Pest Report - August 8, 2008
White Mold - (not to be confused with white rot of garlic and onions that I discussed in the last issue)
White mold is popping up all over this week due to the weather that seems to vary from damp and dreary days to pouring rain. The pathogen, Sclerotinia sclerotiorum, is in most soils usually at some low level because it has so many hosts (over 300 species of crops and weeds). Your job, of course, is to not let the level build and to try to avoid having it attack your crops.
The fungus lives from season to season by what are known as sclerotia. These are hard, black bodies of the mycelium. The sclerotia survive for years (more than five), and when conditions are right (wet soil) they develop these tiny (very tiny) mushroom-like structures on the soil surface that look like a trumpet. Then, when humidity levels drop, spores are released from these fruiting bodies and blow around and land on plant tissue. Then, if the tissue later remains wet for long enough the spores germinate and the fungus starts growing. A white mass of cottony mycelium is what we think of when we think of white mold disease. After about two weeks of growth of the mycelium, new sclerotia form and the cycle starts again. The disease also spreads by plant tissue coming in contact with mycelium. That is why you see clumps of it in a dense canopy.
Crop rotation helps, but not much because almost everything you grow is susceptible. Some things are much more susceptible than others. Beans are probably the most susceptible. Fennel is too. Carrots (although symptoms
don’t usually appear until after the crop is in storage for a while), are very susceptible too. A bit less are tomatoes, peppers, onions, cabbage and celery. I only know that corn is not, but there may be some other things that don't get it too. Not many choices for a rotation crop.
Your best bet to avoid the disease is crop spacing and anything else you can do to keep the plants from staying wet long enough for the spores to germinate. Crowded plants and weedy rows keep the tissue wet. Irrigation should be by drip rather than overhead.
If you have the disease, do not let crowded rows of plants sit in the garden after you are done harvesting. These will just be hosts and eventually lead to piles of sclerotia building up.
Of course, weather like we have had recently defeats all of our efforts at keeping the leaves dry and the fungus often wins.
Fusarium Wilt and Crown Rot
These diseases can be caused by several different species of the genus Fusarium. Fusarium wilt is a serious disease of cantaloupe. Mature plants most commonly show the symptoms caused by this pathogen, with symptoms including yellowing of older leaves and wilting of runners. Vascular discoloration will apparent if the stem is cut along its length near the crown. The fungus is soil borne and can live long periods. Infection is more likely in drier, slightly acidic (pH 5.0-6.0) soils. It is spread primarily by moving soil from field to field. Because other cucurbits are carriers without showing symptoms, and because spores can live in the soil long periods, crop rotation does not work well to prevent Fusarium wilt of melons. The best defense is to use resistant varieties.
Fusarium crown rot is primarily a disease of summer squash and pumpkin but can effect most cucurbits. The first sign is wilting of the leaves. Within days the entire plant may die. Digging around near the base will reveal a rot of the crown and top of the root. Wet soil will lead to much more severe crown rot. This pathogen can survive in the soil for 2-3 years and so crop rotation can be used to avoid the problem.
Downy Mildew Update - (Reprinted and modified from the Umass. Vegetable News letter)
Massachusetts has been at ’high risk’ for downy mildew on several occasions over the past week. The disease has moved closer and this week was confirmed in NJ and eastern NY (Saratoga County). 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 mildes. 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.