Vegetable Notes 2025 Vol. 37:9

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Crop Conditions

Sighs of relief can be heard across the state as the sun has come out for a few days in a row. Time to get out there and kill a few weeds! Most of Massachusetts saw 7-11 inches of rain in May, up to twice the normal precipitation in many areas. Rainy and overcast conditions have led to slower growth and slower sales of spring crops and seedlings or bedding plants. 

We are pleased to announce that Lynne McLandsborough has been named director of the University of Massachusetts Amherst’s Center for Agriculture, Food and the Environment (CAFE) and will also serve as assistant vice chancellor for research and engagement. She brings to her new roles 30 years of experience at UMass Amherst, where she bridged the academic and applied sciences and gained deep expertise in working with the commonwealth’s UMass Extension program, which reaches into every corner of agricultural life in Massachusetts. Follow the link to read the full announcement.

Contact Us

Contact the UMass Extension Vegetable Program with your farm-related questions, any time of the year. We always do our best to respond to all inquiries.

Vegetable Program: 413-577-3976, umassveg[at]umass[dot]edu (umassveg[at]umass[dot]edu)

Staff Directory: https://ag.umass.edu/vegetable/faculty-staff

Home Gardeners: Please contact the UMass GreenInfo Help Line with home gardening and homesteading questions, at greeninfo[at]umext[dot]umass[dot]edu (greeninfo[at]umext[dot]umass[dot]edu).

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Pest Alerts

Alliums

Onion thrips: Eggs are now hatching, and we are starting to see the first nymphs on onions. We have not yet seen thrips numbers above treatment thresholds (1-3 thrips per leaf), but we expect that warmer weather will accelerate their development. Continue scouting weekly and be prepared to treat once populations are at threshold. Organic growers should use the lower 1 thrips per leaf threshold. See our article from this May for more information. 

Asparagus

An Important Window for Weed Management: Weed management in asparagus continues to be difficult, but renovation is another important time to manage weeds. This is your opportunity to apply another pre-emergent herbicide. You’ll notice in the table in this article that you’re only allowed one application per year of many of the pre-emergent products, so rotate to a different product at renovation. See this article for a reminder of the basics of pre-emergent herbicides. To have the best coverage, the ground needs to be clear of vegetation, so you will need to also manage any early-season weed growth. This can be done by including a post-emergent burn-down herbicide or with shallow cultivation. You may also consider using a post-emergent systemic herbicide at renovation, especially if you’re targeting perennial weeds (products such as 2,4-D, clopyralid, dicamba, or glyphosate). Many systemic herbicides are more dangerous to your asparagus crop, so be sure to clean harvest your asparagus (pick all the spears) below the soil. You risk damaging the crown of any asparagus exposed above the soil when applying a post-emergent systemic herbicide. See the New England Vegetable Management Guide for a full list of herbicide options for asparagus.

Asparagus beetle eggs are being laid now in Hampshire County. Treat spears if >10% of the plants are infested with beetles or 1-2% have eggs or damage. Scout ferns for all life stages of both pests and treat if 50-75% are infested. If possible, spot spray along edges of planting where overwintering adults colonize the field and/or band insecticide over the row to help conserve natural enemies. Use selective insecticides on ferns. 

Cucurbits

Striped cucumber beetles are out in force in field cucumbers and squash. Damage from adults and larvae can cause reductions in stand, plant growth, and yield, and higher numbers of beetles increase the risk of bacterial wilt. Early control is most effective. See the article in our last issue for more information including management recommendations.  

Nightshades

Colorado potato beetle: We are now seeing eggs hatching in areas where CPB was first seen this year. Get the most bang for your buck by waiting until 10-50% egg hatch to spray larvae and making a follow-up application in 1 week.

Three-lined potato beetle adults and larvae were seen causing heavy damage on tomatillos in Hampshire County. Severe infestations on tomatillos can be treated organically with Entrust or conventionally with Radiant, Agri-Mek, or pyrethroids. 

Strawberry

Cyclamen mites were confirmed in Worcester County this week, causing stunted and distorted leaves, death of flowers, and shrunken fruit with protruding seeds. Once populations reach a damaging level it can be hard to bring them back under control. Cyclamen mites are smaller than two-spotted and other mites and at least 20X magnification is needed to spot them. Look at the youngest leaves, along the midvein and on the underside of the leaf at the petiole. Thorough spray coverage of the crown leaves is important for good control, so high volumes of water are needed. Horticultural oils can be used if temperatures are below 88°F. Agri-Mek SC (3-day PHI) or Portal XLO (1-day PHI) also can be used for mite control. Predatory mites can be used and work best if cyclamen mite populations are small and confined to scattered hot-spots in a field.

