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Forecasting the Seasonal Maturation of Ascospores of 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. Phytopathology 75:381-385. Accepted for publication 8 October 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-381.

A system is presented that, at the time of apple bud break, can forecast maturity of ascospores of Venturia inaequalis for an entire growing season. The system was developed in two steps. In the first step, a regression equation based on historical weather data was used to forecast degree-day accumulation from a phenological date to some future date. Based on the forecasted degree-day accumulation, ascospore maturity was then estimated by using a second model that described the relationship between ascospore maturity and degree-day accumulation. From 1979 to 1984, the forecasting system predicted the date of 10% ascospore maturity to within 4 days and the date of 90% ascospore maturity to within 2 days in 5 of 6 yr. The forecasting system can be adapted to areas other than New Hampshire and may be used to supplement or replace microscopic assessments of ascospore development in apple scab management programs. The system can identify periods of relatively rapid or slow inoculum accumulation during ascospore maturation. Applications of the forecasting system in timing fungicide sprays to control apple scab are discussed.