This vine disease (also known as dry rot) originated in North America and, after phylloxera and the two types of mildew, reached Europe as a "fourth import" in the early 1880s. It should not be confused with black spot disease. Black rot was identified in 1883 by the French viticulture expert Pierre Viala (1859-1936) in the laboratory of Montpellier. It occurs mainly in France, Italy, southern Switzerland and occasionally in German wine-growing regions and is spreading more and more. The disease is caused by the fungus Guignardia bidwelli (rust fungi). Prolonged rainfall in May and June with warm weather promotes the development and spread of the fungus.
Spots of dead, necrotic tissue appear on the leaves, which are bordered by a dark brown edge. This can also occur on leaves, apples and shoots. Fruiting bodies (pycnidia) form on the necrotic spots, which appear as small black, shiny dots. White spores in white threads are released from the pycnidia. Affected berries first show light brown spots, then the whole grape is captured. The infested berries turn purple to black and mummify. The fungus hibernates on them and attacks the young leaves again in spring. There can be considerable reductions in yield. Fungicides are usually used for effective control.
Grape: From Daniel Molitor, CC BY 2.0 de, Link
Pycnidia: From Clemson University - USDA, forestryimages, CC BY 3.0, Link
Leaf symptoms: From Daniel Molitor, CC BY 2.0 de, Link
Variety resistance: Ludger Hausmann, JKI