First report of powdery mildew caused by Phyllactinia juglandis on Juglans mandshurica in Korea

Juglans mandshurica Maxim., known as Manchurian walnut, is a deciduous tree native to China, Russian Far East, and Korea. It is an ecologically important tree of forest communities in Korea. In October 1999, several trees showing symptoms of powdery mildew were found in Anyang, Korea. Initial symptoms were small, circular, powdery, white colonies on the abaxial leaf surface. Later, light yellow spots formed on the adaxial leaf surface corresponding to powdery mildew colonies of abaxial leaf surface. Each colony enlarged and coalesced to cover the whole abaxial surface, followed by early senescence and premature defoliation. Powdery mildew lesions with abundant chasmothecia were found in October. Similar symptoms had also been found in Hongcheon and Suwon from 1999 to 2014. Seven voucher specimens were deposited in the Korea University Herbarium (KUS). Hyphal appressoria were hook-shaped, often branched, and single or opposite in pairs. Conidiophores were erect, cylindrical, 160 to 280 × 6 to 8 μm, straight in foot-cells, and produced conidia singly. Conidia were obpyriform, papillate at the apex, 45 to 75 × 20 to 30 μm, and devoid of fibrosin bodies. Germ tubes were produced on the terminal position of conidia. Chasmothecia were blackish brown, depressed globose, and 200 to 250 µm in diameter. Chasmothecial appendages numbered 9 to 14, were acicular with a bulbose base, 1 to 1.5 times as long as the chasmothecial diameter, hyaline throughout, and aseptate. Penicillate cells were ampulliform, numerous, 45 to 70 µm long, 10 to 20 µm wide, and with filaments 23 to 55 µm long. Asci were 12 to 20 to a chasmothecium, 82 to 98 × 35 to 40 µm, and stalked. Ascospores were two in an ascus, ellipsoid-ovoid, 32.5 to 40 (average 37.4) × 17 to 20 (average 18.8) µm, and pale olivaceous. The features were compatible with those of Phyllactinia juglandis J.F. Tao & J.Z. Quin (Braun and Cook 2012). The internal transcribed spacer regions of rDNA from KUS-F24775 were amplified using primers ITS1/ITS4, sequenced, and deposited in GenBank (Accession No. KT186246). GenBank BLAST search of the isolate revealed >99% similarity with those of P. juglandis (JF460007 and AB080531). Pathogenicity was confirmed by gently pressing an infected leaf onto 10 leaves of a healthy, potted tree. Ten noninoculated leaves of similar features served as controls. All inoculated leaves developed symptoms after 8 to 10 days, whereas the control leaves remained symptomless. Juglans mandshurica has been known to be associated with P. juglandis-mandshuricae in China (Yu et al. 1979). The Korean isolate, however, differs from P. juglandis-mandshuricae by having much smaller structures (e.g., 200 to 250 µm versus 225 to 330 µm in chasmothecia; 33 to 40 × 17 to 20 µm versus 35 to 50 × 20 to 27 µm), and therefore fit well with P. juglandis (Braun and Cook 2012). To our knowledge, this is the first report of powdery mildew caused by P. juglandis on J. mandshurica in Korea (Farr and Rossman 2015). Our field observations show that juvenile Manchurian walnuts growing under tree canopies suffer from nearly 100% infection of P. juglandis.