Neopestalotiopsis crown rot is suspected in strawberry plants in Middlesex County. Recent rainfall is very likely contributing to the disease development and spread. This pathogen has increasingly been identified in plug transplants sourced from southern nurseries and can lead to progressive decline in newly established fields. Symptoms include stunting, crown discoloration, and patchy distribution. Fruit rot can also occur. Remove symptomatic plants and plants within a radius of affected plants. Limit overhead irrigation and sanitize tools to reduce further spread. For conventional growers, rotate fungicides Switch (cyprodinil + fludioxonil) and Captan. 

Sweet corn

Corn earworm (CEW) numbers are slowly increasing in areas of southeastern MA, but we have not yet received confirmed reports of moths from other parts of the state. 

European corn borer (ECB) caterpillars are starting to be seen in fields of corn grown under plastic, which is already at the whorl stage. This should be a reminder that a) traps should be deployed by June 1st to ensure you get an early warning, and b) pheromone trapping for ECB can underestimate damage in the field, and scouting should still be done to determine when a treatment is necessary—15% infested plants. Using a sequential sampling protocol can make this pretty quick. See our corn scouting worksheet for details.

As vegetable planting season really gets under way, I want to remind everyone to consider incorporating the stale seedbed technique into their weed management plan. This technique can reduce weeds by 43-83% within the cropping season^1,2, leading to increased yields^3,4 and can be useful for managing weeds in almost any vegetable production system: organic, conventional, reduced tillage, large-scale, or small. The idea is to reduce weed seeds within the soil seedbank⁵ by first stimulating germination of as many seeds as possible and then killing them before planting. This article will go into some details about the stale seedbed technique, including the weed species that it best targets, and the tools that are most effective at killing emerged weeds. But if you are pressed for time and just want to jump into using the stale seedbed technique, here are the general steps:

1. Figure out the timing of your stale seedbed.

Starting the stale seedbed technique can be done days, weeks, or months in advance of actually planting your crop⁴. You can experiment with timing, but generally people begin 24-40 days before you plan to plant your crop^5,6. Start now (mid-June) for crops you’re planting in mid-July⁶. If you have less time, that’s ok, even a week can help with early weed control. This technique is somewhat seasonally dependent due to weed species periodicity, which is the period of maximum germination for a species within a year⁷. It is especially useful for crops that you plant later in the summer because more weed species will have emerged. It has been suggested for brassicas, spinach, lettuce, summer squash, beans, beets and herbs, planted in the summer or fall⁶.

2. Prepare the seedbed.

Conduct any plowing, disking, or bed-forming practices that you typically use now, so that you can reduce the need for soil disturbance later. At this stage you want to stimulate germination of weed seeds that are dormant in the soil, called the soil seedbank. Soil disturbance is a useful way to stimulate seed germination, so consider using at least one tillage event at this point. More intensive soil crumbling can introduce more oxygen into the soil, increasing germination and resulting in a greater reduction of seeds in the soil seedbank⁵. But if you are prioritizing reduced tillage, irrigation and soil warming through tarping* may also help.

3. Encourage weed seeds to germinate.

Give the weed seeds within the soil seedbank time to germinate. Make sure they have enough soil moisture to emerge and grow. The more weeds you see at this stage, the fewer will be in your soil seedbank to emerge and manage later.

4. Kill all the emerged weeds.

Once seeds have germinated, kill seedlings with a non-selective tool of your choice⁴. That can be shallow tillage, flaming, herbicides or tarping. Usually, these non-selective tools work better when the weeds are smaller, so don’t wait too long before killing your weeds. On the other hand, the longer you wait, the more seeds will have emerged. Target control somewhere between the cotyledon stage up until about 5 cm tall⁶.

5. Repeat steps 3 and 4 as many times as you have time for.

Repeatedly killing emerged weeds can more dramatically reduce the seeds in the soil seedbank⁵.

