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Ecology and Epidemiology

Powdery Mildew of Sweet Cherry: Influence of Temperature and Wetness Duration on Release and Germination of Ascospores of Podosphaera clandestina. Gary G. Grove, Assistant plant pathologist, Washington State University Tree Fruit Research and Extension Center, 1100 N. Western Ave., Wenatchee, WA 98801; Phytopathology 81:1271-1275. Accepted for publication 12 June 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1271.

The effects of temperature and wetness duration on release and germination of ascospores of Podosphaera clandestina, causal agent of sweet cherry powdery mildew, were determined in controlled-environment studies. Ascospore release occurred in the presence of free water at temperatures between 5 and 30 C and at wetness durations as short as 1 h. Spore release increased rapidly between 2 and 4 h wetness durations, and after 8 h was optimal at 1520 C. Ascospore release did not occur at relative humidities of 90100% at 1525 C; release occurred, however, when cleistothecia previously exposed to high humidities were moistened with water and reincubated at 15 C. A multiple regression equation using temperature and wetness duration as independent variables adequately described ascospore release. Ascospore germination was observed after 8 h at 25 C and after 16 h at 15 and 20 C. Germination was optimal at 25 C, but never exceeded 30%. A multiple regression equation with temperature and wetness duration as independent variables adequately described ascospore germination. Field trapping of ascospores occurred over a range of temperatures at wetness durations as short as 1 h, but generally after 34 h. Ascospores were trapped only during periods of wetness initiated by rains exceeding 2.8 mm.