This photo illustrates a bad case of Powdery Mildew (Erysiphe necator) on Grapes.
This is one of the most common diseases of grapes. Initial symptoms of powdery mildew appear on leaves, then on the fruit, then on the bark of the shoots.
The mildew survives the winter by laying dormant in the buds. It is only when the spring is damp and temperatures rise that spores begin to get going.
Initially the shoots and leaves of the vine all appear to be growing well in the spring. But all the activity starts on the underside of the leaves, out of sight.
There are a number of fungus chemicals on the market, but its normally too late when the fungus has got a hold, as shown in the photo. Spraying very often is continuous throughout the growing season.
The more effective method, is to reduce temperatures and moisture during the spring, by increasing ventilation and air circulation. The only trouble is that many of the older varieties of grapes are not immune, whereas the newer varieties have an inbuilt immunity.
Remeber that where grapes are normally grown in the Mediterranean countries, where there is not the moisture or humidity, and they are grown outside with full ventilation. We have a tendency to grow them in a greenhouse with sat tomatoes, which is not ideal.