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 MOFGA's 2013 Pest Reports - Compiled by Eric Sideman, PhD

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Pest Report - May 29, 2013
Compiled by Eric Sideman, PhD, MOFGA's Organic Crop Specialist

In this report:

Cucumber problems (when planted early)

Colorado potato beetle

Gray mold

Mexican bean beetle


CUCUMBER PROBLEMS

Blame it on global warming, or not, we have experienced extremes that average out to pretty normal, but we don't often see a normal day. After about three weeks of very dry weather, we just went through a week of very wet weather. Now most fields are wet, some are water logged. This is not good for roots.

The roots of plants need oxygen and most plants are not able to move oxygen down to them. Roots get oxygen from the spaces between the soil particles, and if these spaces are filled with water there is none there. Roots without oxygen do not function well and the plant is stressed. This week I have seen plants stressed to the point of wilting, some only to the point that they are weak and diseases win out.

Cucumbers have been the hardest hit. Many growers who are rushing things and have put their cucumber seedlings out are reporting problems. Some years you get away with rushing things, some years you don't. I have seen chilling injury (Chilling injury is damage to plant parts caused by cool temperatures but above the freezing point. Plants of tropical or subtropical origin are most susceptible. Temperatures below 50 degrees F. will set back cucumbers). I have also seen damping off. And, I have seen seedcorn maggots crowded into the stems of cucumber plants. Read the May 7 and April 22 pest reports for a discussion of these early season problems (see menu tabs at the top of this page).

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COLORADO POTATO BEETLE

Colorado potato beetle eggs
Colorado potato beetle at the 4th instar
Adult Colorado potato beetles

Colorado potato beetles (CPB) adults are not yet showing up in potato and eggplant crops, but will soon. The bright yellow eggs are laid in clumps with about 30-35 eggs each, generally on the undersides of leaves. As with most other insects and plants, there is a direct relationship between higher temperatures (in the range between about 55 and 90 degrees F) and faster rate of development. That includes egg-laying, egg hatch, larval growth, and feeding rates. A period of cold, rainy weather slows everything down, but we can expect a surge of adults and shiny yellow eggs to appear after a few hot spells.

Scouting and Thresholds

Walk your fields and look for CPB adults and eggs. A treatment should be considered for adults when you find 25 beetles per 50 plants and defoliation has reached the 10% level. The spray threshold for small larvae is 4 per plant; for large larvae, 1.5 per plant (or per stalk in midseason).

Eggplants are more sensitive. Controls are needed on eggplants when there are 2 small or 1 large larvae per plant.

Potatoes can tolerate 20% defoliation without reduction in yield (or even more, depending on time of the season and cultivar). Damage to eggplant seedlings from adult feeding is often severe enough to warrant control of the adults, but in potato, damage from adults may not be significant, so you may be able to wait until after egg hatch to kill both adults and larvae.

Look on the undersides of leaves for the orange-yellow egg masses. The fresher the eggs, the brighter orange the eggs will appear. Eggs hatch in 7-10 days, depending on temperature. If you want to know when the earliest eggs are hatching, you can flag a few of the earliest egg masses you find with bright tape or flags, and then keep an eye on the hatch.

Hatched larvae go through four stages (called instar) before they become adults. In the first stage, the larvae are about the same size as the eggs, and in the second stage they are about an eighth of an inch long. As the larvae get bigger, they do more feeding. The fourth, or largest, stage does 85% of the feeding damage. It’s a good idea to prevent beetles from ever reaching the fourth instar!

After larvae complete their growth, they drop to the ground and burrow into the ground to pupate. About ten days later the next generation of adults emerges – ready to feed. If they emerge before August 1, they will lay more eggs. After August 1, they feed and head to overwintering sites. Good control in June prevents problems with CPB in August.

Cultural Controls

Crop Rotation. The single most important tactic for CPB management is to rotate potatoes or eggplant to a field that is at least 200 yards from the previous year’s fields. Since the adult that comes out of winter cannot fly, barriers such as roads, rivers, woodlands, and fields with other crops are helpful. Rotated fields tend to be colonized 1-4 weeks later in the season. Also, the total population of adult beetles is lower, producing fewer larvae to control.

