Most grapevines are very susceptible to various fungal diseases. These diseases affect leaves, shoots, and fruit. Leaf diseases can drastically reduce photosynthesis, and, in extreme cases, can cause defoliation.
Fungal diseases can render fruit unusable and can very easily cause severe losses in yield.
It is important to distinguish between the three broad categories of grapevines because of differences in susceptibility to fungal diseases. Generally, vinifera are much more susceptible to bunch rot, with the white varieties more susceptible than the reds. Canadice and Romulus are very susceptible. Any tight clustered variety is likely to have Botrytis problems. Good air circulation throughout the canopy is critical in controlling this disease.