Grapevines are susceptible to a variety of fungal diseases. The diseases can ruin a whole crop or destroy the entire vine. You will need to closely observe your vines, and sweep up any diseased grapes or leaves that fall to the ground. Dispose of diseased leaves or grapes in the trash, not in a compost pile, to limit the spread of the disease.
Black rot is a fungal disease that can decimate grapevines. It will appear as small red-brown spots on the lower leaves around mid- to late June. It is caused when even a tiny amount of rainfall falls on old infected berries and canes. The spots will develop a black border and small black pimples will appear on the spot itself. The fruit will become infected in mid season and develop a white spot. About two weeks later, the grapes will shrivel and drop off.
Downy mildew will cause the loss of a good deal of the crop. It is a fungus that lies dormant throughout the winter in leaves on the ground. When spring rains come, they splash the spores up and onto the leaves and berries. If it is a warm and wet growing season, the fungus can kill whole clusters of grapes. When grapes become infected early on in the season, they will develop a white fungus. If it happens late in the season, the fungus will attack the stems and individual berries instead of the whole cluster. The infected berries will turn red or yellow and drop off.
Eutypa dieback is another serious and deadly fungal disease. In the late spring, before the new shoots have reached full growth, they will become stunted, the leaves will curl up, be smaller than they are supposed to be and either turn all yellow or become streaked with yellow. In most cases, the infection enters the vine at a point where it was pruned and it rots out the inside of the arms or the trunk of the plant. Eutypa dieback progresses slowly. It takes two to three years for the initial signs to appear. After about four or five years, the old pruning wound will develop a dark ring around it, made up of grape bark tissue and fungus, that is called stroma. Eventually, one or more of the arms or the whole vine will die.