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Disease Cycle and Epidemiology
The pathogen survives the winter period as oospores embedded in dead leaves and other host tissue on the vineyard floor. Oospores may be released from the decaying plant material onto the soil surface.
Oospores typically produce sporangia. These sporangia, in turn, produce zoospores. Sporangia and zoospores are splashed by rain or carried by wind to the lower leaves and tissues of the grapevines.
The conditions necessary for oospore germination are wet soils with temperatures above 10ºC (50ºF). An additional rule of thumb in some regions includes rainfall of at least 10 mm (0.04 in.) in a 24-hr period to satisfy the requirement of soil wetness. Once zoospores form and are dispersed to host tissue, infection will occur given a minimum of 45 degree-hours of leaf wetness [“degree hours” is the summation of hourly temperatures (C) until the specified value is achieved]. Zoospores encyst and then germinate and penetrate through stomates. Sporangia are usually inactivated after several hours in sunlight, so most infections occur in the morning. After infection, the pathogen grows intercellularly, producing haustoria.
Sporangia for secondary infections are produced on sporangiophores that emerge through stomates of infected leaves and other grapevine tissues (e.g. bunches). Production of sporangiophores and sporangia requires 95 to 100% relative humidity and at least 4 hr of darkness at temperatures initially exceeding 13ºC (55ºF). Once sporulation has begun, the process will continue, provided temperatures exceed 11ºC (52ºF). Sporangia are dispersed to new infection sites by rain splash and/or wind; the latter occurs when sporangia are released into the air as the humidity decreases. Zoospores from these sporangia are released in water and are further spread by wind or rain splash.
The incubation period (the time from infection to appearance of new symptoms) varies from five to 21 days depending on temperature. The incubation period is shortest (~5 days) at mean temperatures from 20-25ºC (68-77ºF). At mean temperatures of 12ºC (54ºF) or lower, the incubation period is 14 days or longer. The latent period on leaves (the time from infection to first production of sporangia) is one day less than the incubation period, given favorable conditions, i.e. sporulation may occur one day prior to the appearance of oilspots.
Sexual reproduction occurs towards the end of the season. The resulting oospores are thick-walled and serve as survival spores.
Copyright © 2000
by The American Phytopathological Society