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 °C 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 °C. 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.