Dispersal of the sexual stage of Erysiphe necator in northern Italy

Abstract: Either by themselves or in combination with mycelium in the dormant buds, ascosporesproduced in chasmothecia (the sexual fruiting bodies) of Erysiphe necator are an important sourceof primary inoculum for grapevine powdery mildew disease. In northern Italy, E. necatoroverwinters mainly as the sexual stage (i.e., mycelia are not usually present in dormant buds), butno data are available on the abundance of chasmothecia in the vineyards. Therefore, the dispersal ofchasmothecia was studied in commercial vineyards of northern Italy, from 2005 to 2007 (16combinations of 10 vineyards x 3 years); the vines were not sprayed with fungicide during thisstudy. The dispersed chasmothecia were collected on filter papers in funnels that were placed on thetrunk of affected grapevines from mid-August to the end of leaf fall. Each filter paper wasexamined microscopically, and the chasmothecia were counted. The first chasmothecia of theseason were dispersed between late August and early October; cumulative numbers of thechasmothecia dispersed subsequently increased (with different dynamics depending on the vineyardand 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 maturechasmothecia that dispersed roughly depended on the powdery mildew severity on leaves: highnumbers of chasmothecia were associated with disease severity ≥ 80%. According to estimatesbased on the current data, vine bark in vineyards trained with a Guyot, Geneva double curtain, orspurred cordon pruning system could contain till 18, 19, or 44 million chasmothecia/ha,respectively.