Ascospore maturation and discharge in Erysiphe necator

Ascospores of Erysiphe necator are a relevant source of inoculum for spring infections. They form within chasmothecia (formerly named cleistothecia) which develop on the affected grapevine tissue in late summer to autumn, disperse to the bark by rain-splashes, and overwinter. Naturally dispersed chasmothecia were collected at 15-day intervals from ripening to complete leaf fall, in 2005 and 2006. Chasmothecia were placed on the bark of trunk pieces, overwintered outside, and observed weekly for the developmental stage and for discharged ascospores until the end of June. During leaf fall, 56% of chasmothecia had mature ascospores, 26% had immature ascospores, while 18% were unfilled (undifferentiated or empty). During winter (complete leaf fall to bud break) the distribution of chasmothecia in the three classes was 12%, 34%, and 28%, respectively, while after bud break it was 3%, 30%, and 67%, respectively. 35% of ascospores were discharged before complete leaf fall, 7% during winter (especially soon after leaf fall), and 58% after bud break. 55% of the total ascospores discharged before complete leaf fall were released from the earliest group of chasmothecia collected, 38% and 4% from the following groups, while no ascospores were discharged from the latest chasmothecia. These result showed that in northern Italy the development of chasmothecia depends on the period of their formation, resulting in two distinct periods of ascospore discharge in autumn and spring, as previously observed in Australia but not found in the state of New York. These findings may be relevant to disease epidemiology and management.