Pest Report - July 26, 2010
LATE BLIGHT (Phytophthora infestans)
At this time late blight is only in one location in Maine. The center of it seems to be the western part of the Waldoboro area. Similar to the other occurrences this year in PA, CT and NY, it seems to be an isolated breakout. The good weather of this week should support that. But continue to be vigilant and out there scouting. If you are within 50 miles of there, you should consider spraying one of the approved copper fungicides. Check your email at least once a day for updates and don't hesitate to ask questions.
|Powdery mildew damage on a cucumber plant (above). The first symptoms of powdery mildew on underside leaf surfaces are white, powdery fungal patches (below). Photos by Eric Sideman.
POWDERY MILDEW (Podosphaera xanthii)
Powdery mildew is a common disease of pumpkins and winter squash. All cucurbits are susceptible, but many common cucumber and melon varieties are resistant. The disease can cause infected leaves to die prematurely, reducing yields and lowering fruit quality, especially taste. Winter squash from diseased plants won’t store as long as fruits from healthy plants. The fungus that causes the disease does not overwinter in Maine. Spores blow up every year from southern overwintering sites. If they arrive late in the season, you may not need any control; but if they arrive in early to midsummer, exercise some control or you may have no leaves by mid-August.
Powdery mildew reports are now coming in to me. This may warrant attention. Go out and scout.
Check upper and lower surfaces of leaves of older plants every few days starting now. The first symptoms usually are white, powdery fungal patches on the undersides of older leaves. Yellow spots may form opposite these, on the upper leaf surfaces.
No products with systemic activity (products that move through the plant) are approved for organic production, and applying fungicide to the lower leaf surface is difficult. In experiments, foliar applications of sulfur have been more effective than most other organic products for powdery mildew, apparently because sulfur deposited on the upper leaf surface can volatilize and be redistributed to the lower surface. Sulfur can be phytotoxic on melons, especially if applied when temperatures are hot.
ATTRA (National Sustainable Agriculture Information Service, www.attra.ncat.org) reports that a single spray application (to runoff) of 0.5 percent (wt./vol. of water) baking soda, plus 0.5 percent (vol./vol. of water) SunSpray UFP® horticultural oil almost completely inhibited powdery mildew on heavily infected pumpkin foliage; while baking soda without the oil was ineffective, and a 2 percent (wt./vol. of water) solution of baking soda damaged the leaves. But, remember if you are a commercial grower then you must be using EPA registered pesticides and not homemade concoctions.
LEAFHOPPERS (Empoasca Fabae)
The report I presented on leafhoppers a few weeks ago (Pest Report June 24, 2010) warned that they were taking down potatoes, and they did. In my travels last week I saw potatoes on their way down. If leafhoppers where into your beans or potatoes back then, I suspect you see the crop dying now with the edges of the leaves turning brown then crisp and the plant getting weaker and weaker. I have received numerous calls confusing this (or damage from the tarnished plant bug, Pest Report from July 7, 2010) with late blight. Once you see the damage, it is too late to spray.
|Symptoms of black rot of brassicas are irregular, dull, yellow areas along the margins of the leave that expand in a "V" shape. Photo by Eric Sideman.
BLACK ROT OF BRASSICAS (Xanthomonas campestris)
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.
|Purple blotch lesions begin as whitish sunken areas that elongate and develop purplish centers. Photo by Eric Sideman.
PURPLE BLOTCH (Alternaria porri)
Purple is a very common and sometime destructive disease that effects onions, garlic, shallots and leeks. Lesions begin as whitish sunken areas that elongate and develop purplish centers. Under favorable conditions (warm with wet leaves) the purple blotch lesions grow large and oval with concentric rings. The lesions may merge and take down whole leaves, and may become covered with brownish spores. The older leaves are more susceptible than the younger leaves.
Onion residue is the source of the disease in the spring. The fungal mycelium and conidia spores persist as long as the onion debris in the field or in cull piles. New conidia are produce on infected tissue in the spring and wind blown or carried in water to the new crop. The leaves have to have liquid water on them for the spores to germinate, but germination is very quick, less than an hour. Symptoms may appear in less than a week after germination and new conidia spores are quickly produced.
Sanitation is very important to limiting spread. Infected crop debris should be destroyed or buried after harvest. Cull piles should not be kept near the new onion field. Onions should be grown in rotation with non host crops.
Materials Approved for Organic Production
Serenade has been shown to be effective against purple blotch.