6. Plant your crop as close to the time of killing your weeds as possible.

With fewer weeds now in your seedbank, plant your crop as soon after you kill emerged weed seedlings as possible. Sometimes, depending on your non-selective method of killing weeds, people will even plant the crop and then kill the weeds (this can work with flaming and some herbicides). Plant with minimal soil disturbance. Additional soil disturbance at this point will stimulate new weed seeds to germinate with your crop, so be careful not to do any heavy tillage. Be especially mindful about brining soil from deeper than 10 cm within the soil profile to the surface⁵.

7. Have a plan to manage weeds later in the season.

Even if the stale seedbed technique is perfectly executed, weeds will continue to germinate and emerge from the soil while your crop is growing. Be ready to manage those weeds, whether you do that with herbicides, mulch, tillage, or a combination of tools.

The stale seedbank technique is versatile and effective, but there are many factors that affect just how well it will reduce the soil seedbank and improve weed management in your crop. You don’t have control over many of these factors, but understanding them may help you decide when and where to most effectively use this technique, as well as give you a better idea of how well it will work to control weeds on your farm.

There are many factors that contribute to breaking weed seed dormancy

Many weed seeds that successfully mature find their way into the soil. These seeds persist in the soil, collectively called the soil seedbank, as a community of weed species with the potential to germinate, live, and survive into the future⁸. Seeds in the seedbank are dormant. They are alive, but they are not growing. This dormancy is what gives them power to persist across seasons until the growing conditions for that species improve⁹. To stimulate germination, you have to break seed dormancy. Cues for breaking dormancy are different for each weed species. When you’re trying to break dormancy of many different species of seeds all at once, it’s best to rely on the most common and important triggers:

• Temperature is the biggest cue to come out of dormancy⁵. Some seeds that germinate and emerge in April would never emerge in June and vice versa. There’s not much you can do to warm up the soil to encourage greater weed seed emergence, although perhaps tarping would help*.
• Soil moisture is the next biggest cue¹⁰. Luckily you can manipulate this! If you’re using the stale seedbed technique and it hasn’t rained, irrigate to increase weed seed emergence.
• Seed depth within the soil profile greatly affects emergence. Some species will only germinate at the soil surface¹⁰ and most germinate from the top 10 cm. This is probably due to oxygen content within the soil profile, and so factors that increase oxygenation of soil microsites, such as breaking up soil clods, will increase short-term seed germination⁵.
• A flash of light – less than a second – is a common cue for seed germination. This occurs when you cultivate, triggering germination¹.

Certain weed species will be better controlled

The stale seedbed technique will be more effective at controlling certain weed species and groups of species than others. Here are some considerations when thinking about how this tool could be useful for your farm.

• This technique mostly targets annual weed species. The stale seedbed technique works by reducing the number of seeds within the soil seedbank. There is a wide diversity of species within the soil seedbank, but generally, this will be a helpful tool to control annual weeds because they usually produce more seeds and therefore contribute more seeds to the seedbank.
• Grasses may be better controlled. Generally, grasses (plants in the Poaceae family) have fewer dormancy requirements^5,11 and germinate well when irrigated¹⁰. I know a lot of you struggled to control annual grass weeds last season, so consider using the stale seedbed technique to help control those seeds set last year!
• This technique does not work well on perennial species. Once perennials, such as yellow nutsedge, are well-established, they can rely on resources stored in their root systems and can germinate from deeper within the soil profile. They are not well controlled by the stale seedbed technique⁶.
• This technique does not work well on small-seeded species. Weed species with smaller seeds, like horseweed, need to germinate at the soil surface so their germination is often reduced by using the stale seedbed technique⁵.

Weed control tools are variably effective at killing emerged weed seedlings

Once weeds have emerged you need to kill them all with a non-selective weed management tool. Any weed escapes at planting can become well-established and will compete strongly with the crop⁴. Tools range in efficacy and price, but there isn’t one clear “best” tool. Use what will fit best with your farm’s operations. Here are some results from different studies that you may want to consider:

• Killing emerged weed seedlings with flaming or glyphosate, which results in no soil disturbance, was often the most effective method. In most studies comparing the use of glyphosate to kill emerged weeds, it was the most effective management technique^1,2,6 except against yellow nutsedge^3,6.
• A field cultivator was the least expensive tool, and a power cultivator, a bed shaper, and organic herbicides were the most expensive tools^4,12.
• Sometimes more expensive tools were more efficient if they could complete two tasks at once, such as incorporating a pre-emergent herbicide simultaneously⁴.
• Tillage implements should operate at shallow depths—more intensive tillage, such as rototilling, controls more emerged weeds, but can also bring new seeds from deeper in the soil profile up into the germination zone^4,12.
• Tillage implements that are too gentle, such as a flexible tine weeder, can be ineffective at killing weeds depending on the soil conditions⁶.
• The size and species composition of a weed community can interact with the tool to impact efficacy. In one study, a rotary hoe was more effective than flaming when weeds were small (cotyledon to 1-leaf stage), but this trend reversed when the weeds were larger (2- to 3-leaf stage)¹².
• A flame weeder may struggle to control grasses¹².

Good luck and I hope you find this tool helpful on your farm! I’d love to hear how it goes for you.

*I don’t know if many people use tarps in this way, but if you do, please let me know, I would love to hear about your experience and expertise with tarps!

Works Cited

1. Riemens, M., Van Der Weide, R., Bleeker, P. & Lotz, L. Effect of stale seedbed preparations and subsequent weed control in lettuce (cv. Iceboll) on weed densities. Weed Res. 47, 149–156 (2007).
2. Kanatas, P. et al. False Seedbed and Stale Seedbed Against Important Broadleaf Weeds: A Case Study and a Step Closer to Agroecology. Plants 14, 564 (2025).
3. Johnson, W. C. I. & Mullinix, B. G. Jr. Stale seedbed weed control in cucumber. Weed Sci.46, 698–702 (1998).
4. Johnson, W. C. I. & Mullinix, B. G. Jr. Evaluation of Tillage Implements for Stale Seedbed Tillage in Peanut (Arachis hypogaea). Weed Technol. 14, 519–523 (2000).
5. Benvenuti, S. et al. Stale seedbed preparation for sustainable weed seed bank management in organic cropping systems. Sci. Hortic. 289, 110453 (2021).
6. Caldwell, B. & Mohler, C. Stale seedbed practices for vegetable production. HortScience36, 703–705 (2001).
7. Stoller, E. W. & Wax, L. M. Periodicity of Germination and Emergence of Some Annual Weeds. Weed Sci. 21, 574–580 (1973).
8. Schwartz-Lazaro, L. M. & Copes, J. T. A Review of the Soil Seedbank from a Weed Scientists Perspective. Agronomy 9, 369 (2019).
9. Jaganathan, G. K., Boenisch, G., Kattge, J. & Dalrymple, S. E. Physically, physiologically and conceptually hidden: Improving the description and communication of seed persistence. Flora 257, 151413 (2019).
10. Benvenuti, S. & Macchia, M. Seedbank Reduction after Different Stale Seedbed Techniques in Organic Agricultural Systems. Ital. J. Agron. 1, 11–21 (2006).
11. Thompson, K., Band, S. R. & Hodgson, J. G. Seed Size and Shape Predict Persistence in Soil. Funct. Ecol. 7, 236–241 (1993).
12. Boyd, N. S., Brennan, E. B. & Fennimore, S. A. Stale Seedbed Techniques for Organic Vegetable Production. Weed Technol. 20, 1052–1057 (2006).

--Written by Maria Gannett, UMass Vegetable Extension Weed Specialist

Caterpillars in Brassica Crops

There are several species of caterpillars that are major pests of brassicas in Massachusetts. We’re already seeing damage from imported cabbageworm and expect to see diamondback moth, cross-striped cabbageworm, and cabbage loopers start to show up in mid- to late summer. Though they may look similar, these four major brassica caterpillar pests have important distinctions among them that can affect management decisions. They differ in size and feeding habits, as well as their susceptibility to beneficial parasitoid insect species and certain insecticides. Getting acquainted with these pests will help you to know what kind of damage to expect and what to look for when scouting for their different life stages and biocontrols. Feeding damage by any of these caterpillars can reduce yield and marketability of both leafy and heading brassica crops.

