First Report of Powdery Mildew Caused by Podosphaera astericola on Aster spathulifolius in Korea

Aster spathulifolius Maxim. is a perennial herb in the family Asteraceae, which is narrowly distributed and endemic in coastal regions of Korea and Japan (Nguyen et al. 2013). This plant has long been grown in gardens for ornamental purposes in Korea and has recently been valued as a medicinal herb (Won et al. 2013). In August 2016, dozens of A. spathulifolius plants showing symptoms of powdery mildew were observed in the natural habitat of Ulleung island (37°29′30″N; 130°54′39″E), Korea, with a disease incidence of ∼10%. The powdery mildew colonies were circular to irregular, forming white patches on both sides of the leaves. The voucher specimens were deposited in the Korea University herbarium (KUS-F23869 and F29401). Hyphae were flexuous to straight, branched, septate, and 4 to 7 μm wide. Hyphal appressoria were nipple shaped or nearly absent. Conidiophores were straight, 135 to 250 × 10 to 12 μm, and formed apically 2 to 4 immature conidia in chains showing crenate outline. Foot cells of conidiophores were cylindrical and 35 to 65 μm long. Conidia were ellipsoid-ovoid to barrel-shaped, 27 to 40 × 18 to 23 μm (length/width ratio of 1.5 to 2.2), and containing distinct fibrosin bodies. Teleomorph stage of the fungus was not observed. All morphological features described above are typical of the powdery mildew Fibroidium anamorph of the genus Podosphaera. Aiming to confirm the fungus identity, internal transcribed spacer (ITS) region of KUS-F29401 was amplified with the primers ITS5 and P3 as described by Takamatsu et al. (2009) and sequenced directly. A BLAST search of the Korean isolate had a high similarity (>99%) with those of Podosphaera astericola on asteraceous hosts (AB040353, AB040335, AB040341, and KU147187), and the next closest taxon was P. carpesiicola (AB040350). The obtained 775-bp sequence was deposited in GenBank (accession no. MF476989). Morphological characteristics and molecular sequence analysis were consistent with those of P. astericola U. Braun & S. Takam. (Braun and Cook 2012). Pathogenicity was confirmed by gently dusting a diseased leaf onto young leaves of three healthy plants. Three noninoculated plants were used as controls. Inoculated leaves developed symptoms after 7 to 10 days, whereas the control plants remained symptomless. The fungus present on the inoculated plants was morphologically identical to that originally observed on diseased plants, demonstrating pathogenicity on the host. About 15 species of Aster have been known to be associated with powdery mildew fungus of Podosphaera (syn. Sphaerotheca) (Braun and Cook 2012; Farr and Rossman 2017). To our knowledge, this is the first report of powdery mildew caused by P. astericola on A. spathulifolius in Korea or elsewhere. Production of A. spathulifolius seedlings was only recently started on a commercial scale in Korea. The occurrence of powdery mildew on A. spathulifolius in natural habitats, although currently not found in commercial farms, poses a potential threat to the safe production and marketing of healthy plants.