An Outbreak of Leaf Spot Caused by Corynespora cassiicola on Korean Raspberry in Korea.

Abstract

In September and October 2010, leaf spots were observed on Korean raspberry (Rubus crataegifolius Bunge) plants in farmers' fields in Hapcheon, Gyeongnam Province, South Korea. Disease incidence ranged from 50 to 80% among fields. Circular- to irregular-shaped spots surrounded by yellow halos occurred frequently on the leaves of Korean raspberry plants. Brown spots became dark with wavy borders and ranged from 20 to 300 mm in diameter. Infected leaves became chlorotic, blighted, and eventually died. Fungal hyphae covered the lesions with abundant conidia and conidiophores. Fresh leaf specimens were collected from infected plants and the putative causal pathogen was isolated onto potato dextrose agar (PDA). A total of 30 isolates of the fungus were collected from diseased plants collected in the field. Fungal colonies were gray to brown on PDA. Colonies formed conidia, 38 to 210 × 8 to 20 μm, which were solitary or catenary, obclavate to cylindrical, smooth, straight or curved, and subhyaline to pale brown or brown. Conidiophores, 98 to 840 × 4 to 12 μm, were slightly or conspicuously swollen at apex, single, simple, straight or slightly flexuous, pale to midbrown, smooth, septate, thick, monotretic, and determinate or in tufts. Morphological characteristics of the fungal specimens were similar to descriptions of Corynespora cassiicola (1). A representative isolate of the pathogen was used to inoculate leaves of Korean raspberry plants for pathogenicity testing. Five leaves of a 3-month-old potted plant were sprayed with a suspension of conidia in water. Conidia were harvested from PDA cultures and adjusted to 2 × 104 conidia/ml with a hemocytometer. Five leaves sprayed with sterile distilled water served as controls. Inoculated plants were placed in a humid chamber with 100% relative humidity at 30°C for 24 h and then moved to a greenhouse. Symptoms similar to those observed in the farmers' fields developed on the inoculated leaves within 12 days, whereas the controls remained asymptomatic. The causal fungus was reisolated from the lesions of inoculated plants to satisfy Koch's postulates. To confirm the identity of the fungus, the complete internal transcribed spacer (ITS) rDNA region was amplified and sequenced (3). Amplification of the ITS region generated a 559-bp sequence (GenBank Accession No. JQ340026) with 100% similarity to sequences of C. cassiicola in GenBank (Accession No. GU138988) causing leaf spot on cassava (2). Based on the symptoms, morphological characteristics, pathogenicity, and molecular identification, this fungus was identified as C. cassiicola (1). To our knowledge, this is the first report of leaf spot caused by C. cassiicola on Korean raspberry. The recent occurrence of leaf spot on Korean raspberry suggests that C. cassiicola is spreading widely and posing a serious threat to these plants in Korea.