P. viticola is an obligately biotrophic oomycete placed into the kingdom Chromista.
All green parts with stomata of the host plant are infected. On young leaves, lesions appear as translucent 'oil spots'. Oil spots become dry and necrotic, first in the centre and later throughout the entire lesion. Lesions are restricted by veins to form angular, yellow to reddish-brown spots which combine to form a patchwork. Sporulation only occurs on the lower leaf surface, where the sporangia on sporangiophores appear as a white growth. On older leaves sporulation occurs primarily on the margins of the lesion.
Infected inflorescences and young berries become yellow or gray and may be covered with cottony spores under favorable conditions. Berries infected later in the season become discolored and shrivel but do not support sporulation. This stage is sometimes referred to as the 'brown rot' phase. Potential yield losses remain high, ranging from 50 to 100% under favorable conditions.
P. viticola is heterothallic with two mating types. An antheridium fertilizes an oogonium to form the sexual oospore in fallen leaves infected in the previous season. In mild climates the pathogen overwinters as mycelium in buds and canes of wild grape species. Sporangial dispersion was observed only in wind-blown rain, not in the air. However, there is some evidence of long-distance (500-600 km) spore dispersal in regional air currents.
Oospores are spherical, 28-40 µm in diameter, covered by two inner oospore membranes and an outer wrinkled oospore wall. They germinate in spring when temperatures reach 10°C and vineyard soils are wet. The germ tube terminates in a macrosporangium which releases an average of 8-20 and up to 60 zoospores. Zoospores require surface wetness to infect the host and infection takes place only through the stomata. Zoospores swim on the tissue surface, encyst near stomata, and each spore forms a single germ tube which penetrates the stomata. In the substomatal cavity the germ tube swells, forming a substomatal vesicle from which a single hypha arises growing intercellularly. In as little as 3.5 hours the first haustorium forms where the pathogen contacts the host cells. Later additional haustoria form parasitizing the mesophyll cells. The incubation time, the period between infection and the first appearance of symptoms, depends on temperature and ranges from 4 to 21 days, with an average of 7-10 days. The pathogen sporulates through stomata during warm, humid nights.
The sporangiophores are hypophyllous, arborescent, 130-250(-700) x 11-14 µm, branching monopodially in the upper third at right angles to the main axis, and with a base tapering to a conical point; branches in a whorl of 4-5, 35-45 µm long, often with two opposite secondary branches 15-20 µm long, all having 3-4 conical tips 10 µm long, 6 µm wide at base, diverging at right angles and tapering to a terminal swelling.
The sporangia are ovoid, colorless, 20 x 14µm, each producing 1-6 zoospores.
For sporulation, P. viticola requires at least 95-98% RH, temperatures between 10 and 30°C and at least 4 hours of darkness. Individual lesions resporulate a number of times under favorable conditions, and can retain the potential to sporulate for several months. Secondary cycles of infection occur repeatedly throughout the growing season if weather conditions are favorable.