Homalocladium platycladum (F. Muell. ex Hook.) L.H. Bailey [syn. Muehlenbeckia platyclada (F. Muell.) Meisn.], commonly known as centipede plant or ribbonbush, is a plant in the Polygonaceae from New Guinea and the Solomon Islands. In Korea, this plant is grown in greenhouses for ornamental purposes. In October 2015, several plants were found damaged by a powdery mildew in a greenhouse in Gongju (36°26′ N; 127°14′ E), Korea. In January 2016, powdery mildew symptoms were found on H. platycladum plants grown in a greenhouse in Seocheon (35°01′ N; 126°43′ E), Korea. Symptoms first appeared as thin white patches on the upper surface of leaves, which progressed to abundant hyphal growth on the both side of the leaves. Infected leaves, especially infected when young, often turned yellow or chlorotic with twisted margins and resulted in early defoliation. Voucher specimens were deposited in the Korea University Herbarium (KUS-F19990, F29116, F29129). Hyphal appressoria were well-developed, lobed, and solitary or in opposite pairs. Conidiophores were cylindrical, 70 to 160 × 7.5 to 9.0 µm, and composed of 3 to 4 cells. Foot-cells of conidiophores were straight, cylindric, and 25 to 40 µm long. Singly produced conidia were cylindric-oval to oblong, 30 to 47 × 15 to 25 µm with a length/width ratio of 1.3 to 2.3, with angular/rectangular wrinkling of outer walls, and devoid of distinct fibrosin bodies. Germ tubes were produced in the perihilar position of conidia. No chasmothecia were found. These structures are typical of the powdery mildew Pseudoidium anamorph of the genus Erysiphe. The measurements and host range match with those of E. polygoni DC. (Braun and Cook 2012). The complete internal transcribed spacer (ITS) regions of KUS-F29116 were amplified with primers ITS5/P3 and sequenced (Takamatsu et al. 2009). The resulting 558 bp sequence was deposited in GenBank (accession no. KX098506). The ITS sequence showed >99% sequence identity with the sequences of E. polygoni on Polygonum spp. (LC010013, AB104522, AF011307, etc.). Pathogenicity was confirmed through inoculation by dusting conidia onto leaves of five healthy potted H. platycladum. Five noninoculated plants served as controls. Inoculated plants were isolated from noninoculated plants in separate rooms in a greenhouse maintained at 26 to 30°C. Inoculated plants developed symptoms after 6 days, whereas the controls remained symptomless. The fungus present on the inoculated plants was identical morphologically to that originally observed on diseased plants, fulfilling Koch’s postulates. Association of H. platycladum with E. polygoni has been known in the United States, Europe (United Kingdom, Czech Republic, Romania, Finland, and former Soviet Union), and Asia (China, Taiwan, Nepal, and India) (Lebeda et al. 2007; Farr and Rossman 2016). To our knowledge, this is the first report of powdery mildew caused by E. polygoni on H. platycladum in Korea. Our observations in two greenhouses showed that this powdery mildew expanded rapidly on young leaves and detracted from the aesthetic value of plants. © 2016, American Phytopathological Society. All rights reserved.
