Morphology and Phylogeny of Cytospora erumpens, Unreported Species Isolated from Apple Trunk in Korea

A fungal strain was isolated from a darkened apple tree trunk and designated as KNUF21-C5. The cultural and morphological characteristics of the isolated strain were evaluated using potato dextrose and malt extract agar (MEA). After 7 days, the colonies on MEA were circular, dense, rough, lacking aerial mycelia, olivaceous-yellow at the center with white margins, and reached 90 mm in diameter. Morphologically, the conidiophores were straight and hyaline, and the conidiogenous cells were enteroblastic, phialidic, hyaline, and cylindrical. The conidia were unicellular, elongated-allantoid, and measured 5.1–6.6 × 1.3–1.6 µm in diameter. These characteristics were consistent with those of the Cytospora species. To identify the species, molecular analysis was performed using sequences of the internal transcribed spacer regions, large subunit of 28S rRNA, actin, translation elongation factor 1-alpha, and RNA polymerase II subunit gene, it showed 98.0–100% similarity to C. erumpens CFCC 53163. The KNUF-21-C5 strain was clustered with C. erumpens CFCC 53163 in phylogenetic trees, and the conidial size was similar to the type strain of C. erumpens (5.1–6.6 × 1.3–1.6 µm vs. 5.6–6.7 × 1.3–1.7 µm). Based on fungal characteristics and phylogenetic analysis, KNUF-21-C5 was identified as C. erumpens. The pathogenicity of KNUF-21-C5 in apples was confirmed by inoculation of apple twigs. This is the first record of C. erumpens associated with Cytospora canker on apples in Korea.