Black rot, like botrytis bunch rot, targets the ripening fruit on the grapevine. It is caused by an ascomycete, Guignardia bidwellii. While black rot can lead to infections on leaves and shoots, it causes significant damage to the clusters in the canopy. Most importantly, it can lead to mummified berries, which in turn, become the primary source of inoculum for future infections. Those mummies that are left hanging in the canopy are significantly more dangerous for future infections than even those that are dropped on the ground because they can become the source of secondary inoculum in the form of conidia that can be dispersed directly into the canopy. The ascospores are aerially dispersed and are released into the air about one to two weeks before bloom. The spores can overwinter in any part of the plant, which may also serve as inoculum for the subsequent two years. The ascospores of Guignardia bidwellii cause the primary infection on the leaves. In turn, the picnidia develop in the lesions on the leaves contain conidia which are rain dispersed.

This disease causes significant damage to the quality of fruit and impacts the saleable and usable nature of the fruit. Black rot-infected fruit is commercially undesirable, and it is therefore a serious problem for the grower to ensure its control. However, as fruit matures it becomes increasingly more resistant to the pathogen, so spraying at the beginning of the season when the disease is developing, as an attempt to reduce the amount of initial inoculum as well as the rate of development, is very important.

As with the other pathogens discussed previously, moisture is a significant concern in the spread of Guiguardia bidwellii. Spores spread through rain splash, and will travel between berries with the help of moist, humid weather. What makes black rot unique is its very long latent period, which can stretch from three to five weeks after infection, if the disease develops at the end of the susceptibility period. This extended latent period means once again that control at the beginning of the season is crucial, because if the initial amount of inoculum is not reduced, it may produce secondary inoculum that may not appear until the end of August, right before harvest. In New York State, the disease pressure is relatively high throughout the season, and with a long latent period that favors humid weather, growers cannot depend on a dry period to eradicate the disease. Black rot will outlast a dry period and will spread when the humidity returns.