Perimeter treatments or perimeter trap cropping can be applied to potato. One approach is to plant a barrier crop between overwintering sites and this year’s crop and get it in earlier than the main crop; then control early-arriving beetles.

Early planting. Green sprouting, also know as chitting, prepares whole seed potatoes to emerge rapidly, gaining about 7-10 days to harvest. This early start makes it easier for the crop to put on growth and size before CPB adults and larvae arrive. It can be combined with raised beds and plasticulture. While it won’t avoid damage altogether, it may reduce the need for insecticides. Refer to the New England Vegetable Management Guide (www.nevegetable.org) for more details.

Late planting. Another strategy for beating the beetle is to plant late. CPB adults that do not find food leave the field in search of greener pastures. Planting after mid- June, using a short season variety, often avoids CPB damage and eliminates the need for controls.

Straw mulch. It has been well documented that when potato or eggplants are mulched with straw, fewer Colorado potato beetle adults will settle on the plants and fewer eggs will be laid. This can be accomplished on larger plantings by strip planting in a rye mulch, followed by mowing and pushing the rye straw over the plants after they emerge. For smaller plots, straw may be carried in.

Biological Control. There are numerous predators and parasitoids that attack CPB adults (a tachinid fly) and larvae (12-spotted ladybeetle, spined soldier bug, ground beetles).

Organic pesticides. Spinosad (Entrust) and azadiractin (AZA-Direct) are two options. Recent studies have shown very good results with spinosad, but please use reluctantly because resistance will build up in populations of CPB. The azadiractin has shown some efficacy, but neem works slowly. Generally it is used to reduce overall damage and reduce numbers but it is not a rescue treatment like spinosad is. Beauvaria bassiana (Mycotrol O) has been shown to suppress CPB populations though does not provide immediate control. And, using Beauvaria bassiana does not jive well with using crop rotation because you need the overwintered adult that remain infected with the fungus to eliminate buying the product new every year. But, if you are in a situation where rotation is not possible anyway, Beauvaria may help manage this pest.

[NOTE: There is no longer a registered Bt product for potato beetle management in organic crops.]

(Modified from the Umass Vegetable Notes, an article by R. Hazzard; (sources include: D. Ferro (UMass Amherst), J. Mishanec (NYS), J. Boucher (CT), J. Whalen (DE), T. Kuhar (VA), , G. Ghidhu (NJ), New England Vegetable Management Guide, Ohio Vegetable Production Guide)

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GRAY MOLD (Botrytis cinerea)

Botrytis (gray mold) on strawberries
Gray mold on peas
Botrytis sporulating on a tomato leaf

Gray mold is a fungus that you cannot run and hide from. The spores are blowing everywhere. It is a very common problem on soft fruits (ex. strawberries) and all sorts of vegetables. 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. It is time to worry, especially for strawberries because the initial infection is usually on senescent tissue, for example the old petals and sepals senescing right now. This is true for tomatoes (high tunnel), peas, and other vegetables too where flowers are now senescing. Wounded stems or leaves are also points of infection. The disease then spreads from there if the conditions are right to favor the fungus over the crop.

Even though you may not spot the problem until you are harvesting strawberries and you see the typical gray fuzzy mass of fungi and spores, the initial infection took place during bloom. And, during bloom this year it is wet.

Botrytis is often 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. I often get many people calling with fear they have late blight when really they have Botrytis gray mold. If you get the typical gray fuzzy growth it is easy to tell. 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 gray mold lesion is usually associated with yellowing of the leaf (see picture).

After the rainy weather most of us are experiencing 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.

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MEXICAN BEAN BEETLE

Plan Ahead for Biological Control
(Reprinted and modified from Mass. Veg. Notes, written by Ruth Hazzard and A. Brown)

Mexican bean beetle larva
Adult Mexican bean beetles

If Mexican bean beetles have historically been a problem on your farm, you will very likely see them again this year. They may be pests on snap beans, soybeans, and lima beans. While they are not a pest on every farm, some farms report repeated significant damage from these pests and have to take action to prevent crop loss. Using biological control can reduce the need for insecticides.

