Viticultural Information

Inadequate sulfur dioxide has allowed the Botrytis to spread from the field-infected berry

Botrytis cinerea, which also causes Botrytis bunch rot in the field, severely affects stored table grapes because it can infect the grape in the field and then continue to grow in the berries during storage at 0°C (32°F). Because of its ability to grow at such low temperatures and its broad host range, Botrytis is a severe postharvest pathogen of many crops, including flowers, strawberries, and lettuce.

Degree of susceptibility of stored grapes to Botrytis depends upon the level of infection in the field before storage, the variety of grapes, condition of berries at harvest, and effectiveness of fumigation practices for control.

Initial infection begins in the field before harvest. Berries that do not show symptoms or are missed during packing contaminates the rest of the lug during storage. Usual spread of the fungus is not from lug to lug bur originates from infections within the lug. The spread of fungal mycelium from berry to berry is referred to as contact infections. Usually conidia from the fungus are not as important in the disease cycle in storage as they are in the field.

Several other fungi can cause decay in stored table grapes. The most significant after Botrytis cinerea is blue mold rot caused by Penicillium sp. It affects 1.5 percent of the shipments inspected in New York by USDA. The blue-green fungus destroys the entire berry, turning it into a watery consistency. Rhizopus sp. affects less than 1 percent of the lots inspected in New York. It produces no readily visible symptoms; however, when slight pressure is applied to an infected berry, it "explodes," turning into a watery mass. Aspergillus niger is easily identified when sporulating by its black, sooty spores and is often referred to as "smut." Black spot caused by Cladosporium herbarum can also be a problem during storage of table grapes.

Although powdery mildew, caused by Uncinula necator and covered elsewhere, does not develop or spread during storage, it can decrease the storage potential of infected clusters if lesions occur on the rachis. Rachis infections cause an increase in water loss from the berries. Dried berries pull away from the pedicle and are more susceptible to bleaching during fumigation or may fall off the cluster. The rapid drying also reduces the marketability of the fruit. Rachis infections must be controlled in the field before harvest.

Pearson, R., and Goheen, A. 1988. Compendium of Grape Diseases. APS Press, St. Paul, Minnosota. 93 pages.