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Factors Affecting Germination of Conidia of Podosphaera clandestina on Leaves and Fruit of Sweet Cherry. Gary G. Grove, Assistant plant pathologist, Washington State University Tree Fruit Research and Extension Center, Wenatchee 98801; Robin J. Boal, Agricultural research technologist, Plant Pathology New Series 0097, Project 0795, Agricultural Research Center, Pullman, WA. Phytopathology 81:1513-1518. Accepted for publication 30 July 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1513.

The effects of temperature and vapor pressure deficit on germination of conidia of Podosphaera clandestina on sweet cherry foliage were determined under controlled conditions. Germination occurred at vapor pressure deficits between 0 and 851 Pa and increased with decreasing vapor pressure deficit. Germination occurred at 1530 C during 8-h incubation periods and 1030 C during 16- and 24-h incubation periods. The optimum temperature for germination after 16 h of incubation was 25 C, but after 24 h, shifted downward to 20 C. After 24 h, germination exceeded 50% under optimum conditions of 0 Pa at 20 C. At 20 C, germination decreased from 51% at 0 Pa to 29% at 466 Pa. After 24 h, germ tube length was greatest at 25 C and was not affected significantly by increasing vapor pressure deficit. Germination on immature (green) fruit increased with incubation time and decreased vapor pressure deficit (e.g., at 0 Pa germination ranged from 8% after 8 h to 48% after 24 h). Germination on ripening fruit was lower than on green fruit and was suppressed by increasing soluble solid concentrations (e.g., at 0 Pa the mean proportion of conidia germinated equaled 26% and 2% on fruit with soluble solid concentrations of 714.9% and ?15%, respectively. Multiple regression equations using temperature and vapor pressure deficit, and temperature, vapor pressure deficit, and incubation time as independent variables adequately described germination on foliage and green fruit, respectively. Multiple regression equations using vapor pressure deficit and fruit soluble solid content adequately described germination on maturing fruit.

Additional keywords: Prunis avium, quantitative epidemiology.