Mexican bean beetle (MBB) adults are coppery brown with black spots. They look very much like large ladybeetles and in fact are closely related – but unlike lady beetles they feed on leaves, not other insects. Adults lay yellow-orange egg masses on the underside of bean leaves. These hatch into bright yellow, spiny oval larvae, which feed, molt several times as they grow, and pupate on the underside of leaves. Feeding damage from adults and larvae can reduce yield and injure pods if numbers are high. There are several generations per season, often with increasing populations in each generation.

Pediobius foveolatus is a commercially available biological control agent for Mexican bean beetle control and has a good track record in the mid-Atlantic states and among New England growers who have tried it. (Pediobius is pronounced “pee-dee-OH-bee-us”). It is mass-reared and sold by the New Jersey Dept of Agriculture and is also available from other beneficial insect suppliers. This small (1-3 mm), non-stinging parasitic wasp lays its eggs in Mexican bean beetle larvae. Wasp larvae feed inside the MBB larva, kill it, and pupate inside it, forming a brownish case or ‘mummy’. About twenty five adult wasps emerge from one mummy. Control continues and in fact gets better as the season progresses and successive generations of the wasp emerge and search out new bean beetle larvae. Planning 2-3 releases at 7-10 day intervals will help ensure good timing. and coverage on several plantings. This makes it well suited to our succession-planted snap bean crops. After a release in the first plants, it is advisable to leave that planting intact for a while, until the new generation of wasps has emerged from their mummies. As with any biological control, make releases as soon as the pest is present – not after it has built up to damaging numbers.

The New Jersey Dept of Agriculture Beneficial Insect Rearing Laboratory recommends two releases, two weeks in a row, coinciding with the beginning of Mexican bean beetle egg hatch. Wasps will lay their eggs in larvae of any size, but it is best to target the newly-hatched young MBB larvae. This will give control before damage has been done. Thus, timing is important. Watch for eggs and time the shipment for the first hatch of eggs into larvae. If in doubt about the timing of the hatch, release as soon as you see the eggs – if you wait for the larvae you may be playing catch-up. The release rate should be at least 2000 adult wasps per field for less than an acre, or 3,000 per acre for fields of one acre or more.

The 2009 cost from NJDA is $40 plus shipping for 1000 adults, or $20 for 20 mummies (pupal parasites inside dead MBB larvae) from which about 500 adults will emerge. Order adults if you already have MBB larvae in the field. Ship for overnight delivery. Instructions for handling and release will come with the wasps. Wasps reproduce in the field and will still be around when the second generation of MBB hatches out. Thus, it should not be necessary to make more than two releases. Like beans, Pediobius wasps are killed by frost.

Plan ahead by contacting a supplier to inform them of your expected release dates and acreage.

Contact information for New Jersey source:
Tom Dorsey, 609-530-4192;
NJDA, Phillip Alampi Insect Lab, State Police Drive, W. Trenton, NJ 08628.
http://nj.gov/agriculture/divisions/pi/prog/beneficialinsect.html.

You’ll also get advice on how to use the wasps from this office.

Pediobius is also available from the following suppliers:

Green Spot Ltd. (N.H.)
www.greenmethods.com
603-942-8925;

IPM Laboratories (N.Y.)
www.ipmlabs.com/
315-497-2063;

ARBICO (Ariz.)
800-827-2847
www.arbico.com/

Network (Tenn.)
615-370-4301
www.biconet.com/.

If you would like assistance in using these biocontrols in your bean crops please call the UMass Extension Vegetable Program at 413-577-3976 or 413-545-3696 or email at umassvegetable@umext.umass.edu

Materials Approved for Organic Production

Moderate control can be achieved with Entrust as well as mixtures of pyrethrin (Pyganic EC5.0) and Neemix.

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