Dispersal of the sexual stage of Erysiphe necator in northern Italy

Abstract: Either by themselves or in combination with mycelium in the dormant buds, ascospores produced in chasmothecia (the sexual fruiting bodies) of Erysiphe necator are an important source of primary inoculum for grapevine powdery mildew disease. In northern Italy, E. necator overwinters mainly as the sexual stage (i.e., mycelia are not usually present in dormant buds), but no data are available on the abundance of chasmothecia in the vineyards. Therefore, the dispersal of chasmothecia was studied in commercial vineyards of northern Italy, from 2005 to 2007 (16 combinations of 10 vineyards X 3 years); the vines were not sprayed with fungicide during this study. The dispersed chasmothecia were collected on filter papers in funnels that were placed on the trunk of affected grapevines from mid-August to the end of leaf fall. Each filter paper was examined microscopically, and the chasmothecia were counted. The first chasmothecia of the season were dispersed between late August and early October; cumulative numbers of the chasmothecia dispersed subsequently increased (with different dynamics depending on the vineyard and year) but dispersal stopped at the end of leaf fall. Over all three years and 16 vineyards, chasmothecia averaged 3/cm2 of trap surface, with a maximum of 15/cm2. The numbers of mature chasmothecia that dispersed roughly depended on the powdery mildew severity on leaves: high numbers of chasmothecia were associated with disease severity ≥ 80%. According to estimates based on the current data, vine bark in vineyards trained with a Guyot, Geneva double curtain, or spurred cordon pruning system could contain till 18, 19, or 44 million chasmothecia/ha, respectively.