​​​​​​Imported cabbageworm or cabbage butterfly (Pieris rapae) is a very familiar white butterfly that can be seen during the day fluttering around brassica fields. Each forewing has a dark border and one or two round black spots. Eggs are laid singly on the underside of leaves, standing upright. They are bullet-shaped, about 1/8-inch in length, and initially pale white, turning yellow as they mature. Larvae are gray-green, slightly fuzzy, and sluggish. They can be very well camouflaged, especially when they lie along the midrib of a leaf. Feeding and resting occur on the underside of leaves, and in cabbage or broccoli, the larvae feed more heavily in the head than on the other leaves. The overwintering stage is the chrysalis (pupa), which is green or brown and smooth, with three pointed ridges on its back. There are 3-4 generations per year.

Click on images to enlarge.

Diamondback moth (Plutella xylostella) adults are tiny (< ½ inch long) brown moths that rest with their wings folded together like a tent. Historically, DBM did not overwinter in the Northeast, but caterpillars have been reported as early as May in recent years, suggesting that they may be overwintering in high tunnels or other warm spots. Regardless, this pest also blows in every year from warmer areas to our south. Adults are weak fliers, but populations are known to disperse long distances on wind and annually reinvade areas well into Canada. Eggs are laid singly or in small clusters. Caterpillars go through four instars and are small (< ½ inch when fully grown), light green, and appear segmented, with a forked end and pointed shape. When disturbed, they wiggle vigorously and may drop off the plant on a string of silk. Feeding causes small, round holes that don’t break through the top layer of leaf tissue, leaving translucent films across holes, called “windowpane” damage. Feeding tends to be spread across the foliage and is not necessarily concentrated in the head.

Cabbage looper (Trichoplusia ni) usually does not survive the winter in New England and arrives in migratory flights from farther south. Generally, cabbage looper numbers are not high until late July or August, though earlier flights may occur in some years or they may not be found at all in others. Adult moths are mottled gray-brown, about ¾ inch long, with a distinct round silver-white mark on each forewing. Since they fly at night, they are rarely seen unless monitored with pheromone traps. If you want to know when moths arrive, use a wing trap baited with Trichoplusia ni lure, placed near the canopy. Eggs are round, pale green or yellow, and are laid singly underneath the foliage. The cabbage looper caterpillar is light green and smooth, with wavy white or light yellow lines down the back and sides and stubby legs at the tip of the abdomen. Full-grown larvae reach 1½ to 2 inches. Cabbage loopers of any size move like inchworms—by raising the middle of their body in a characteristic “loop” shape. Feeding tends to create large, ragged holes in foliage, on both frame leaves and heads. Cabbage looper also feeds on many non-brassicas including lettuce, celery, spinach, and chard, so when they do arrive, scout those crops as well as brassicas.

Cross-striped cabbageworm (Evergestis rimosalis): Formerly restricted to the South, this insect is now a serious problem on brassica crops in southeastern New England. Cross-striped cabbageworm does not generally overwinter in New England but we saw a very early infestation on one farm in MA last spring, suggesting that the caterpillars overwintered in high tunnel brassica crops. One of the major differences between this insect and the other brassica caterpillars is that the eggs are laid in a cluster, and caterpillars feed in a group on one plant so that it’s riddled with big holes like buckshot. CSCW is closely related to European corn borer, and the adults are similar in shape and color: straw-colored with a little purple, and crossed by wavy lines. Since the adults fly at night, you will likely only notice the caterpillars and their damage. The clusters of 3 to 25 eggs are yellow, flattened, and attached to the lower leaf surfaces. The caterpillars are light bluish-grey on top and green underneath, with numerous black transverse bands across their backs and a yellow line down each side. Larvae grow to ¾ inch long in 2 to 3 weeks. There are 2 to 3 generations per year, but generally, numbers do not reach damaging levels until late summer. Larvae can produce small holes in leaves until only veins remain, feed in terminal buds and sprouts, or burrow into heads. Plants with larvae are often completely skeletonized while adjacent plants may be left undamaged.

Scouting

It is especially important to check cabbage or broccoli plantings as they begin forming heads. Leafy greens such as collards and kale should be scouted earlier, since all leaves are marketed. Randomly select 25 plants throughout the field and check for caterpillars or fresh feeding damage on the top or underside of leaves. Look for black or green frass and tiny clustered feeding holes. It is often easier to spot the frass and feeding damage first before finding the caterpillar. Classify plants as infested (one or more caterpillars present) or non-infested, and calculate the percentage of plants infested. In the Northeast, there is generally no need to treat young plants until at least 35% are infested with caterpillars. Once heading crops (cabbage, broccoli, cauliflower) begin to form the head, a lower threshold should be used to protect the marketed head; treat heading crops after head formation if 15-20% or more of the plants are infested. The most critical time to scout and apply controls is just prior to head formation. For leafy crops like kale and collards, a 10-15% threshold should be used. Because cross-striped cabbageworm can be so destructive, a lower threshold should be used—treat when 5% of plants are infested with this pest.

