First report of powdery mildew caused by Erysiphe aucubae on Aucuba Japonica in Korea

Aucuba japonica Thunb. (Garryaceae) is native to rich forest soils in China, Korea, and Japan. Recently, it has been widely planted in gardens and parks for ornamental purposes. In July 2016, A. japonica ‘variegata’ was found damaged by a powdery mildew with approximately 10 to 30% disease incidence in a recreational park in Busan, southern Korea (35°07′24″N; 129°01′07″E). Powdery mildew symptoms first appeared as circular to irregular, white to gray patches on abaxial surfaces of both young and old leaves. In case of severe infection, fungal patches caused brown discoloration of abaxial leaf surfaces, markedly detracting from the aesthetic value of glossy evergreen leaves and resulting in premature leaf fall. Chasmothecia were not found in the collected samples. Two voucher specimens were deposited in the Korea University Herbarium (KUS-F29313 and F29638). The hyphal appressoria were well-developed and lobed. Conidiophores (n = 20) were straight, 70 to 110 × 8 to 10 µm, and composed of 3 to 4 cells. Foot-cells of conidiophores were straight, cylindrical, and 25 to 45 µm long. Singly produced conidia (n = 20) were cylindrical to oblong-elliptical, 37 to 48 × 15 to 22 µm (length/width ratio = 2.1 to 3.1), showed angular/rectangular wrinkling of outer walls, and devoid of distinct fibrosin bodies. Germ tubes were produced in the perihilar position of the conidia. The fungus was initially identified as Erysiphe aucubae S. Takam. & Siahaan, based on the host and the anamorphic traits (Siahaan and Takamatsu 2016). To confirm the identification, the internal transcribed spacer (ITS) regions of KUS-F29313 were amplified with primers ITS5/P3, and sequenced directly (Takamatsu et al. 2009). The resulting 558 bp sequence was deposited in GenBank (accession no. KY406744). A GenBank BLAST search of the Korean isolate showed 100% identity with E. aucubae on A. japonica from Japan (LC121921). Pathogenicity was confirmed by gently pressing a diseased leaf onto young leaves of three asymptomatic, potted plants of 2-year-old cuttings. Three noninoculated plants of the same age were used as controls. Plants were maintained in a greenhouse at 24 to 30°C. Inoculated leaves developed symptoms after 7 days, whereas the control plants remained symptomless. The fungus present on the inoculated leaves was morphologically identical to that observed on the original infected leaves. Powdery mildews of A. japonica have been recorded as Oidium sp., Microsphaera sp., and E. aucubae from Japan (Amano 1986; Braun and Cook 2012; Farr and Rossman 2017; Siahaan and Takamatsu 2016). To our knowledge, this is the first report of powdery mildew caused by E. aucubae on A. japonica in Korea. Our field observations suggested that powdery mildew infections pose a serious threat to the health of this plant, especially in shady areas of the gardens in southern Korea.