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First Report of Powdery Mildew Caused by Erysiphe michikoae on Celtis sinensis in Korea

Celtis sinensis Pers., commonly known as the Chinese hackberry, belongs to the family Cannabaceae. In Korea, the leaves and bark of this tree are used for the treatment of gastrointestinal disorders and lung abscess. In October 2015 to 2018, symptoms of powdery mildew were observed on the leaves of C. sinensis in a garden at Chonnam National University, Gwangju (35°10′0″N, 126°55′0″E), Korea. The symptoms included white powdery colonies on the surfaces of young leaves and brown spots on older leaves. The three specimens were deposited in the Environmental Microbiology Laboratory Herbarium, Chonnam National University, under accession numbers CNUFC PWS1, CNUFC PWS2, and CNUFC PWS3, respectively. The chasmothecia were most abundant on the lower surfaces of the leaves. The hyphal appressoria were lobed, single, or opposite in pairs. The conidiophores were cylindrical, 65 to 87 × 9 to 12 μm, followed by two or three straight cells. The conidia were ellipsoidal or cylindrical, 11.5 to 14.5 × 29 to 33.5 μm. The chasmothecia were 64 to 94.5 × 76.5 to 98.5 μm, globose, and black to dark brown. Each chasmothecium had eight to 16 appendages and was 98.5 to 167.5 μm long and 5.5 to 7.5 μm wide, containing four asci; each ascus contained four to five ascospores. The ascospores were ellipsoidal, 10.5 to 14 × 15.5 to 24.5 μm. On the basis of these characteristics, the fungus is morphologically similar to Erysiphe michikoae, as described by Meeboon et al. (2013). To confirm the morphological identification, genomic DNA was extracted directly from the mycelia and chasmothecia, using a DNA prep kit (Solgent, Daejeon, Korea). The internal transcribed spacer (ITS) regions of the rDNA of three specimens were amplified using the primers ITS5 and PM6, as described by Lee and Nguyen (2016). The ITS sequences from CNUFC PWS1, CNUFC PWS2, and CNUFC PWS3 were deposited at GenBank under accession numbers MK757876, MK757877, and MK757878, respectively. BLASTn with the ITS sequences showed 99% similarity with the sequences of E. michikoae (AB693963) and E. kusanoi (AB475120). Phylogenetic analyses based on the ITS rDNA indicated that the three specimens were identical to E. michikoae. Therefore, the causal pathogen was identified as E. michikoae based on the morphological and molecular analyses. Pathogenicity tests were performed by gently pressing the infected leaves onto 10 healthy leaves of C. sinensis. Powdery mildew symptoms were observed on the inoculated leaves after 10 to 15 days, whereas no symptoms were found on the control. The fungus on the inoculated leaves was morphologically identical to that on the naturally infected plants. Powdery mildew on C. sinensis has been previously reported as E. kusanoi (synonyms: E. kusanoi var. kusanoi, Uncinula kusanoi, and U. kusanoi var. kusanoi) and Pleochaeta shiraiana (synonyms: Uncinula shiraiana and Uncinulopsis shiraiana) in Japan, China, Taiwan, Korea, and South Africa (Farr and Rossman 2019). E. michikoae has been found in only C. jessoensis from Japan (Meeboon et al. 2013). To our knowledge, this is the first report of powdery mildew caused by E. michikoae on C. sinensis in Korea as well as worldwide. This information indicates that E. michikoae is able to infect other Celtis species and may pose as a serious threat to the health of C. sinensis in Korea. Further studies on the host range of E. michikoae are required.