A system is presented that, at the time of apple bud break, can forecast maturity of ascospores of V. inaequalis [apple scab] for an entire growing season. The system was developed in 2 steps. In the 1st 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 2nd model that described the relationship between ascospore maturity and degree-day accumulation. From 1979-1984, 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 Hamsphire [USA] 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.