Chemical Control

Use selective insecticides to protect beneficial insects that keep aphids under control, eat insect eggs and small caterpillars, and parasitize imported cabbageworm or diamondback moth. Selective products are often most effective when consumed with foliage, so coverage is important. Use at least 50 gallons of spray material per acre; higher volumes provide better coverage. Better coverage of lower leaf surfaces can also be achieved by using drop nozzles. Use a spreader-sticker to prevent sprays from rolling off of waxy leaves. The most effective materials include:

• Diamides (Group 28) including chlorantraniliprole (e.g. Coragen), cyantraniliprole (e.g. Exirel), and cyclaniliprole (e.g. Harvanta)
• Spinosyns (Group 5) including spinetoram (e.g. Radiant) and spinosad (e.g. Entrust) - also effective against flea beetles and onion thrips
• Bacillus thuringiensis (Group 11) including Bt aizawi (e.g. XenTari) and Bt kurstaki (e.g. Dipel DF, many other products) – these materials are highly selective and will ONLY affect caterpillars.

These materials and the aizawai strain of Bt will usually provide better control of resistant diamondback moth than older products. See the cabbage insect control section of the New England Vegetable Management Guide for additional synthetic and naturally derived products and more details.

Cultural and Biological Controls

Incorporate crop residues shortly after harvest to decrease movement to successive plantings and reduce overwintering populations. Populations are suppressed by a wide range of natural enemies, including several species of parasitic wasps. Diamondback moth eggs are parasitized by the ichneumonid wasp, Diadegma insulare, which occurs naturally in eastern North America. D. insulare females require sources of nectar, so maintain wildflower stands near brassica fields. Imported cabbageworm eggs are parasitized by the braconid wasp, Cotesia rubecula, which was introduced to New England from China in 1988 and is now established in Massachusetts. You may see their small white cocoons on brassica leaves. The chalcid wasp, Trichogramma brassicae, will lay its eggs in many species of caterpillar, including all of the brassica pests above (as well as non-target caterpillars, so be cautious if you are maintaining wildflowers that might attract endangered moths or butterflies). T. brassicae wasps are not found in New England but can be purchased from several biological control companies for release in brassica fields. The wasps arrive as pre-parasitized caterpillar eggs that are glued to cards that can be distributed throughout the crop. As of 2022, each card costs around $20-23, and contains about 100,000 wasps, which is enough for up to 1 acre. According to one source of T. brassicae wasps, IPM Labs Inc., some growers will release the wasps in lieu of using any kind of pesticide. Some growers release one card per acre per week for about 4 weeks, while others will release every week for the life of the crop. These biological controls are compatible with many selective and lower impact sprays used for control of caterpillars (Bt, oils, soaps), especially because the wasps are protected from sprays when they are inside of host eggs. Another source, Evergreen Growers Supply, notes that Trichogramma wasps are more effective against moth species that lay their eggs in clusters, so they may be a good option if cross-striped cabbageworm has been a particular problem on your farm.

--Written by R. Hazzard, S.B. Scheufele, and L. McKeag

Clubroot of Brassicas

Three cases of clubroot have been diagnosed at the UMass Extension Plant Diagnostic Lab this spring. Clubroot infects most crops in the Brassica genus including broccoli, cabbage, Brussels sprouts, turnip, and kale. The disease is caused by Plasmodiophora brassicae, a type of single-celled organism unrelated to plants, animals, bacteria, and fungi. Symptoms of clubroot include swelling and gall formation on roots and lower stems resulting in poor growth, yellowing, and wilt. Decay of the galls causes most of the root system to be left behind when plants are pulled from the soil. Roots and stems are often invaded by secondary bacteria. Disease development is favored by cool temperatures, wet soils, and acidic soils. P. brassicae produces thick-walled resting spores that can persist in the soil for several years.

