Black rot of Grapes

Guignardia bidwellii

Black rot is an important fungal disease of grapes that originated in eastern North America, but which now occurs in portions of Europe, South America, and Asia as well. It can cause complete crop loss in warm, humid climates, but is virtually unknown in regions with arid summers. There is a wide variation in susceptibility to this disease among native American and hybrid cultivars, whereas all common cultivars of Vitis vinifera appear to be highly susceptible.
Symptoms and Signs. All young green tissues of the vine are susceptible to infection. Relatively small, brown circular lesions develop on infected leaves, and within a few days tiny black spherical fruiting bodies (pycnidia) protrude from them. Elongated black lesions on the petiole may eventually girdle these organs, causing the affected leaves to wilt. Shoot infection results in large black elliptical lesions. These lesions may contribute to breakage of shoots by wind, or in severe cases, may girdle and kill young shoots altogether.
Infection of the fruit is by far the most serious phase of the disease and may result in substantial economic loss. Infected berries first appear light or chocolate brown, but quickly turn darker brown, with masses of black pycnidia developing on the surface. Finally, infected berries shrivel and turn into hard black raisin-like bodies that are called mummies.

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