Phialophora malorum (Kidd & Beaum.) McColloch
Occurrence and importance
Side rot has been reported on apples grown in the northwestern part of the United States and on apples grown in Indiana, Virginia, West Virginia, and Maryland. It has been observed on a wide range of varieties, but more frequently on Winesap and Delicious than on other varieties.
During the years that Bordeaux mixture was used as a standard fungicidal spray, side rot frequently caused serious losses in apples both during storage and on the market. Since the change to organic fungicidal sprays, the problem of side rot has practically disappeared.
Typical side rot lesions are oval, but often they have slightly irregular margins. They are usually less than 1 inch in diameter, are slightly sunken, and have a dull appearance (see photo). The color of the lesions varies from brown with a pale center to dark brown. The surface is frequently cracked, but when it is not it breaks with a pop under slight pressure. The texture of the decayed tissues when the rot is active is wet and slimy. It separates cleanly from the healthy tissues, leaving a saucer-shaped cavity usually about 1/16 to 1/4 inch deep. In certain cases where the rot is less active, especially where the surface has been broken, the texture is dry and spongy. The rot is tender, not firm, and gives readily under pressure.
The tendency of side rot lesions to have pale centers is the reason that they are sometimes confused with bull's-eye rot. Although there is considerable variation, the lesions of bull's-eye rot are usually larger and paler. If the bull's-eye rot spots are large, the causal fungus may be fruiting on them. They are fairly firm and do not separate readily from the healthy tissues; in cross section bull's-eye rot is found to penetrate toward the core in a U-shaped pattern.
The fungus, Phialophora malorum, which causes side rot of apple, is primarily a saprophyte living in surface soil and upon the bark and in cankerous woody tissues of apple trees. Apples become contaminated while on the trees, and if conditions are favorable the fungus may develop. In the past, side rot has caused serious and unpredictable losses in fruits in storage and on the market. Most lesions on apples appear to develop around lenticels, but it is difficult to determine in many cases whether the point of entrance was a lenticel or a cuticular crack. The fungus enters also through insect injuries and mechanical punctures.
The orchard sprays used to control bull's-eye rot in the Northwest also control side rot. Recommendations of the State extension service or experiment station should be followed.