Michael A. Ellis
Botrytis bunch rot is caused by the fungus Botrytis cinerea. This fungus is very common in nature, and causes diseases on a variety of unrelated crops. Bunch rot can cause serious losses on highly susceptible grape varieties. Although berries of all grape varieties are susceptible to bunch rot, losses generally are greater on tight-clustered varieties of Vitis vinifera and French Hybrids. Losses result from the rotting of berries in the field or in storage.
Infection of ripe berries is the most common and destructive phase of this disease. Infected berries first appear soft and watery. The berries of white cultivars become brown and shriveled, and those of purple cultivars develop a reddish color. Under high relative humidity and moisture, infected berries usually become covered with a gray growth of fungus mycelium. One or a few berries within the bunch or the entire bunch may be affected. Generally, healthy berries touching infected berries will become infected. Rotted berries generally shrivel with time and drop to the ground as hard mummies. The fungus also can cause a blossom blight that can result in significant crop loss early in the season. Although uncommon, leaf infections also occur, but appear to be of no economic importance in Ohio. Leaf infection begins as dull, green spots, commonly surrounded by a vein. The spots rapidly become necrotic lesions.
Botrytis bunch rot is caused by the fungus Botrytis cinerea. The fungus overwinters in grape mummies, dead grape tissues, and other organic debris in and around the vineyard, as well as on a multitude of alternate plant hosts. Because of its wide host range, growers always should assume that the fungus is present in the vineyard. In spring, the fungus germinates from small, dark, hard resting structures known as sclerotia. The fungus then produces spores (conidia) that spread the disease. These spores are produced throughout the growing season. As blooms die, the spores germinate and colonize dead flower parts. Using the dead tissue as a food base, the fungus invades living tissue. After penetrating the berry, the fungus may remain dormant until the fruit sugar content increases and the acid content decreases to a level that supports fungus growth. Symptoms then develop readily under warm, moist conditions.
Berries that escape bloom-time infection may become infected at or near harvest under favorable environmental conditions. Any wound on the berry provides an excellent infection site for the fungus even in the absence of favorable environmental conditions. Birds, insects, hail, and powdery mildew are common causes of wounds. Swelling during ripening in tightly packed clusters causes pressure that also can rupture the berries. Wet and humid conditions around the berries and leaves greatly enhance disease development. The longer wet conditions persist, the greater the probability of infection, even to undamaged berries. Warmer temperatures also favor infection. At 54 to 75 degrees F, infection occurs in 12 to 24 hours, while at 37 degrees F, 60 to 72 hours are required.
|Figure 3. Disease cycle of Botrytis bunch rot. We wish to thank the New York State Agricultural Experiment Station for the use of this figure. Figure taken from Grape IPM Disease Identification Sheet No. 3.|
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