A spore producing canker showing guitar string symptoms.
The disease is found to a lesser extent on pear, quince, and crabapples. Bull's eye rot of apple fruit and perennial canker of apple are similar diseases, but are usually found in the drier, colder areas east of the Cascade mountains.
Infection generally occurs during fall rains but can take place throughout the winter and early spring during mild, wet weather. Spores of the fungus, Pezicula malicorticis (Crytosporiopsis curviospra), formed on the dead bark of older cankers, are splashed to other twigs and branches and infect through wounds or natural openings in the bark. These infection areas appear as small, circular, reddish brown spots on the bark in late fall. The discoloration extends into the tissue as far as the sapwood. Canker growth is very limited during the winter, but they begin enlarging rapidly in the spring.
The cankers usually form on young branches, and are generally less than 2 inches in diameter, but some may develop on larger branches or young tree trunks. By June, the cankers have grown to full size and subsequent branch growth results in formation of a crack around the canker . Fullgrown cankers are elongate, from 1-10 inches in length, and up to several inches wide but most are 2-5 inches long. There may be one or many on a single branch. Girdling of smaller branches frequently occurs.
By midsummer, the canker surface is sunken, and as the summer progresses, some of the dead bark in the canker falls out. A callus layer forms and produces a ridge around the canker. By fall, the canker generally has many lengthwise slits, the shredded bark sometimes having the appearance of many guitar strings. Larger cankers on main branches or trunks may not have this latter appearance.
This canker lacks the pattern of overlapping concentric rings so characteristic of the perennial canker and European canker diseases. In the fall, one year following infection, the fungus forms spores on the dead bark, and these spores cause new infections. Though the canker has attained full size, the fungus can live for several years in the bark, producing spores each fall for new infections.
Infection of the fruit occurs in the fall. The rot is sometimes apparent at harvest, but often it only appears after the apples have been in storage for a time. The rot is first seen as small, circular, light brown areas. As the spots enlarge they become darker in color and sunken, but remain circular in shape. The tissue beneath the spots is spongy. As the spots continue to develop, small bumps or cushions are formed in the centers. These bumps contain spores. Spots may measure 3/4 inches or more in diameter. Although the spots enlarge slowly, other rot-producing fungi may enter the damaged tissue and cause rapid rotting.