MOFGA's Pest Report - June 1, 2011
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist
Wet: Although it seems as if we are out of the woods because it has not rained in a while and soils are mostly dry enough to work, there is still plenty of time to worry. Last week I discussed the loss of nutrients to leaching and other paths. Now, looking at my yellow, stunted severely and some dead spinach I thought I should mention that roots need air as much as leaves do. Because the spaces between the soil particles were filled with water for too long, the roots stopped functioning, and some may have died. You may look at your pale plants and make a decision to hope for the best, or replant.
Grey mold: (Botrytis cinerea) is a fungus that you cannot run and hide from. The spores are blowing everywhere. It is a very common problem on soft fruits (e.g. strawberries), all sorts of flowers, and sometimes leaves and stems. We organic growers must be aware of the conditions that favor the fungus over our crops and do our best to manage them. Botrytis is favored by cool, rainy, foggy and generally moist situations. The reason there is still time to worry even though the weather has improved is that the initial infection is usually on senescent tissue, for example the old petals and sepals of strawberry flowers, wounded stems or leaves, etc. And then it can spread from there if the conditions are right, or get right again to favor the fungus over the crop.
Even though you may not spot the problem until you are harvesting strawberries when you see the typical gray fuzzy mass of fungi and spores, the initial infection took place during bloom. And, during bloom this year it was wet. Also, Botrytis can be a problem in weedy fields or where the foliage is too dense, e.g., the lower leaves of a lush row of tomatoes (especially in a high tunnel) or lower leaves in the dense growth of potatoes. Essentially, where the plant tissue stays damp and cool for a while there is a risk of Botrytis, especially if there is senescent tissue. Last year I was getting many people calling with fear they had late blight when really they had Botrytis gray mold. If you get the typical gray fuzzy growth it is easy to tell. On leaves before spores are produced it is a bit harder. Gray mold will form lesions on leaflets of potatoes or tomatoes and progressively expand to include the whole leaf and then the petiole and stem. Lesions on lower parts of the stem may girdle it and the plant will wilt. A key distinction from late blight is the lesion usually is associated with yellowing of the leaf.
After the rainy weather most of us experienced this spring, I suggest you pay close attention to anything that would lead to damp conditions. What I think is really important now is to get the weeds under control, do not allow foliage to get too dense, and use wide spacing between rows of crops.
Tomato leaves with gray mold. Photo by Eric Sideman.
Tomato with gray mold. Photo by Eric Sideman.
Tomato blossom with gray mold. Photo by Eric Sideman.
Strawberry with gray mold. Photo by Eric Sideman.
Potato Seed Problems: Once again I am getting reports about Fusarium dry rot on potatoes bought for seed. This is a very common potato storage problem, and also on potato seed. Conventional growers commonly treat potatoes going into storage, or to be used for seed, with fungicides to control the problem. The problem begins at wounded spots on the potato. Brown lesions form and the infection slowly develops into cracks and cavities in the potato. White cottony growth may occur. And, if the problem persists then secondary bacterial infections occur and the potato becomes soft and watery.
Fusarium spores are really common and are a cause of seed piece decay. The most common cause of the disease in the new crop is though planting diseased seed pieces. Do not plant any potato seed that is obviously displaying Fusarium dry rot. When cutting seed pieces, do not cut diseased seed pieces and then go to use the same knife for healthy seed. Planting whole seed pieces is safer.
The one piece of good new is that it is not late blight, which is the worry of the people calling me.
Potato seed pieces with Fusarium
dry rot. Photo by Eric Sideman.
Late Blight in tubers. Photo by Steve Johnson.
Striped Cucumber Beetle: (Modified from article in Mass. Veg. Notes written by Ruth Hazzard & Andrew Cavanagh, University of Massachusetts, updated for 2010.)
Striped cucumber beetle is our most serious early-season pest in vine crops. I have already had reports of the first ones showing up in southern Maine. These beetles spend the winter in plant debris in field edges, and with the onset of warm days and emergence of cucurbit crops, move rapidly into the field. Densities often are very high, especially in non-rotated fields or close to last year’s cucurbit crops. Management cannot wait because adult beetle feeding on seedlings (cotyledons and young leaves) is the most serious, and can cause stand reduction and delayed plant growth. Also, the striped cucumber beetle vectors Erwinia tracheiphila, the causal agent of bacterial wilt, and this may be more damaging than direct feeding injury. Crop rotation, transplants, and floating row cover are cultural controls that help reduce the impact of cucumber beetles. By far, floating row covers offer the best results. You must get the row cover on at transplanting or seeding. Early management is important to avoid early season infection with wilt. Cucurbit plants at the cotyledon and first 1-4 leaf stage are more susceptible to infection with bacterial wilt than older plants.
|Striped Cucumber Beetles. Photo by Eric Sideman.
