Abstract
Although bitter rot is distributed widely in the United States, the disease is virtually unknown in Michigan. Fruit samples of cvs. Empire, McIntosh, Sunrise, Paulared, and Jonagold with bitter rot symptoms were collected from eight apple orchards in 1995 and the Colletotrichum spp. determined. The orchards were located in western Michigan, extending from the Indiana border north to Grand Rapids and east to East Lansing. C. acutatum and C. gloeosporioides were recovered from 81.2 and 18.8% of 165 fruit samples, respectively. All isolates except two of C. gloeosporioides grew on potato dextrose agar amended with 5 μg of benomyl per ml. Sequence similarity for expected polymerase chain reaction-amplified internal transcribed spacer region 1 (ITS 1) products was high (>95%) among four strains of C. acutatum and four strains of C. gloeosporioides, but low (62 to 67%) between strains of the two species. All isolates were virulent on apples of cv. Golden Delicious. Colletotrichum appears to be endemic in Michigan apple orchards but outbreaks of bitter rot are rare, suggesting that environmental conditions in Michigan are normally unfavorable for disease development.
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