WSU Tree Fruit Research & Extension Center

Tree Fruit Market Diseases

Saturday, September 23, 2017

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



Glomerella cingulata (Ston.) Spauld. & Schrenk

Occurrence and importance
Bitter rot is principally a disease of the fruits of apple but it may also occur on pear, peach, quince, and cherry. It may occur in practically all apple-growing sections east of the Great Plains, but it is potentially destructive only in hot, humid districts. The disease appears in northern orchards only when warm, wet growing conditions prevail. Delicious, Golden Delicious, Jonathan, Yellow Newtown, Northwestern Greening, and Grimes Golden are the more susceptible varieties. Normally resistant varieties, however, may be seriously affected in hot, wet weather. Bitter rot appears most frequently on apples approaching their full size, and for that reason it is sometimes called "ripe rot."

Symptoms
Bitter rot is characterized by brown to dark brown, definitely limited spots varying in size from mere specks to lesions involving the whole side of an apple. Rots developed in the orchard about harvest time are very firm, are not sunken at first, and are generally marked by narrow concentric zones of light and brown color. Spots 1/2 inch or more in diameter may exhibit small, sticky, salmon pink or cream-colored spore masses that may or may not be concentrically arranged. Eventually the spore masses darken (top photo) and weather off, exposing dark brown to black underlying tissues. In the flesh, the diseased tissues are cone-shaped, brown, and somewhat moist.

Infrequently, small dark spots on a fruit may number in the hundreds, but they remain small, giving the fruit a distinctive peppered appearance.

The bitter rot that is sometimes found on apples in storage and on the market following storage results from small lesions and incipient infections overlooked at harvest. The decayed spots in storage are usually small, 1/4 to 5/8 inch in diameter, firm, flat or slightly sunken, and uniform brown. Rotten spots found after the fruits have been removed from storage are usually larger and more sunken (second photo) and often produce wet pink or cream-colored spore masses. The more or less concentric arrangement of the spore masses distinguishes the disease from black rot, in which the spore-producing bodies (pycnidia) are always black and scattered irregularly over the diseased area.

Causal Factors
Glomerella cingulata, the causal agent of bitter rot, overwinters in fruit mummies and in dead and diseased parts of trees. Spores produced on these parts are disseminated by rain, wind, and insects to initiate infections, which usually occur fairly late in the growing season. In periods of hot muggy weather, extensive losses can occur in the orchard.

Control measures
Bitter rot must be controlled in the orchard, therefore the recommendations of the State agricultural experiment station or extension service should be followed. Apples should be stored promptly and adequately refrigerated immediately after harvest, especially where possibility of late infection exists.

Bitter rot in orchard
Bitter rot in orchard

Bitter rot developed after storage
Bitter rot developed after storage

Bitter rot
Bitter rot

Bitter rot
Bitter rot

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