Once established in the wound, the fungus can quickly spread from infected fruit to adjacent healthy fruit during storage. Photo by K. Peter.
The disease develops very quickly at cold storage temperatures. Also known as nest rot or cluster rot, gray mold can cause large losses because of its ability to spread from infected to adjacent healthy fruit in storage.
Gray mold lesions first appear as pale tan areas without sharp margins. Infections may originate from wounds, stem punctures, or the stem or calyx end of the fruit. As the decay enlarges, the older portions of the decay may turn darker brown, but the edges often remain a paler color. On red apples, dark areas may persist around lenticels even after the fruit are completely decayed, and these give the fruit a speckled appearance. On firm green pears, the decay is often water-soaked and gray-green, but it is brown on riper fruit and at higher temperatures. On less mature apples and pears, the rotted tissue is firm and does not separate easily from healthy tissue, but the rot is softer on riper fruit. In advanced stages, the decayed flesh has a sweet, cider-like odor. Under high humidity, white or gray-white mycelium may develop on the surface of decayed areas. Little sporulation occurs at cold storage temperatures.
Gray mold is caused by Botrytis cinerea. Gray mold rot is seldom seen in the field, although B. cinerea is a common saprophyte on decaying organic matter on the orchard floor.
Wounds or injuries are the primary infection points for initiation of gray mold. Conidia from the orchard soil or other organic debris are brought in via storage bins and containers.
Fruit decay during storage produce additional inoculum. The conidia are most typically water-dispersed in flumes in packing houses. Once established in the wound, the fungus can quickly spread from infected fruit to adjacent healthy fruit during storage. The fruit-to-fruit spread causes the phenomenon known as nesting and can result in the loss of many fruit as a consequence of a single original infection.
Bin sanitation is an important measure for control of gray mold in order to minimize spores brought in from the field. Postharvest fungicides are also effective for controlling gray mold; however, fungicide resistance is an issue and rotating chemicals with different FRAC groups is recommended.