![]() The boundary between healthy and infected tissue is discontinous. Tuber infections appear initially as reddish brown, dry lesions that can extend an inch or more into tubers. Petioles and stems also become infected and produce water-soaked, dark green to black lesions. Under humid conditions, white downy mycelium is visible surrounding necrotic lesions. As lesions develop, they become more circular with a necrotic center surrounded by a yellow or pale-green margin. Late blight symptoms first appear on foliage as small, irregularly-shaped necrotic spots. Survival in soil for months or possibly years occurs by oospores in certain areas of the world where P. ![]() Therefore, cull potatoes, volunteers, and contaminated seed are the primary means of pathogen survival and dissemination. In the High Plains, the pathogen requires a living host to survive. Tuber infections may also occur during harvest and handling. Tuber infections are thought to occur when sporangia are washed from leaves and contact buried tubers through cracks in the soil. Lesions can develop in as little as three days after infection, producing additional sporangia that are responsible for secondary spread of the disease in a field. Infection begins when sporangia or zoospores form germ tubes and infect foliage when temperatures are cool to moderate (46 to 75✯) and free moisture is available. ![]() infestans can infect both potato foliage and tubers. ![]() The pathogen can infect several Solanaceous plants, including tomato, eggplant, petunia, and hairy nightshade. Late blight, caused by the fungus-like organism Phytophthora infestans, is a potentially devastating disease of potato, and may be the most important disease of potato worldwide. ![]()
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