Survival and viability of ascospores of Erysiphe necator in Austrian vineyards

Grape powdery mildew caused byErysiphe necatoris a limiting factor in grape production. In order to develop effective management strategies, the factors influencing the survival of the fungus need to be identified and characterized accordingly. For this purpose, we focused on the effect of weather conditions during overwintering on the survival and viability of ascospores. In spring 2017 and 2018, grape leaf litter and bark samples were collected and examined to determine the density of chasmothecia and the viability of ascospores in various Austrian vine growing regions. There were obvious differences in the amounts of chasmothecia between both years and all examined vineyards. Lower quantities of chasmothecia were detected on the exfoliating bark compared to leaf litter, with up to 37% of chasmothecia containing viable ascospores. In comparison, chasmothecia from leaf litter showed a lower viability (up to 5%). The number of viable ascospores per head of vine ranged from 0 to 351 and from 0 to 251 in 2017 and 2018, respectively, and showed partly a strong variation within one location in both years. The infectivity of ascospores on detached leaves was confirmed. In a survival experiment, chasmothecia, when incubated at 7 °C, released more viable ascospores than chasmothecia incubated at 17 °C. After an incubation period of 30 weeks, only chasmothecia stored at the lower temperature contained viable ascospores. However, the mean temperature differences of 0.1 to 1.2 °C during the period of formation of chasmothecia to bud break in both years and six investigated areas did not explain differences in the viability of the ascospores. Differences in vineyard management seem to be of particular importance here.