WSU Tree Fruit Research & Extension Center

Tree Fruit Market Diseases

Sunday, February 17, 2019

WSU-TFREC/Postharvest Information Network/Tree Fruit Market Diseases/peargraymold

Botrytis cinerea Pers. ex Fr.

Occurrence and importance
Gray mold rot is an important storage disease of pears, especially varieties that are stored for long periods. The disease is present in all pear-growing areas of the country but is particularly troublesome in the commercial pear storages of Washington and Oregon.

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.

The appearance of gray mold rot varies with the variety of pear, the stage of ripeness, and the temperature at which the rot develops. On firm, green-skin pears, the storage rot may appear watersoaked and grayish green (top photo). At higher temperatures and on riper fruits, the color is usually a medium, but not necessarily a uniform, brown (second photo). Because of the rough-textured brown skin of Bosc pears, gray mold rot identification should be based on characteristics other than color.

Gray mold rot is firm to relatively firm on unripened fruits. At this stage the decayed tissues cling tightly to the healthy flesh. Gray mold rot becomes increasingly softer as the fruits ripen. Even when decaying pears are ripe, the decayed flesh is never as soft and glassy as that of blue mold rot. Nor does the decayed flesh separate as cleanly from the healthy flesh as it does in blue mold rot, unless the pears are overripe. Gray mold rot of pears, especially in advanced stages, has a pleasantly fermented odor that distinguishes it from other storage rots.

Under humid conditions the characteristic gray surface mold of the disease appears, especially when the skin over the lesion is broken or when the rot is advanced. In spreading from an infected fruit to surrounding healthy fruits, the fungus mycelium grows from one pear to another and thus binds the rotting fruits together to form clusters or "nests" of decay. Small, hard, black resting bodies of the fungus, called sclerotia, may appear on the moldy surface of fruits in advanced stages of decay.

Causal factors
Botrytis cinerea, the cause of gray mold rot, is widely distributed in nature and thrives on decaying plant matter. Cool, moist weather before and during harvest favors an abundance of Botrytis spores in the orchard. The fungus becomes established on the stems and calyxes of pears before harvest and thus has an opportunity to invade the edible fruits while in storage. Mechanical injuries can provide an entry for the spores to infect the fruits, but as stated earlier the first fruits to decay in storage are nearly always infected through the stem or calyx. These fruits may become completely decayed in 3 to 4 months at 30° to 31°F, and the decay may spread to surrounding fruits.

Control measures
Packinghouse sanitation is always a good practice, but that alone is not enough to control gray mold rot.

In Washington and Oregon, practically all graded pears are washed in a formulation of sodium orthophenylphenate as a standard practice. This disinfecting wash is especially effective in reducing surface spores and perhaps in reducing infection of the calyxes. It appears to be less effective, however, in killing the infection in the stems. This becomes evident from the decay that develops in the stems of fruits, in which the fungus is favored by the high relative humidity afforded by ventilated polyethylene liners in the boxes of pears in commercial storages. Stored pears should be observed frequently for stem decay. The presence of many infected stems in a lot makes it advisable to market the lot early before the fungus penetrates the fleshy portion of the pears and causes decay.

Sodium orthophenylphenate is highly toxic to fruits and workers, and precautions are necessary in using it. The formulation provides a highly alkaline solution that reduces the hazard of injury to the fruits, but the solution does not remain stable and requires frequent analysis to indicate the need for adjusting the alkalinity. Pears remain in the disinfecting bath for about 1/2 to 1 minute and are then thoroughly rinsed. Rinsing is necessary to prevent chemical injury to the pears. (See Pear Chemical Injury.)

Formulations of sodium orthophenylphenate are sold under various trade names, but all must be used with care. Workers should not come in contact with the solution nor breathe the fumes; therefore, the disinfecting tank should be located in a place with adequate ventilation or with fans to draw off the fumes.

Treated pears are also wrapped in copper-impregnated, oiled paper wrappers to prevent the spread of gray mold rot from one fruit to another in case the fungus is not killed by the disinfecting treatment.

These treatments, however, do not replace good practices of careful handling, prompt storage, and rapid cooling to 30° to 31°F. This temperature range should be maintained throughout storage.

Pear gray mold rot, at stem end
Pear gray mold rot, at stem end

Pear gray mold rot, at stem end
Pear gray mold rot, at side

Pear gray mold rot
Pear gray mold rot

Pear gray mold rot
Pear gray mold rot

Pear gray mold rot
Pear gray mold rot

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