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Pest Report - August 12, 2009

LATE BLIGHT: What to do with a field infected with late blight
[Reprinted and modified a bit from an article by Becky Grube Sideman, UNH Cooperative Extension, Sustainable Horticulture Specialist]

Late blight is now in potato and tomato fields throughout the state. From what I have heard, those using preventative fungicide programs have kept the disease at bay thus far, but many who did not apply fungicides are having fields go down. Once symptoms are widespread in the field, it is the time to cut your losses and kill the plants to prevent the disease from spreading into other fields or high tunnels. For tomatoes, this will mean a crop loss. For potatoes, however, it may still be possible to get a decent crop.

Killing potato vines before harvest will reduce the chance of infecting tubers. Late blight spores do not survive on dead vegetation. Vines can be killed by mowing, burning, or using dessicants (herbicides). Mowing can be a challenge in a hilled potato field. After killing the vines, wait at least two weeks before harvesting to give the vines time to die completely. This helps avoid dragging the potatoes through spore laden foliage. The ground is a good place to store potatoes while waiting to dig them; it is cool, moist and dark, and the skins will harden on those tubers that are not already infected. That said, ground storage poses some risks: wireworms, grubs, and rodents may find and feed on potatoes, and the longer the tubers stay in the ground, the higher the risk. Digging test hills in a few locations to check for evidence of these pests may help you evaluate the tradeoffs.

Harvest when soil is not excessively wet, and do not wash tubers. Tubers should be dry when placed in storage. Remove any obviously infected tubers before storage. Curing tubers (10-20 days at 50-60 F, 95% relative humidity) generally heals wounds and help tubers store longer. These conditions will also cause those tubers infected with late blight to decay rapidly, so that you can cull rotted tubers. In storage, forced air ventilation through the storage bin can help minimize spread from tuber to tuber. Storage of potatoes with small amounts of late blight should be at 38 F to retard late blight tuber rot. If you must wash tubers before sale, organic growers can use Storox (Oxidate) or chlorine (dilute to 4ppm before discharge) at labeled rates in wash water.

Some common questions…

How do I get rid of infected tubers? If you end up with infected tubers, it’s important to dispose of them properly. Do not make cull piles. In a cull pile, the late blight pathogen can survive the winter and be a source of inoculum early next year. No matter how you dispose of tubers, carefully monitor for (and destroy) and volunteer sprouting potatoes next spring. Options for safe disposal include the following:

Burial. Trenches filled will culls should be covered with at least 18” of clean soil to prevent tubers from sprouting.

Livestock feed. This can be done during late fall and winter, when freezing temperatures will kill tubers that are not eaten within one day. During summer, this is not recommended unless all culls are eaten within one day.

Composting. Compost piles must be turned and mixed routinely and may require special equipment for large piles. The University of Maine fact sheet ‘Composting Cull Potatoes’ has more detailed information: http://www.umaine.edu/umext/potatoprogram/Fact%20Sheets/Composting%20Cull%20Potatoes.pdf

Spreading. Cull potatoes may be spread during late fall and early winter on fields that are not destined to be potato fields the next year. Culls should be left on the surface to ensure they are killed by freezing temperatures.

Can I save my potatoes for seed? If you have late blight symptoms in your field, saving seed tubers from that field is very risky. Even when there’s no evidence of tuber blight, up to 20% of the tubers in a seed lot can have latent infections with the late blight pathogen. Tubers with latent infections can cause foliar symptoms very early in the season, and set you up for needing to apply foliar late blight fungicides. The best way to avoid seedborne transmission of late blight is to purchase certified disease-free seed next year and make sure to scout for and remove any volunteer potato plants that come up next spring.

My tomato plants are going down, but fruit look OK. Can I harvest and sell healthy fruit? Once plants are infected, symptoms continue to develop in fruit even after they are harvested. In previous years, it has not been uncommon to see infected fruits in markets. Fruits harvested from infected plants are definitely at risk of developing symptoms during storage before they are sold and consumed.

Information compiled from many sources including Universities of Vermont, Maine, Massachusetts, and Idaho.

DOWNY MILDEW UPDATE. (Reprinted and modified from the University of Massachusetts Vegetable News letter). Downy mildew is being reported in Massachusetts. This makes it very possible that downy will be showing up in your fields soon. Cucumbers are especially at risk. SCOUT OFTEN. Downy mildew first appears as yellow polka-dots on the leaves. These yellow spots quickly turn into square or almost square brown lesions that are restricted by the small leaf veins. Next the older leaves begin to curl upwards at the margins and finally all the foliage in the field dies. If you see the yellow polka-dots or brown mosaic-like lesions you will need to respond quickly with an effective material because it only takes a week to 10 days for the foliage to die. Resistant varieties, selecting sites with good air movement and other practices that promote quick drying and drip irrigation rather than overhead are the most important cultural practices to avoid downy mildew. Copper compounds are probably the best material organic growers have, but they are not very effective. See the ATTRA publication for more information on copper and other management ideas (http://attra.ncat.org/attra-pub/downymildew.html).

BLACK ROT OF BRASSICAS. Black rot is one of the most important diseases of brassicas world wide and is much more of a problem in warm, humid climates than in the cool northeast. Still, it is very common here and does result in crop loss. The disease is caused by a bacterium called Xanthomonas campestris. The initial infection is most often through seeds and infected crop debris. The bacteria are spread by rain splash, insects, workers, and equipment. The symptoms are irregular, dull yellow areas along the margins of the leaves that expand in a "V" shape (see picture on MOFGA website). The veins in the lesions often look dark. Sanitation is the key to avoiding this disease, but the problem is that sanitation is most important during seed production. Buy seeds from a reputable source. Begin sanitation when raising seedlings. Any yellowing seedlings or plants with "V" shaped lesions should not be planted in the field as they will serve as a source of bacteria that may spread to the whole field. There are resistant varieties. Crop rotation (3 years) is important so if you have this problem pay attention to which fields and clean equipment so you do not drag crop debris to new fields. Avoid overhead irrigation, but of course this year that would make no difference. Do not use manure from livestock that have been fed diseased Brassicas on fields intended from Brassica crops. Hot water treated seed is recommended. For cabbage and Brussels sprouts soak seed for 25 minutes in 122 F water, and for broccoli and cauliflower soak for 20 minutes. Precise time and temperature control is essential to minimize damage to the seed.

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