The fungus Guignardia bidwellii causes black rot of grape. It is the most common and serious disease of grape in Maine and during years when the weather is favorable losses can range up to 80% of the crop.

The fungus infects leaves, fruit, canes, shoots, and tendrils. Infections are usually first noticed on the leaves where they appear as small spots which may enlarge to ¼ inch in diameter. The spots are commonly light tan in the center circled by a dark tan band (Fig. 1). As the infection ages, tiny dark spore producing structures (pycnidia) form within the spots. Spots formed by infection of the shoots are larger and darker and these also produce pycnidia.

Symptoms of infected fruit begin as small brownish spots that quickly expand to involve the entire fruit in a matter of days. As the infection continues, the grape shrivels from a soft brown rotted fruit to a small, black, hard mummy (Fig. 2). A few, many, or most of the fruit may be infected in each cluster. Once again, pycnidia will form in these infected fruit.

The spores that begin the disease cycle are released in the spring. The spores are of two types. Ascospores (sexually produced spores) are produced in pseudothecia that develop in infected, over-wintered leaves and fruit. These spores are ejected after bud-break in the spring after about 1/100 inch or more of rain has fallen. Conidia (asexual spores) from pycnidia in the over-wintered fruit and cane lesions (and later from new infections) may be released two to three weeks earlier following 1/10 inch or more of rain. After the spores land on tissues susceptible to infection they require sufficient time in

Spore dispersal continues through the middle of the summer and then declines until by the end of the summer no new infections occur. Through this period only new growth is susceptible to infection except for the fruit, which can be infected until the onset of color.

3) Select resistant varieties when planting.