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Environmental Factors Influencing the Discharge of Basidiospores of Gymnosporangium juniperi-virginianae. R. C. Pearson, Assistant professor, New York state Agricultural Experiment Station, Cornell University, Geneva, NY 14456; R. C. Seem(2), and F. W. Meyer(3). (2)(3)Assistant professor and research technician III, respectively, New York state Agricultural Experiment Station, Cornell University, Geneva, NY 14456. Phytopathology 70:262-266. Accepted for publication 24 September 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-262.

Discharge of basidiospores by Gymnosporangium juniperi-virginianae was studied under field conditions during three growing seasons. Discharge usually began within a few hours of the start of rainfall, continued throughout the rain period, and ceased when rainfall ended, leaves dried, and relative humidity (RH) dropped below 85%. The duration of the spore release period was most highly correlated with hours RH ≥85% (r = 0.935). Generally, environmental parameters had higher correlation coefficients with duration of spore discharge than with spore numbers. Delays in discharge of basidiospores following the start of rainfall were not significantly correlated with any of the environmental parameters studied. Multiple regression equations were developed to predict the amount and duration of spore discharge based on easily measured environmental parameters. On several occasions after a rain-induced spore discharge period, basidiospore release resumed 1224 hr later in the absence of rainfall or leaf wetness. This phenomenon was highly correlated with hours RH ≥85% (r = 0.869).

Additional keywords: cedar apple rust, epidemiology, Juniperus virginiana, Malus pumila.