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Epidemiological Patterns of Phytophthora infestans Under Semi-Arid Conditions. J. Rotem, Division of Plant Pathology, The Volcani Center, ARO, Bet Dagan; Yigal Cohen, Department of Life Sciences, Bar Ilan University, Ramat Gan, Israel. Phytopathology 64:711-714. Accepted for publication 7 December 1973. DOI: 10.1094/Phyto-64-711.

Epidemic patterns of potato late blight during a period of heavy nocturnal dew followed by hot and dry days, were studied by inducing epidemics in growth chambers and assessing the influence of environmental factors on epidemic development. In all but the hottest regimes of temp, epidemics were enhanced by low daytime relative humidity (RH). The chief factor facilitating epidemic development was abundant sporangial dispersal enhanced by low RH. However, in days with high temp the survivability of dispersed sporangia was reduced; more under low, than under high, conditions of humidity. In such instances, survivability overrides dispersal effect and becomes a major factor in epidemic development. Under extremely unfavorable (but transient) conditions of temp and humidity, the high survivability of mycelium in leaves, and especially in stem lesions, provided the opportunity for subsequent renewal of an epidemic.