Effect of different overwintering conditions on the germination dynamics of Plasmopara viticola oospores

Germination of the oospores of Plasmopara viticola (Berk. et Curt.) Berl. and De Toni, the agent of grapevine downy mildew, provides the inoculum required for the onset of primary infections in vineyards. To ascertain the influence of different water supplies during overwintering on the germinability of P. viticola oospores, leaf fragments containing the sexual structures of the pathogen were overwintered in the vineyard and under controlled conditions, at 5 \ub0C and three different water availability regimes: constant, fluctuating and dry (no water supply). From November to the end of June, oospore germination was tested every 4 days on water agar at 25 \ub0C. The germination percentage of oospores overwintered in the vineyard increased from November until January, showed ramakable fluctuations afterwards, and fell to zero from the beginning of June onwards. Only very few oospores overwintered in constantly dry conditions and seldom formed macrosporangia, while reduced and fluctuating germination percentages were observed for the oospores kept under variable water regimes. Oospores constantly provided with water germinated abundantly from November to the end of June even if their germination percentages greatly varied during the overwintering period. Therefore, it seems that oospore germinability is increased and prolonged when overwintering is at low temperatures and constant water supply. However the observed fluctuation in germination percentage suggests that other factors, probably endogenous, are strongly influencing germinability

Evolution of QoI resistance in Plasmopara viticola oospores

QoI resistance in P. viticola was first detected in France and Italy in 1999. Molecular and biological assays have been carried out since 2000 in order to provide reliable methods of detecting and quantifying resistance. Oospores were collected in vineyards located in northern and southern Italy. QoI resistance was evaluated by the germination rate of oospores on azoxystrobin amended medium and the frequency of mutant alleles in the DNA extracted from oospores. Both methods correlated to each other and were used side by side to test QoI resistance. Due to the spontaneous occurrence of the G143A mutation in wild type populations and the immigration from surrounding vineyards, resistance frequencies up to 10% were found in samples collected from vineyards never treated with QoIs. Particularly high values, about 90%, were associated with the application of five to six QoI treatments within the same season, while lower percentages, about 30%, were detected in vineyards treated with QoI used in mixture with fungicides belonging to a different resistance group. A progressive decrease of resistance frequency was observed when QoI applications were reduced in number or completely suspended for at least one season. Therefore, a full recovery of sensitivity may be achieved even in vineyards characterized by high levels of resistance, if particular care is taken during disease control by using QoIs only in mixtures and reducing the number of QoI treatments