First report of powdery mildew caused by Erysiphe trifoliorum on Indigofera amblyantha in Korea

Indigofera amblyantha Craib (Fabaceae), known as tall indigo or Chinese indigo, is a deciduous shrub native to central and eastern China. In Korea, it was introduced from China in the 1980s for planting on slopes for nature restoration in man-influenced areas (Kang et al. 2014). Due to its aesthetics and drought tolerance, some horticultural varieties are commercialized as garden shrubs in western countries. In July 2014, an indigo shrub with symptoms of powdery mildew was found in a public garden in Wando, Korea. White superficial mycelia and conidia were present on both sides of the leaves, as well as on stems, forming circular to irregular colonies. Symptoms included leaf distortions and plant stunting. Similar symptoms were also found in Hongcheon and Seoul in October and November 2015, respectively. Three voucher specimens were deposited in the Korea University Herbarium (KUS-F27946, F29018, F29107). The hyphae were septate, branched, and 3 to 8 µm in width. Appressoria on the mycelium were well developed, lobed, solitary or in opposite pairs. Conidiophores were cylindrical, 55 to 130 µm, and composed of 3 to 4 cells. Foot-cells of conidiophores were straight to substraight, cylindrical, and 25 to 45 µm long. Conidia produced singly, were ellipsoid to cylindrical, 28 to 48 × 15 to 20 µm (length/width ratio of 1.7 to 2.6), and devoid of distinct fibrosin bodies. The surface of the conidia revealed an angular/rectangular wrinkling pattern. Germ tubes were formed on the perihilar position of the conidium. No chasmothecia were found. These structures are typical of the powdery mildew Pseudoidium anamorph of the genus Erysiphe. The features were compatible with those of E. trifoliorum (Wallr.) U. Braun (Braun and Cook 2012). The internal transcribed spacer rDNA region of isolate KUS-F29018 was amplified using primers ITS5/P3, and sequenced (Takamatsu et al. 2009). The resulting 939 bp sequence was deposited in GenBank (Accession No. KU886243). The amplified ITS sequence shared >99% similarity with the sequences of E. trifoliorum on many fabaceous plants including Indigofera pseudo-tinctoria from Japan (LC009968). Pathogenicity was confirmed by gently pressing an infected leaf onto 10 leaves of a healthy, potted I. amblyantha. Ten noninoculated leaves of similar age served as controls. All inoculated leaves developed symptoms after 7 to 10 days, whereas the control leaves remained symptomless. Previously, three species of Erysiphe, viz. E. pisi, E. indigoferae, and E. trifoliorum, have been recorded on several species of Indigofera in Asia, Africa, and Australia, but not on I. amblyantha (Takamatsu et al. 2015; Farr and Rossman 2016). To our knowledge, this is the first report of powdery mildew caused by E. trifoliorum on I. amblyantha. Our field observations suggest that the disease would be a potential threat to the health of this shrub, especially under the condition where the shrubs are densely planted or grown in the shade.