First Report of Bitter Rot Caused by Colletotrichum acutatum on Apple in China

Bitter rot of apple caused by Colletotrichum gloeosporioides was first reported in China in 1985 (3). In China, apples are grown on approximately 2 million ha, and bitter rot occurs in almost all production areas, with crop damage ranging from 30 to 70%. During the summer of 2007, fungi were isolated from apple fruit exhibiting bitter rot symptoms in 12 and 9 orchards in Shaanxi and Henan provinces, respectively, in China. Symptoms included 2- to 3-cm-diameter, sunken, brown lesions on the fruit surface that contained black, pinhead-size fruiting structures producing orange conidial masses under high humidity, similar to that of C. gloeosporioides. On potato dextrose agar (PDA), colonies were white, pale gray, or pale orange when grown at 25°C. Conidia were 8 to 16 x 2.5 to 4 μm, fusiform, pointed at one or both ends, one celled, thin walled, aseptate, and hyaline. Appressoria were 6.5 to 11 x 4.5 to 7.5 μm, clavate to circular, and light to dark brown. These characteristics matched published descriptions of C. acutatum (2). To confirm pathogenicity, three mature, healthy apples (cv. Fuji) were surface disinfested with 70% ethanol and then wounded with a sterile needle. After being inoculated with a spore suspension (1 x 10⁵ conidia/ml) prepared from a 2-week-old culture on PDA, these apples were sealed in a plastic bag and incubated at 25°C. Symptoms appeared 3 to 5 days after inoculation and began to enlarge 7 days later, forming lesions with fruiting structures. Under high humidity, cream-to-salmon pink spore masses were produced on lesions. As the lesions enlarged, the rot progressed to the core of the fruit in a V-shaped pattern. When the pathogen was reisolated from lesions of inoculated fruit onto PDA and incubated at 25°C, colony and conidial morphology were identical to those of the original isolates. Tests were performed three times with similar results. PCR with species-specific primer pair CaInt2/ITS4 (1) of genomic DNA from the isolates resulted in an amplification product of approximately 490 bp, which is specific for C. acutatum. The sequences exhibited 99% similarity with those of C. acutatum isolates AB273195 from GenBank. Approximately 20 of 103 symptomatic fruit from the field survey yielded fungal cultures whose morphology was consistent with that of C. acutatum, whereas the other cultures were C. gloeosporioides and Botryosphaeria dothidea. To our knowledge, this is the first report of bitter rot of apple caused by Colletotrichum acutatum in China.