In 2010 and 2011, crab apples in Andong Province, Korea were found with dark brown spots on the fruit and mummified fruit on a tree. The fruit surface had red, circular spots that contained smaller, white spots; the color of the inner spots later changed to brown or black. Eventually, the rotten fruit dried and became mummified. Microscopic examination revealed the presence of acervuli and dark brown-to-black, needle-shaped setae. To isolate potential pathogens from infected fruit, small sections (5 to 10 mm2) were excised from the margins of lesions. These sections were surface sterilized with 70% ethanol and 1% NaOCl for 1 min and then rinsed three times with sterile distilled water. The fungus that was isolated produced whitish mycelia when grown on potato dextrose agar (PDA); the mycelia later became gray to dark gray with aerial mycelia in tufts and numerous conidia were produced. The conidia were straight, cylindrical with an obtuse apex and a truncated base, and measured 11.4 to 17.5 × 4.2 to 7.1 μm. The measurements and taxonomic characteristics coincide with those of Colletotrichum gloeosporioides (Penz.) (1). The isolated fungus was tested for pathogenicity on crab apples and cv. Fuji apples by inoculation with a conidial suspension (105 conidia/ml) prepared from 20-day-old PDA cultures. A 20-μl drop of the conidial suspension was placed onto crab apple and apple fruits that had been wounded by piercing them 1 to 2 mm deep with a pin. Small, dark lesions were observed on the artificially inoculated fruit 3 days after inoculation. Nine days after inoculation, dark lesions with salmon-colored masses of conidia were observed on fruit, which were also soft and sunken. Abundant masses of conidia were produced in the decayed tissues. The fungus was reisolated from the parts of the fruits showing the symptoms. The internal transcribed spacer (ITS) rDNA of the isolated fungus was amplified and sequenced by PCR as described by White et al. (2). The resulting 582-bp of ITS rDNA sequence was deposited in GenBank (Accession No. JQ405742). A BLAST analysis for sequence similarity of the ITS region revealed 100% identity with nucleotide sequences for C. gloeosporioides isolates (Accession Nos. HQ645080 and AB458667). The results obtained on morphological characteristics, pathogenicity, and molecular data corresponded with those of C. gloeosporioides described by Sutton (1). To our knowledge, this is the first report of the presence of C. gloeosporioides on crab apple in Korea (3). Crab apple is used as a pollinator for single-cultivar apple orchards and may become a possible source of inoculum for cultivated apple.
References: (1) T. B. Sutton. Compendium of Apple and Pear Diseases. The American Phytopathological Society, St. Paul, MN, 1990. (2) T. J. White et al. PCR Protocols: A Guide to Methods and Applications, Academic Press, Inc., New York, 1990. (3) S. H. Yu. List of Plant Diseases in Korea. 5th ed. (in Korean). The Korean Society of Plant Pathology, 2009.