First Report of Powdery Mildew Caused by Erysiphe quercicola on Quercus robur in Korea

Quercus robur L. (English oak or pedunculate oak) is native to Europe, western Asia, and northern Africa. It is cultivated as an ornamental tree in the temperate regions of most countries. In Korea, it is grown in botanical gardens and parks. In May 2015, a powdery mildew was observed on several Q. robur in a public garden of Daegu, Korea (35°48′00″N; 128°31′19″E). Symptoms first appeared as thin white colonies, which subsequently developed into hyphal growth on both sides of the leaves and stems. Severe infections often caused necrotic discoloration of the leaves. A voucher specimen was deposited in the Korea University Herbarium (KUS-F28616). The hyphae were septate, branched, and 3 to 8 µm in width. Conidiophores were cylindrical, 60 to 100 × 7 to 9 µm, and composed of 3 to 4 cells. Foot-cells of conidiophores were cylindrical, straight or slightly flexuous at the base, and 25 to 50 µm long. Conidia produced singly were ellipsoid or doliiform, 25 to 40 × 17 to 23 µm with a length/width ratio of 1.5 to 2.0, and devoid of distinct fibrosin bodies. Germ tubes were formed on the perihilar position of the conidia. The surface of the wrinkled conidia had an angular/rectangular wrinkling pattern. No chasmothecia were observed. These structures were typical of the powdery mildew Pseudoidium anamorph of the genus Erysiphe. The structure and measurements were consistent with the anamorphic state of Erysiphe quercicola S. Takam. & U. Braun (Braun and Cook 2012; Takamatsu et al. 2007). The internal transcribed spacer regions of KUS-F28616 were amplified using primers ITS1/ITS4, and sequenced directly (Takamatsu et al. 2009). The obtained sequence of 576 bp was deposited in GenBank (accession no. MG564340). An NCBI BLAST search of the Korean isolate showed >99% similarity with that of E. quercicola on Q. crispula (AB292690 and AB292691 from Japan) and Quercus sp. (AB292692 from Thailand and AB292693 from Iran). A pathogenicity test was performed through inoculation by gently pressing a diseased leaf onto five healthy leaves of potted 2-year-old seedlings. Five noninoculated leaves served as controls. Inoculated leaves developed signs and symptoms after 5 days, but the noninoculated leaves remained symptomless. The fungus presented on the inoculated plants was morphologically identical to that originally observed on diseased plants. Powdery mildews of Q. robur have been recorded as Cystotheca lanestris, Erysiphe spp., and Phyllactinia spp. (Braun and Cook 2012; Farr and Rossman 2017). Powdery mildew caused by E. alphitoides is one of the major foliar diseases of Q. robur in Europe (Desprez-Loustau et al. 2011). Of those, powdery mildew infections of Q. robur associated with E. quercicola have been recorded in Australia (Farr and Rossman 2017; Takamatsu et al. 2007). To our knowledge, this is the first report of powdery mildew caused by E. quercicola on Q. robur in Korea. The powdery mildew infections pose a potential threat to health and beauty of this ornamental tree.