Occurrence and importance
Gray mold rot of apples is primarily a storage disease that occurs anywhere apples are grown and stored. The disease is second to blue mold rot in losses it causes during storage and marketing of eastern apples. It is not as common or as destructive on apples as on pears grown in Washington and Oregon. In these states the Winesap variety is affected more of ten than other varieties.
Gray mold rot develops faster at cold-storage temperatures than any other fungus rot of apples. Apples carrying infection in the fall when placed in storage at 31° to 32 °F are often completely decayed by February or March.
The appearance of gray mold rot varies somewhat with the variety of the apple, the stage of ripeness, and the temperature at which the rot develops. The color is somewhat paler on green apples than on red ones. On any hard variety in cold storage, gray mold rot appears first as a firm, pale, translucent area without sharp margins. As the rot enlarges, it becomes pale brown to brown, but the margin of the rot is often a paler color than the main part (top photo). Some completely decayed apples have the appearance of baked apples. Frequently certain red varieties that develop gray mold rot in cold storage have a dark reddish-brown area about 1/8 of an inch in diameter around the lenticels (second photo). The dark areas persist even when decay is complete and the contrast between the dark spots and the paler decayed surface give the fruits a freckled appearance. As the decayed area enlarges, affected tissues soften but do not separate readily from the healthy flesh unless the apple is soft-ripe or the decay is in an advanced stage. The decayed flesh in an advanced stage usually has a pleasantly sweetish fermented odor. Apples decayed late in the season may have a stale, fermented odor, but it is quite distinct from the musty odor of advanced stages of blue mold rot.
Gray mold rots that develop in storage from incipient infections at harvest occur principally at the stem or calyx end. From these initially rotted fruits the rot then spreads to fruits in contact with them to produce "nests" or "pockets" of decaying fruits in the stored package. Hence the disease is often called nest rot or cluster rot.
The Botrytis fungus can also cause a condition known as blossom-end rot of apples in Northeastern orchards.
In general, gray mold rot of apples throughout the United States is caused by Botrytis cinerea Pers. ex Fr.
In Washington a new species was reported as Botrytis mali Ruehle in 1931 from stored apples. This species has not been reported since and appears to cause little loss.
It is believed that the source of gray mold infection is in the orchard. The fungus grows and sporulates abundantly on dead and dying plant material found in orchard cover crops, especially during cool, moist weather. Fruits hanging near the ground are more likely to become infected than those farther from it. As mentioned earlier, fruits infected at the time of storage nearly always decay at the stem or the calyx end. The spores lodge in these cavities and under favorable conditions may develop and attack the stem or calyx parts. From such superficial infection the fungus spreads to the edible part of the fruit. Infected fruits may be completely decayed after 3 or 4 months in cold storage, then they begin to infect fruits touching them.
Some measure of control may be obtained by keeping the cover crops closely clipped around the trees, permitting better air circulation and reducing dampness and other conditions favorable to fungus development.
Apples should be handled carefully, stored and cooled promptly, and maintained at 31° to 32 °F throughout storage.
In the Northwest, most apples are treated with a formulation of sodium orthophenylphenate as a fungicidal wash at packing time. This should aid in controlling gray mold rot. (See Pears, Gray Mold Rot).
Gray mold rot, medium stage
Gray mold rot, advanced stage
Gray mold rot, Fuji
Gray mold rot, Fuji
Gray mold rot, Golden Delicious
Gray mold rot, Elstar