Cultural practices are the most effective management tools:

• Use soilless potting media when starting seedlings in the greenhouse. Do not reuse pots, flats, or media if the crop is contaminated with P. brassicae.
• In the field, rotate away from Brassica crops every four years as a rule; when club root is detected, increase this to at least 7 years. Some rotational crops such as corn and alfalfa can reduce inoculum of P. brassicae.
• Plant in well-drained soil. Avoid overwatering.
• Add lime to raise the soil pH to 7.2. There is some evidence that benefits from liming are not solely due to the effect on soil pH, but also the increase in soil calcium.
• Prevent the movement of infested soil and contaminated equipment to uninfected areas.
• Use caution when applying animal manures, as resting spores can pass through the digestive system of cattle.
• Tolerant cultivars are available for some crops: see Cornell's list of disease resistant vegetable varieties for an updated list.
• Green manuring, repeated deep cultivation, soil solarization, and soil fumigants may reduce the incidence of clubroot.
• Ranman (cyazofamid), Blocker 4F (PCNB), and Omega 500 (fluazinam) are registered for P. brassicae management in the Northeast. None of these fungicides have systemic activity; labels for Ranman and Omega 500 advise applying by soil drench at planting. See product labels for more information.
• Serenade (B. subtilis) is also labeled for clubroot management in organic systems.
• Avoid planting Brassica cover crops.
• Manage weeds, especially Brassica species, as they may harbor the pathogen.

--Written by Angie Madeiras, UMass Plant Diagnostics Laboratory

Focus on Fungicides: The QoIs (AKA strobilurins) 

The Quinone outside Inhibitor (QoI) fungicides belong to FRAC Group 11. Their story begins in the 1970s, in a laboratory where scientists were studying Strobilurus tenacellus, a mushroom-producing fungus that grows on decaying pinecones. The scientists noticed that S. tenacellus produced something that inhibited the growth of other fungi. In time, they were able to isolate two anti-fungal substances, naming them strobilurins A and B. Further experimentation revealed that strobilurin A was a powerful fungicide, but it broke down quickly when exposed to sunlight (photolysis). After several years of work, chemists produced a form of strobilurin A that was much less susceptible to photolysis. This compound was named azoxystrobin. Azoxystrobin entered the market in 1996 and rapidly became one of the most widely used fungicides in the world. There are now 20 fungicides in Group 11.

The fungicides in this group are active against a broad range of fungal species, but importantly, have no effect on oomycetes. They are especially effective in preventing spore germination and are best used as protectants. The QoIs have translaminar activity within plant leaves. Azoxystrobin and fluoxastrobin have also demonstrated locally systemic activity.

The QoI fungicides work by disrupting the electron transport chain (ETC), the biochemical reaction that enables fungal cells to produce energy. They bind to one specific part of the ETC, the Quinone outside (Qo) site. This action disables the ETC, preventing energy production and leaving fungal cells unable to function. 

The QoIs in general have a high resistance risk due to their single site mode of action. There is also strong cross-resistance in the group: when a fungus becomes resistant to one QoI, it is typically resistant to all of them. Resistance most often occurs when a genetic mutation known as G143A occurs in fungi. This mutation changes the shape of the Quinone outside binding site. This does not inhibit the function of the ETC, but the change in shape prevents the fungicide from binding to it. 

Because the QoIs carry a high risk of resistance development, it is very important to rotate among fungicides from different FRAC groups. Many QoIs are available in combination products with active ingredients from other FRAC groups, e.g., Pristine (11 + 7), Quadris Top (11 + 3), and Tanos (11 + 27). When using these products, be sure to rotate away from both groups. Always read and follow label directions carefully. For more information on QoI resistance management, see the Fungicide Resistance Action Committee resources.

--Written by Angie Madeiras, UMass Plant Diagnostics Laboratory

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Vegetable Notes. Maria Gannett, Genevieve Higgins, Lisa McKeag, Susan Scheufele, Alireza Shokoohi, and Hannah Whitehead, co-editors. All photos in this publication are credited to the UMass Extension Vegetable Program unless otherwise noted.

Where trade names or commercial products are used, no company or product endorsement is implied or intended. Always read the label before using any pesticide. The label is the legal document for product use. Disregard any information in this newsletter if it is in conflict with the label.

The University of Massachusetts Extension is an equal opportunity provider and employer, United States Department of Agriculture cooperating. Contact your local Extension office for information on disability accommodations. Contact the State Center Directors Office if you have concerns related to discrimination, 413-545-4800.