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Ecology and Epidemiology

Effects of Temperature and Relative Humidity on Sporangium Production of Pseudoperonospora humuli on Hop. Dennis A. Johnson, Associate plant pathologist, Washington State University, Irrigated Agriculture Research and Extension Center, Prosser 99350-0030; C. B. Skotland, plant pathologist, Washington State University, Irrigated Agriculture Research and Extension Center, Prosser 99350-0030. Phytopathology 75:127-129. Accepted for publication 27 July 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-127.

The effects of night temperature and relative humidity (RH) on sporangium production of Pseudoperonospora humuli on systemically infected hop shoots were studied. Sporangium production was inhibited or was very light in hop yards when either the nightly minimum temperature was ≥5 C, or the mean RH at night was ≥64%. Production of sporangia increased with increases of nightly minimum temperature, nightly mean RH and temperature, and number of hours in which RH was ≤80%. Multiple regression analysis with minimum nightly temperature x hours with RH ≤80%, and nightly mean RH as independent variables accounted for 84% of the variation in sporangium production.