A Guide to Grapevine Abnormalities in South Africa - P.G. GOUSSARD

Downy mildew.

entire bunches are usually destroyed before and until pea berry stage and even later than that (Photos 49 & 50). Early signs of oidium on leaves and young bunches may sometimes be confused with those of downy mildew. It is nevertheless simple to distinguish between the respective diseases by wetting the plant matter using a spray canister and keeping it overnight in a moist, inflated plastic bag at a mild temperature (20 - 30°C). In the event of downy mildew, vigorous fungal growth will be noticeable the next day (Photo 51), whereas no visible fungal growth will occur if oidium is present.

PHOTO 31. The first signs of downy mildew are associated with irregular, pale yellow spots on the upper parts of leaves. Note the oily, shiny appearance – hence the name “oil spots”.

A Guide to Grapevine Abnormalities in South Africa • 29