First report of powdery mildew caused by Erysiphe sedi on Orostachys Japonica in Korea

Rock pine (Orostachys japonica (Maxim.) A. Berger) is a perennial succulent belonging to the Crassulaceae. It is widely cultivated in Korea for medicinal uses (Park et al. 2015). Potted rock pines are also marketed as ornamentals. In August 2014, several pot-grown rock pines in an outdoor hobby garden were observed showing typical symptoms of powdery mildew in Suwon (37°15′46.3″ N; 126°59′27.4″ E), Korea. Similar symptoms were found on potted rock pines in gardens in Jinju (35°12′40.8″ N; 128°07′00.5″ E) and Hongcheon (37°40′54.0″ N; 127°52′18.9″ E), Korea. Three voucher specimens were deposited in the Korea University Herbarium (KUS-F28101, KUS-F28195, and KUS-F28777). Mycelial colonies were white, conspicuous, and epiphytic on leaves and stems. Hyphae were septate, branched, and 3 to 6 µm wide. Appressoria on the hyphae were well-developed, lobed, and mostly positioned in pairs. Conidiophores were cylindrical, 75 to 145 × 7 to 11 µm, and composed of 3 to 4 cells. Foot cells of conidiophores were straight, cylindrical, and 25 to 45 µm long. Conidia produced singly were variable in shape, oval to cylindrical oval, or oblong-elliptical, 30 to 50 × 16 to 22 µm, 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. The morphological characteristics were consistent with descriptions of Erysiphe sedi U. Braun (Braun and Cook 2012). To confirm the identity of the causal fungus, the complete ITS region of rDNA from KUS-F28777 was amplified with primers ITS5/P3 (Takamatsu et al. 2009) and sequenced. The resulting sequence was deposited in GenBank (Accession No. KT748731). A GenBank BLAST search using the present data revealed that the ITS sequence shares >99% similarity with those of E. sedi (Accession Nos. LC010046, LC010047, and KR091961). Pathogenicity was confirmed through inoculation by dusting conidia of isolate KUS-F28777 onto leaves of five healthy potted rock pine 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 6 days, whereas the control plants remained symptomless. The fungus on the inoculated plants was morphologically identical to that originally observed on diseased plants, fulfilling Koch’s postulates. There have been numerous records of E. sedi on many crassulaceous plants, but not on Orostachys spp. (Farr and Rossman 2015). To our knowledge, this is the first report of E. sedi infections of O. japonica in Korea and probably in the world. According to our observations, potted rock pines in partly shaded areas and indoors were commonly affected by powdery mildew, reducing the aesthetic value of the plants. On the other hand, six surveys conducted during 2014 and 2015 in commercial farms of rock pines grown for medicinal purposes failed to detect powdery mildew. At present, the economic importance of powdery mildew can be considered low in the case of field-grown commercial rock pines.