DISPARITY BETWEEN MORPHOLOGICAL MATURITY OF ASCOSPORES AND PHYSIOLOGICAL MATURITY OF ASCI IN VENTURIA-INAEQUALIS

Records from 1984 to 1990 of the morphological maturity of ascospores of Venturia inaequalis in New York, New Hampshire, and Vermont indicated that discharged asci occurred rarely in pseudothecia until approximately 10-15% of asci contained ascospores rated as mature, even though numerous rain events occurred, in most years, before the time that this level of maturity was reached. Furthermore, laboratory discharge tests and field trapping of ascospores showed that few or no ascospores were detected before 10% of the asci contained morphologically mature ascospores and that a substantial number of asci containing morphologically mature ascospores were retained in pseudothecia after rain. The number of ascospores discharged per ascus containing morphologically mature ascospores increased through early spring, thus indicating that the ability of asci to release ascospores (physiological maturity of the ascus) lags behind morphological maturity of the ascospores by several days. In subsequent laboratory tests, nearly all ascospore release occurred within 90 min of the initial wetting of overwintered leaves. However, even after 5 hr of continuous wetting, over half of the pseudothecia examined contained between 10 and 50 asci with morphologically mature ascospores. The impact of these findings on the development and evaluation of predictive models of ascocarp development, development of apple scab epidemics, and the use of morphological assessments of ascospore maturity in grower advisory programs are discussed.