Pezicula malicoeticis (Jacks.) Nannf.
Occurrence and importance
Bull's-eye rot, a disease that occurs late in the storage season on pears grown in Washington, Oregon, and British Columbia, can cause serious losses. It occurs on fruits grown in the irrigated valleys on both sides of the Cascade Range.
Bull's-eye rot infections occur in the orchard, becoming established in the fruits at any stage of development from petal fall onward. The rot usually arises at the lenticels, but can develop in wounds and skin cracks, as well. The organism grows slowly at cold storage temperatures, producing the first symptoms about 5 months after harvest. The rot, which is most common on Anjou, Comice, and Winter Nelis, does not spread from one fruit to another in storage.
Symptoms
The rot develops at the lenticels, at breaks in the skin, or at the calyx in early infections. The spots, which may occur singly or may be numerous, range from 1/8 inch to more than 1 inch across. They vary from yellowish-cream to uniformly brown, but are usually brown with a pale center, suggesting a bull's eye (see photos). The spots may be flat or slightly sunken. The skin does not break readily under slight pressure. The rotted tissues are relatively firm, somewhat mealy, and do not separate readily from the healthy tissues. The rot may be shallow or nearly as deep as wide. In the deeper rots the penetration is more-or-less U-shaped. Spore tufts of the causal fungus may or may not be present on the surface of the rot, but when present are short, wet, and cream colored, and protrude through the skin.
Bull's-eye rot on pears can readily be distinguished from side rot, in which the spots are dark-brown to black, with a tender skin that breaks with a pop under slight pressure. In contrast to the firm, somewhat mealy texture of bull's-eye rot, the rotted tissues of side rot are soft and slimy in active lesions, and dry and stringy where the skin has been broken. The diseased tissues of side rot separate readily from the healthy tissues, leaving a shallow saucer-shaped cavity.
Causal factors
Since the last revision of this publication, the sexual stage of the causal fungus has been reclassified as Pezicula malicorticis (Jacks.) Nannf., and the asexual stage as Cryptosporiopsis curvispora (Pk.) Gremmen.
The fungus attacks the trees, producing cankers on the branches. Spores of the asexual stage of the fungus develop on the cankers, and are carried by dripping or blowing water during rain storms.
Control measures
Bull's-eye rot is best controlled by an effective orchard spray program. In years when excessive rainfall interferes with spray applications, the hazard of bull's-eye rot after harvest increases. Precautionary measures such as prompt storage, rapid cooling, and maintaining the storage temperature at 30° to 31 °F are recommended. This retards ripening and markedly delays the onset of the disease. Recommendations of the State agricultural extension service or agricultural experiment station should be followed for the most effective spray program.