Perimeter trap cropping gives excellent control with dramatic reduction in pesticide use (see following short article).
Beetle numbers should be kept low, especially before the 5-leaf stage. Scout frequently (at least twice per week for two weeks after crop emergence) and treat after beetles colonize the field. Early spot treatments of field edges can be helpful. The threshold depends on the crop. To prevent bacterial wilt in highly susceptible crops, we recommend that beetles should not be allowed to exceed one beetle for every 2 plants. Less wilt-susceptible crops (butternut, most pumpkins) will tolerate 1 or two beetles per plant without yield losses. Spray within 24 hours after the threshold is reached. Proper timing is key. Pyrethrin should be spray very early in the morning, at dawn or before, to catch the beetles before they become active with the day’s warmth, and to give the pyrethrin time to work before the sun degrades it. Evening spraying can work for that, but often the beetles are flighty then and you see them flying up and away as you move down the row.
OMRI-listed insecticides available for use in striped cucumber management include kaolin clay (Surround WP), pyrethrin (Pyganic Crop Spray 5.0 EC), and spinosad (Entrust). In 2009 spray trials comparing these three products at the UMass Research Farm, kaolin was the most effective in reducing beetle numbers and feeding damage. There was a trend toward Surround WP being more effective when Pyganic or Entrust was mixed with it, but never significantly better than Surround alone. Other studies have shown more efficacy from pyrethrin and spinosad. Remember, Surround should be applied before beetles arrive because it acts as a repellent and protectant -- beetles do not “recognize” the plant and so do not feed -- not a contact poison. With direct-seeded crops, apply as soon as seedlings emerge if beetles are active. Transplants can be sprayed or dipped into the solution before setting out in the field.
I just heard from a grower who tried out a vacuum yesterday and reported that it seemed to work well. He suggests using a vac with a low setting and/or a broad nozzle so that you don't damage plants. Also, do this earlier in the morning when they are sleepy and slow and be sure to look in their hiding places under the plants.
Perimeter Trap Cropping For Cucumber Beetle Management: (Reprinted from Umass Veg. Notes)
Over the past years, you have probably heard a lot from us [Ruth and Andy at UMass] about perimeter trap cropping to manage striped cucumber beetle in cucurbit crops. The system has proven itself as an effective, cost-saving method for managing this pest. Systemic or foliar insecticides in the trap crop border are effective in halting the beetles in the border and protecting the main crop. PTC systems can reduce insecticide use by over 90% if implemented correctly, but this is not the only benefit. By spraying only the border of your crop you’re leaving the main part of the field as a refuge for pollinators and natural enemies of insect pests. Leaving the main crop unsprayed may also help to delay the development of insecticide resistance in the striped cucumber beetles – a few beetles will always bypass the border, and thereby escape selection for resistance. The first trap crop that we looked at was Blue Hubbard, but many growers told us that Blue Hubbard is difficult to market and other border trap crops were needed. We evaluated buttercup and kabocha squash as alternative border crops, and they worked just as well as Blue Hubbard. Markets for these crops are strong. Any Cucurbita maxima variety is likely to be very attractive. This species includes many giant and specialty pumpkin varieties; the only one we do not recommend as a border crop is Turk’s Turban because unlike most C. maxima varieties it is highly susceptible to bacterial wilt which is vectored by the beetles. You can even plant a border of mixed C. maxima around your butternut squash, acorn squash, and other winter squashes that are C. pepo, or C. moschata types. This will provide you a wide variety of interesting squash to market. We’ve tested this system extensively and found that as long as the trap crop border is planted on good land and remains intact the system works remarkably well. In most cases, growers who use this system never need to apply insecticides to their main butternut crop at all. In Connecticut, they’ve found the system to work equally well with cucumbers and summer squash. Zucchini tends to be more attractive than summer squash, and some varieties such are so attractive that they could be used as a trap crop. We’ve also seen PTC work well in pumpkin crops, as long as the pumpkins in the main crop are C. pepo and not C. maxima. Remember, many giant and specialty pumpkins are actually C. maxima species, and would make good trap crops. On organic farms, growers often treat the main crop with kaolin clay (Surround WP) which serves as a repellent. For transplants, using this before planting is very efficient and lasts for a week or so if there are not heavy rains. Spinosad or pyrethrin could be used in the border. Every year we talk to more growers who adopt this system. The reduction in pesticide costs can be dramatic, and more than offset the small amount of time and care it takes to plant and treat a solid perimeter trap crop. If you would like to try this system and have any questions, or just want to find out more about how it works, please call Andy Cavanagh at 413-577-3976.