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P. viticola infects all green parts of the host plant that bear stomata. It generally causes yellow discoloration, necrosis and distortion.
On young leaves, lesions appear as yellow, translucent 'oilspots' with a chocolate-brown halo (see Pictures). On cultivar Ruby Cabernet, oilspots are reddish instead of yellow (Nicholas et al., 1994). Multiple oilspots can coalesce to cover much of the leaf surface. Oilspots become dry and necrotic as they age, first in the centre and later throughout the entire lesion. On older leaves, the lesions are restricted by veins to form small, angular, yellow to reddish-brown spots which combine to form a patchwork or mosaic-like pattern.
Sporulation only occurs on the lower leaf surface, where the stomata reside. The sporangiophores and sporangia appear as a white, downy, cottony growth. Under highly favourable conditions, sporulation may appear on the undersides of leaves before the yellow oilspot becomes visible on the upper leaf surface. On older oilspots, sporulation occurs primarily on the margins of the lesion.
Infected shoot tips and rachises of young inflorescences distort into a curl or corkscrew.
Infected inflorescences and young berries appear yellow or grey and may be covered with cottony spores under favourable conditions. Sporulation occurs on pedicels and berries. Clusters infected at an early stage can result in individual berries, sections of the cluster, or even entire clusters turning brown, drying and falling off the vine.
Berries infected later in the season (after 2-3 weeks post-bloom) become discoloured and shrivel but do not support sporulation. This stage is sometimes referred to as the 'brown rot' phase. Berry stems continue to sporulate after sporulation ceases on the berries.
Disease pressure varies significantly with weather conditions. Management must be rigorous in wet climates such as eastern North America and parts of Europe, and during unusually wet seasons in dry locations such as California or Australia.
Chemical Control
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any
specific chemical control recommendations. For further information, we recommend you visit the following resources:
P. viticola has caused significant impacts on grape production since the 1800s. During the early culture of European varieties in the USA, yield losses were commonly 75% (Viala, 1893). Catastrophic losses arose in Europe in the late 1800s when P. viticola was first introduced on American rootstocks. The disease caused severe losses in favourable seasons and growers abandoned the use of several highly susceptible varieties (Viala, 1893).
Potential yield losses remain high, ranging from 50 to 100% under favourable conditions. The pathogen directly attacks the young inflorescences and fruit. Indirect damage occurs when severe foliar infections cause early defoliation, exposing the fruit to sunburn and reducing winter hardiness (Emmett et al., 1992). The estimated annual crop loss in an average year in Australia is $22.5 million (Australian dollars) with an additional $10 million spent on control measures. In wet years, direct crop losses may be as high as $64 million (Magarey and Butler, 1998).
