Factors Affecting Dispersal of Conidia of the Apple Scab Fungus

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Abstract
Dispersal of conidia of Spilocaea pomi, the imperfect stage of Venturia inaequalis, was monitored in air during the summers of 1973 and 1974 with a Burkard recording volumetric spore trap and in rainwater during 1974 using funnel traps. Rain was an important mechanism for disseminating conidia; however, significant numbers of conidia also were detected in air. Aerial dissemination was diurnal with peak conidium frequency occurring at 2-4 p.m. Eastern Standard Time (EST) and with a subsidiary peak around 9-10 p.m. EST. Correlation and multiple regression analyses were used to relate counts of trapped conidia to several meteorological factors and to determine their relative importance in the release of conidia. Trap counts of conidia were not related to any dominant environmental parameter, but to the interaction of several. Release of conidia was positively correlated with temperature, wind velocity and solar radiation, but negatively correlated with leaf wetness and relative humidity. Analysis of disease progress data in relation to availability of conidia and to infection periods suggests that airborne conidia contribute to disease increase, particularly during dry periods.

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