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Ecology and Epidemiology

Patterns of Ascospore Discharge by Venturia inaequalis. William E. MacHardy, Professor, Department of Botany and Plant Pathology, University of New Hampshire, Durham 03824; David M. Gadoury, research associate, Department of Botany and Plant Pathology, University of New Hampshire, Durham 03824, Present address: Department of Plant Pathology, New York Agricultural Experiment Station, Geneva 14456. Phytopathology 76:985-990. Accepted for publication 28 February 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-985.

Discharge of ascospores of Venturia inaequalis was monitored with a Burkard volumetric spore trap for 4 yr in an apple orchard with a large overwintering population of the apple scab pathogen. The major discharges occurred during clearly defined intervals. When rain began before 0700 hours, the leaves remained wet through 2400 hours, and temperatures were above 10 C, 97.9% of the ascospores trapped by 2400 hours were trapped between 0700 and 1800 hours. When rain began between 0700 and 1300 hours, 98-100% of the spores were trapped before 2100 hours. Even when the rain began later in the day, between 1400 and 1800 hours, 90% or more of the ascospores were trapped before 2100 hours. No spores were trapped until the following morning when the rain began after 1800 hours. The extremely low density of ascospores in the air between 1800 and 0700 hours has significant implications for determining Mills' infection periods and for selecting and scheduling postinfection fungicides, because spore discharge and the subsequent initiation of infection often occur several hours after rain begins rather than at the start of rainfall.