Indirect Effect of Fungicide Treatments on Chasmothecia of Erysiphe necator Schwein Overwintering on Grapevine Bark

Erysiphe necator Schwein overwinters in Hungary primarily as large numbers of chasmothecia providing primary inoculum for grapevine (Vitis vinifera L.) infection in the next year. In two field trials, the effect of fungicide treatments on powdery mildew severity and the number of chasmothecia produced on leaves and washed to the bark were studied. In 2005, the number of chasmothecia harvested from the bark showed a limited correlation with disease severity (r = 0.553) and number of chasmothecia on leaves (r = 0.600). In 2006, using a refined sieving technique, a substantial increase in the number of harvested chasmothecia could be achieved resulting in a much closer correlation between the variables above (r = 0.750 and r = 0.886, respectively). Among the fungicides applied, boscalid and fluquinconazole (SC formulation) showed the most significant activity by decreasing both the autumn leaf infection and the formation of chasmothecia on the leaves and providing the lowest number of chasmothecia on bark. The research underlined the importance of fungicide applications made in the previous year to decrease the amount of overwintering inoculum and to help protect grapevines from powdery mildew infection in the next year.