Dynamics of ascospore maturation and discharge in Erysiphe necator, the causal agent of grape powdery mildew

Dynamics of ascocarp development ascospore maturation and dispersal in Erysiphe necator were studied over a 4 year period from the time of ascocarp formation to the end of the iscosporic season at the end of June in the following spring Naturally dispersed chasmothecia were collected from mid August to late November (when led fall was complete) the different collections were used to form three to five cohorts of chismothecia per year with each cohort containing ascocarps formed in different periods Chasmothecia were exposed to natural conditions in a vineyard and periodically sampled Ascocarps were categorized as containing mature or Immature ascospores or as empty mature ascospores inside chasmothecia were enumerated Starting from late February Ascospore discharge was determined using silicone coated slides that were placed 3 to 4 cm from sections of the vine trunk holding the chasmothecia Before complete leaf fall 34% of the chasmothecia had mature ascospores 48% had immature ascospores and 18% were empty in the same period the trapped ascospores represented 56% of the total ascospores napped in an ascosporic season (I e from late summer until the next spring or early summer) The number of viable chasmothecia diminished over time 11 and 5% of chasmothecia had mature ascospores between complete leaf fall and bud break and after bud break respectively These ascocarps discharged approximate to 2 and 42% of the total ascospores respectively All the ascocarp cohorts released ascospores in autumn survived the winter and discharged viable ascospores in spring neither ascospore numbers nor their pattern of temporal release was influenced by the time when chasmothecia were collected and exposed in the vineyard Abundance of mature ascospores in chasmothecia was expressed as a function of degree days (DD) (base 10 C) accumulated before and after bud break through a Gompertz equation (R(2) = 0 92) Based on this equation 90% of the ascospores were mature when 153 DD (confidence interval 100 to 210 DD) had accumulated after bud break Most ascospores were trapped in periods with >2 mm of rain however a few ascospores were airborne with <2 mm of rain and occasionally in wet periods of 5 h not initiated by rain