rotated. The Bt product Trident, which was developed a few years ago for CPB control, is still off the market due to formulation issues. With few products to choose from, it’s more important to time pesticide applications so that they are as effective as possible and therefore reduce the need for subsequent applications.

Ledprona is a new insecticide that was registered this year by the EPA for controlling CPB and is manufactured under the brand name Calantha. It is not yet registered in Massachusetts but is registered for use in the rest of New England. It is the first sprayable insecticide utilizing a novel mode of action based on RNA interference (RNAi) with the goal of providing effective and specific control of CPB. RNAi is a natural biological process which exists in most organisms and regulates the translation of messenger RNA, a step leading to the synthesis of proteins. Ingestion of certain manufactured double-stranded RNA (dsRNA) can activate this interference process in some organisms such as CPB, disrupting the expression of a vital enzyme in the insect’s metabolism and resulting in mortality within a few days of consumption. While the potential for dsRNA technology to cause non-target effects has been shown in other organisms, field studies of Ledprona have not found any significant negative effects on non-target arthropods. This, along with dsRNA’s rapid degradation in the environment, may allow for a decrease in environmental impacts stemming from CPB management. However, like with any other insecticide, resistance to RNAi-based biopesticides is likely to emerge if resistance management strategies are not used. Cautious use of this insecticide in combination with other IPM principles shows promise as a method of providing effective control of CPB while mitigating environmental risks.

Do not try to kill every beetle in the field. Potato crops can withstand 15% defoliation without affecting yields. Avoid treating potatoes for CPB late in the season, as defoliation less than two weeks before senescence will have little effect on final tuber bulking.

Resistance management must be part of every potato grower’s plan. CPB has a remarkable capacity to develop resistance to insecticides. Based on a fifty-year track record, we can expect that any insecticide that is used repeatedly on the same population of CPB (that is, those in the same field or farm) will lose its efficacy in less than five years. Where potato production is concentrated and rotation has been limited, resistance may develop on a region-wide basis. It’s up to you to manage resistance in the population of beetles on your farm, and keeping insecticides effective with careful rotations is a worthwhile investment. In the New England Vegetable Management Guide, as well as on pesticide labels, each insecticide has an IRAC (Insecticide Resistance Action Committee) Group Number, which identifies chemistries with the same mode of action. Growers should note the resistance group number of each insecticide, rotate classes of insecticides, and avoid using the same chemistry more than once per year, or even better, once every other year. Do not use the same chemical class on successive generations in the same year. Use newer chemistries first. For conventionally managed fields, there are enough different products to do a two-year rotation that will effectively control CPB while effectively delaying resistance to any one product.

See the potato and eggplant insect control sections of the New England Vegetable Management Guide for full lists of labeled materials.

--UMass Vegetable Program