BOTRYTIS BUNCH ROT, GRAY MOLD
Description
Symptoms
Disease Cycle
Control

Description:
Botrytis bunch rot is the most important bunch rot disease of grapes worldwide, and can cause serious losses in the vineyard and in transit or storage. Botrytis infection is favored by cool weather and free moisture on the surface of fruit, and bunch rot is most severe on cultivars with thin skins or tight fruit clusters, under heavy canopies, and in areas of high humidity.

Symptoms:
Leaf infection by the bunch rot pathogen, B. cinerea, occurs under cool, moist conditions in spring prior to bloom, and appears as a dull green spot that turns into a reddish-brown necrotic lesion. Young shoots and blossoms may also become infected, resulting in significant yield losses. Small brown patches may appear on pedicels or rachises that later turn black, causing portions of the cluster to shrivel and drop off (Fig. 1). The fungus infects and rots ripening berries, causing the fruit of white cultivars to become brown and purple cultivars to become reddish. The most common symptom of the disease appears when fluffy, gray-brown growth containing spores becomes visible (Fig. 2), eventually spreading throughout the cluster.

Disease Cycle:
The fungus Botrytis cinerea overwinters on canes, bark, dormant buds, and debris on the vineyard floor as dormant mycelium, or as hard, resting structures (sclerotia) in berry mummies or on canes which are resistant to adverse weather conditions. The sclerotia germinate in the spring, producing spores (conidia) that reside in the vineyard throughout the growing season. Conidia produced in the spring are watersplashed and windblown to newly emerging leaves. Infection may occur at temperatures as high as 86 F, but the optimum temperature for infection is 59-68 F. Tissue that is dead or has been injured by hail, wind, birds or insects is usually colonized before healthy tissue. Early-season powdery mildew infected fruit are also more susceptible to infection. Since spore production and infection are favored by wetness and high humidity, fruit infection in the southeastern US may occur throughout the season from bloom to closing, and after veraison when sugar concentrations increase in fruit. As harvest approaches, spores from infected fruit may spread to other fruit in the cluster as well as to other clusters.

Control:
Cultural - Good Botrytis control starts with good sanitation practices. Before spring arrives, it is extremely important to remove all of last year's fruit from the trellis, as well as canes, bark, and debris from the vineyard floor. Because B. cinerea thrives under moist conditions, good canopy management practices including shoot thinning, leaf removal, pruning, cluster thinning, and shoot positioning are essential for reducing humidity and increasing air circulation. Cultivars of bunch grapes vary in their susceptibility (Table 1). The disease tends to be more severe on cultivars and clones with tight clusters.

Chemical - Refer to the Winegrape Spray Program in the North Carolina Agricultural Chemicals Manual for current recommendations for Botrytis bunch rot control on bunch grapes http://ipm.ncsu.edu/agchem/chptr7/706.PDF and muscadines http://ipm.ncsu.edu/agchem/chptr7/707.PDF. Fungicide applications are most critical in the veraison and preharvest sprays, and if the season is wet, sprays may be necessary at bloom and closing as well. Additionally it is important to prevent early season powdery mildew infections since infected fruit are more susceptible to infection by B. cinerea. If conditions become wet at harvest, picking early can reduce the amount of fruit lost to Botrytis bunch rot.