Botrytis Bunch Rot

Botrytis bunch rot (gray mold) and blight of leaves, shoots, and blossom clusters occurs throughout the viticultural world. The fungus causing the disease grows and reproduces on senescent or dead plant tissue. Botrytis bunch rot is especially severe in grape cultivars with tight, closely packed clusters of fruit. Botrytis is also responsible for storage losses of grapes picked for fresh market.

Symptoms

Botrytis infection of leaves begins as a dull, green spot, commonly surrounding a vein, which rapidly becomes a brown necrotic lesion. The fungus may also cause a blossom blight or a shoot blight, which can result in significant crop losses. Debris, i.e. dead blossom parts, in the cluster may be colonized by the fungus which can then move from berry to berry within the bunch prior to the beginning of ripening, and initiate development of an early season sour rot. However, the most common phase of this disease is the infection and rot of ripening berries (Figure 78). This will spread rapidly throughout the cluster. The berries of white cultivars become brown and shriveled and those of purple cultivars develop a reddish color. Under proper weather conditions, the fungus produces a fluffy, gray-brown growth containing spores (Figure 79).

Figure 78: Botrytis bunch rot of grape.

Figure 79: Close-up showing Botrytis sporulating on infected berries.

Botrytis Bunch Rot Disease Cycle (Figure 80)  

Botrytis overwinters on debris in the vineyard floor and on the vine. The fungus produces small, dark, hard, resting structures called sclerotia. Sclerotia are resistant to adverse weather conditions and usually germinate in spring. The fungus then produces conidia, which spread the disease. Sporulation may occur on debris left on the vine during the previous growing season, such as cluster stems remaining after mechanical harvest or mummified fruit, or it may occur on sclerotia on canes. The fungus usually gains a foothold by colonizing dead tissue prior to infection of healthy tissue. Tissue injured by hail, wind, birds, or insects is readily colonized by Botrytis. Ripe berries that split because of internal pressure or because of early season infection by powdery mildew, are especially susceptible to infection by Botrytis. Botrytis conidia are usually present in the vineyard throughout the growing season. Moisture in the form of fog or dew and temperatures of 59 to 77°F are ideal for conidia production and infection. Rainfall is not required for disease development, although periods of rainfall are highly conducive to disease development.

Figure 80: Botrytis bunch rot disease cycle. We wish to thank the New York state Agriculture Experiment Station for the use of this figure. Figure taken from Grape IPM Disease Indentification Sheet No. 5.

Control of Botrytis bunch rot

Organic fungicides (copper and sulfur) are not effective for control of Botrytis. There are several biological control agents or products that are registered for control of Botrytis on grapes (Table 1). The efficacy of these products under moderate service disease pressure needs to be determined. Cultural practices that open the canopy such as leaf removal are very important for disease control. Cultivar selection is also very important. Cultivars with tight clusters that are highly susceptible to Botrytis bunch rot should be avoided in organic production in the Midwest.