Tomato: The late blight pathogen is NOT transmitted by tomato seed.On organic farms, early epidemics have a high likelihood of destroying the entire crop. Do not plant infected tubers as they are an excellent source of primary (initial) inoculum to start an early epidemic. If late blight is suspected, have diseased tubers diagnosed by a crop consultant or plant disease diagnostician. Inspect your seed potatoes for late blight symptoms upon arrival, and if seed is stored before planting, inspect it again before planting as late blight can spread during storage. Certified seed is not guaranteed to be late-blight free, but in general, certified seed should be less risky because certified seed production is monitored by seed professionals and certified growers tend to be well informed about significant potato pests affecting seed quality. Purchase certified seed potatoes (potato SEED certification, not organic certification) from high quality sources. Planting potato seed that you have saved or obtained from neighbors increases late blight risk. Potato: Phytophthora infestans, the pathogen that causes late blight, is easily transmitted by diseased potato tubers. Seed selectionīy law, certified organic farmers must buy certified organically-grown seed if available. Do not plant susceptible crops in fields: 1) in which host plants may volunteer/grow (volunteer seedlings of host crops or weeds/hairy nightshade) 2) next to cull piles of tomatoes or potatoes or 3) shaded by trees or structures. Site selectionĬhoose fields with good air movement and well drained soils. Volunteer potatoes and tomatoes should be removed by cultivation. Waste potatoes can be fed to animals, buried 2 feet deep, composted, or spread on the surface of the soil to be frozen during the winter months. This means destroying all cull piles and volunteer hosts (potato, tomato, eggplant, pepper), including hairy nightshade. Therefore, destruction of all living hosts of the pathogen is an important control strategy. In most areas of the US, the late blight pathogen is an obligate parasite, meaning that it can only survive in living plant tissue. Photo credit: Lane Selman, Oregon State University. For more information on Phythophthora infestans/late blight diagnostics and life cycle, see the Michigan State (Kirk et al., 2004) Extension bulletin referenced at the end of this article. The first steps in managing any pest are 1) identifying it when you encounter it in the field, and 2) understanding its life cycle so you can exploit its weaknesses. Organic farmers should employ all strategies available to reduce late blight risk in host crops. Late blight is a very difficult disease to manage organically. Late blight affects foliage of both potato and tomato as well as potato tubers and tomato fruit. When the environment is favorable, the disease can spread quickly and can defoliate fields within 3 weeks. Late blight is favored by high humidity, dew, wet weather and moderate temperatures (50 to 80☏). Late blight is a serious disease of potato family (Solanaceous) crops worldwide, caused by the pathogen Phytophthora infestans. Alex Stone, Oregon State University Introduction
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