Black rot caused by the fungus Guignardia bidwellii is the most prevalent and important grape disease in Kentucky. Without an adequate disease control program, both home and commercial grape production statewide is often severely limited.

Symptoms
The black rot fungus can infect the leaves, young canes, flowers, tendrils and fruit of grapes. All green tissues of the vine are susceptible. Generally the youngest tissues are most susceptible, e.g., leaves are most susceptible during the first week of their emergence, however, the berries may become infected any time before being fully grown.
On the leaves, black rot first appears as minute, round, reddish brown spots. As the spots enlarge to a maximum diameter of 3 to 7 mm (1/8 to 1/4 in.), they develop black margins and tan brown centers. Within spot centers, numerous small, black spore-bearing structures of the fungus, called pycnidia, can be seen. Pycnidia are the source of infective spores and are just visible to the naked eye.
On stems, black rot appears as irregular or sometimes diamond-shaped, somewhat sunken, tan brown lesions also with scattered pycnidia.
On the fruit, symptoms start as light brown soft spots which rapidly enlarge to involve the entire berry. Affected berries then shrivel into black, wrinkled mummies which either drop to the ground or remain in the cluster. As with leaf spots and stem lesions, black rot mummies are covered with pycnidia.

Spread
The fungus survives the winter in mummies both on the vine in the trellis and on the ground, and in fallen leaves and stem lesions. Spore production starts in the spring as temperatures increase and wet weather prevails. Spring rains trigger release of airborne ascospores from overwintering perithecia and/or rain-splashed conidia from overwintering pycnidia. Primary infection takes place if plant surfaces stay wet long enough for spores to germinate and penetrate host tissues. After infection, symptoms generally take about 2 weeks to develop. Once the fungus has become established in susceptible tissue, it starts producing secondary spores which, in turn, are capable of infecting. This cycle of spore production and infection continues the rest of the season, except when the environment becomes limiting.
A single ascospore or conidium can cause a primary leaf infection. Within each leaf lesion, many pycnidia form, each of which can produce hundreds of thousands of conidia, each of which can cause another secondary infection later in the season. The majority of ascospores from mummies on the ground are discharged during the period from one inch shoot growth to 10-14 days after bloom. If mummies are allowed to hang on the trellis, they can discharge ascospores and conidia throughout the growing season. Thus, it is important to control the primary infections so that fewer black rot control measures would be needed later in the season. If early season infections are not controlled, additional chemical protection may be needed later in the season.

Control
Sanitation is critical to successful black rot control. If all mummies and infected canes are removed from the vineyard, there is no source of primary inoculum in the spring, and thus, the disease is controlled. It is especially important to remove mummies from the trellis because they are a threat to the crop all season long whereas the mummies on the ground are only a threat early in the season.
Other cultural practices that will enhance black rot control efforts include removing weeds and basal sprouts to increase air circulation within the vines and hasten the drying of dew and rain from around black rot-susceptible tissues. When establishing a vineyard, try to select a site with inherently good air circulation and drying characteristics. Avoid low lying, poorly drained sites. If possible, select cultivars with some level of black rot resistance (Table 1) and train vines on high trellises to promote good air circulation and to help fungicide sprays penetrate.
Fungicide sprays are an important component to a successful black rot management program, especially in an established vineyard. Fungicides for black rot control are either protective or eradicative; both groups require proper timing of application to be effective.
To use a protective spray program, sprays must generally begin at bud break (after 1/2-inch of new shoot growth) and continue through berry maturity. Note that in a protective program, early season sprays must be applied enough times so that rapidly-growing, susceptible tissue is always protected. Vines not protected early in the season may become infected and result in inoculum build-up which could infect the berries later in the season.
First spray applications can often wait until just before bloom or longer if an eradicant fungicide is used or if black rot is not traditionally a problem in the vineyard. Generally, eradicant fungicides must be applied within a certain time after the infection period to be effective. Contact your county Extension office for a copy of the current "Kentucky Commercial Small Fruit and Grape Spray Guide," ID-94 which contains information on the currently recommended fungicides.

Table 1. --Cultivar Black Rot Susceptibility Ratings. ^{1, 2}

Cultivar	Disease Reaction
Aurora	ES
Baco Noir	HS
Bath	HS
Cabernet Sauvigon	HS
Canada Muscat	HS
Canadice	HS
Cascade (Siebel 13053)	MR
Catawba	HS
Cayuga White	HS
Challenger	ES
Chambourcin	MR
Chancellor (Siebel 7053)	MS
Chardonnay	MS
Chelois (Siebel 10878)	MR
Concord	HS
DeChaunac (Siebel 9549)	MS
Delaware	HS
Dutchess	HS
Foch	MS
Fredonia	MR
Gewurtztraminer	HS
Glenora	HS
Himrod	MS
Interlaken	HS
Leon Millot	HS
Mars	R
Missouri Riesling	MR
Niagara	HS
Pinot Noir	HS
Reliance	HS
Remaily Seedless	MR
Riesling	HS
Rougeon (Siebel 5989)	MS
Seyval (Seyve-Villard 5-276)	HS
Steuben	MS
Vidal Blanc	MS
Vidal 256	MR
Vignoles (Ravat 51)	HS
Villard Blanc	HS
Vinered	HS

¹ Disease ratings compiled from the Missouri and Cornell Grape Pest Control Guides.
² Disease ratings are for a "typical" black rot year. Significantly more or less disease pressure may modify the above. R = Resistant, MR = Moderately Resistant, MS = Moderately Susceptible, HS = Highly Susceptible, ES = Extremely Susceptible.