First report of powdery mildew caused by Podosphaera tridactyla on sweet cherry in Korea

Sweet cherry, Prunus avium (L.) L., is currently a minor fruit crop in Korea, with 120 ha planted for domestic consumption. In September 2012, typical powdery mildew symptoms were observed on sweet cherry trees (cv. Seneca) planted in a plastic greenhouse in Iksan, Korea. Powdery mildew colonies were circular to irregular white patches on the leaves. Three voucher specimens were deposited in the Korea University Herbarium (KUS-F27263, F28367, F28534). Hyphae were flexuous to straight, 4 to 7 µm wide, and had nipple-shaped appressoria. Conidiophores were 110 to 250 × 9 to 11 µm and produced 2 to 8 immature conidia in chains with a crenate outline. Foot-cells of conidiophores were straight, cylindric, and measured 60 to 105 µm long. Conidia were hyaline, ellipsoid-ovoid, measured 30 to 48 × 12 to 20 µm with a length/width ratio of 1.8 to 3.1, and had distinct fibrosin bodies. Chasmothecia were amphigenous, abundantly hypophyllous, scattered, spherical, and 80 to 100 µm in diameter. Chasmothecial appendages were 2 to 4 in number, positioned in the upper half, stiff or slightly curved, 1 to 4 septate, and had branched apices with 3 to 5 bifurcations. Each chasmothecium contained a single ascus and had a terminal oculus 20 to 28 µm wide. Asci were sessile, 65 to 92 × 65 to 72 µm, and contained eight ascospores, each 20 to 30 × 14.0 to 17.5 µm. The structures and measurements were in agreement with the descriptions of Podosphaera tridactyla (Wallr.) de Bary (Braun and Cook 2012). The sequence analysis of the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) of isolate KUS-F28534 was performed to confirm identity. The complete ITS rDNA sequence was amplified using primers ITS1/ITS4. The resulting 477-bp sequence was deposited in GenBank (Accession No. KP710960). A BLAST search in GenBank using this sequence revealed 100% identity with the sequences of P. tridactyla on Prunus species from Japan (AB026147) and Korea (AY833659) (Cunnington et al. 2005). Pathogenicity was confirmed through inoculation tests by gently pressing diseased leaves onto young leaves of three asymptomatic trees. Three noninoculated trees were used as controls. Inoculated leaves developed symptoms after 8 days, whereas the control trees remained symptomless. The symptoms observed on the inoculated leaves were identical to those observed on the diseased leaves, fulfilling Koch’s postulates. Powdery mildew infections of sweet cherry associated with P. tridactyla have been recorded in Europe, North America, South America, Australia, and New Zealand (Amano 1986; Farr and Rossman 2015). To our knowledge, this is the first report of P. tridactyla on P. avium in Korea. Occurrence of powdery mildew on sweet cherry was found only on cv. Seneca in a greenhouse during 2012 to 2014. Seneca as well as other varieties planted outdoor were free from powdery mildew. Though the economic importance of this disease is currently limited, presence of powdery mildew could pose a potential threat to the sweet cherry industry in Korea.