First report of powdery mildew caused by Erysiphe cruciferarum on wasabi (Wasabia japonica) in Korea

Wasabi (Wasabia japonica (Miq.) Matsum., family Brassicaceae) is a commercially important crop in Japan, Korea, China, and Taiwan. In June 2013, dozens of wasabi plants showing typical powdery mildew symptoms were found with approximately 10% disease incidence at an experimental plot in a plastic greenhouse in Pyeongchang, Korea. Symptoms first appeared as thin white colonies, which progressed to abundant growth on the leaves and stems. A representative sample was deposited in the Korea University Herbarium (Accession No. KUS-F28696). Similar symptoms were also observed in 2014 and 2015 at experimental plots in plastic greenhouses in Taebaek, Korea. Appressoria on the mycelium were well-developed, lobed, and solitary or in opposite pairs. Conidiophores were cylindrical, 80 to 110 × 7.5 to 10.0 µm, and composed of 3 to 4 cells. Foot-cells of conidiophores were straight to substraight, cylindrical, 25 to 40 µm long and generally equal to or shorter than the upper cells. Singly produced conidia were oblong to cylindrical or oval, 35 to 47 × 15 to 20 µm (length/width ratio = 2.1 to 2.7), lacked distinct fibrosin bodies, and showed angular/rectangular wrinkling of outer walls. Germ tubes were produced on the perihilar position of conidia. No chasmothecia were found. These structures are typical of the powdery mildew Pseudoidium anamorph of the genus Erysiphe. The morphological characteristics match with those of E. cruciferarum Opiz ex L. Junell (Braun and Cook 2012; Choi et al. 2013). To confirm the identification, the complete internal transcribed spacer (ITS) region of rDNA was amplified with primers ITS5 and P3 (Takamatsu et al. 2009) and directly sequenced. The resulting 702-bp sequence was deposited in GenBank (Accession No. KT372142). The amplified ITS sequence shared >99% similarity with the sequences of E. cruciferarum on several brassicaceous hosts (e.g., EU140958 and KC878683). Pathogenicity was confirmed through inoculation by dusting conidia onto leaves of five 4-month old, healthy wasabi plants. Five noninoculated plants served as controls. Inoculated plants were isolated from noninoculated plants in separate rooms in a greenhouse. Inoculated plants developed signs and symptoms after 8 days, whereas the control plants remained symptomless. The fungus present on the inoculated plants was morphologically identical to that originally observed on diseased plants, fulfilling Koch’s postulates. Previously, the disease was reported in Japan (Oku et al. 1993) and Taiwan (Lo and Wang 2000). To our knowledge, this is the first report of powdery mildew caused by E. cruciferarum on wasabi in Korea. Wasabi is cultivated mainly for fresh roots and partly for fresh leaves. In Korea, production of fresh wasabi leaves was only recently started on a commercial scale. Occurrence of wasabi powdery mildew in experimental plots, though currently not found in commercial farms, poses a potential threat to the production of fresh leaves. The plant is mostly grown using organic farming methods with limited chemical control options; therefore, alternative control measures should be considered.