P. viticola is an obligate parasite. It needs green, fresh vine organs to grow. During the vegetation free period it persists forming fruiting bodies, so called oospores. Oospores of oomycetes can survive very long periods in soil. Therefore we can find downy mildew in places where infections are not possible in every year. In spring when the top soil is moist and warm enough, the oospores will form so called macrosporangia which can release up to 200 zoospores into free water. The zoospores are moved up to the leaves and clusters by wind in water droplets. They do have two flagella and they move in a water film on the downside of the leaves or the clusters and young berries to find a stoma. They enter and germinate into the stoma, in which they transfer all their plasma within less than one hour. In microscopic studies the finding of stoma, encystation and the germination into the stoma was finished within 90 minutes.
P. viticola grows in the inter cellular space and it feeds itself with haustorias penetrating the epidermic and parenchymal cells. In dependence of temperature and relative humidity it develops enough intercellular growth with enough haustorias to form a substomatel body which fills up the whole substomatel area and which lifts up the epidermal tissue from the parenchymal tissue. This leads to the visible symptom of the oil spot.
Oomycetes are sporulating in the absence of light when relative humidity is very high. In P. viticola there is no sporulation if temperatures are below 12°C and relative humidity is below 95%. Sporangiaphores are formed by the substomatel vesicle and they will come out of the stoma. The fresh formed sporangia are sticky and can only be removed from the sporangiaphores by water. During the decrease of relative humidity the sporangia become try and it can be removed by wind too.
Sporangia will release up to 20 zoospores into free water. This zoospores have to be distributed by wind in water droplets too to come to fresh leaves, or the sporangia can be distributed by rain or wind itself. The infection process of primary and secondary infection is the same.
Do to the big importance of the sexual stage for the hibernation of the pathogen we can assume mating types fitting in all vineyards where grape vine downy mildew occurs. The zoospore formation takes place on older leafs during summer and early autumn.
Table of contents: