First Report of Powdery Mildew Caused by Podosphaera xanthii on Ricinus communis in Korea

Ricinus communis L. (Euphorbiaceae), known as castor bean, is widely planted in the tropical and warm temperate areas. The seed is a source of castor oil, which has plenty of applications in medicine and as a motor lubricant (Worbs et al. 2011). In September 2016, powdery mildew symptoms were observed on several castor bean plants grown as a leafy vegetable in a private garden in Gimcheon (36°07′09″N, 128°07′46″E), Korea. White patches consisting of hyphae, conidiophores, and conidia were distributed on both sides of leaves. Infections were weak and did not cause early defoliation or leaf distortion. A representative specimen was deposited in Korea University Herbarium (KUS-F29541). Hyphal appressoria were nipple shaped. Conidiophores were erect, 110 to 230 × 10 to 12 μm, and produced two to five immature conidia in chains. Foot cells of conidiophores were cylindrical, 40 to 70 μm, and somewhat constricted at the basal parts. Conidia were ellipsoid to ovoid, 28 to 35 × 16 to 19 μm with a length/width ratio of 1.6 to 2.2, and contained distinct fibrosin bodies. Germ tubes produced from the lateral position of conidia were simple to forked. Primary conidia were apically conical or rounded, mostly subtruncate at the base, and generally smaller than secondary conidia. No chasmothecia were observed during the growing season. These structures are typical of the powdery mildew Fibroidium anamorph of the genus Podosphaera (Braun and Cook 2012). The internal transcribed spacer (ITS) regions of the specimen KUS-F29541 were amplified and sequenced with primers PMITS1/PMITS2 (Cunnington et al. 2003). The ITS sequence was deposited in GenBank (accession no. MG462908). It showed over 99% similarity with Podosphaera xanthii sequences obtained from Cucurbita pepo (KJ698669), Calendula officinalis (AB525914), and Verbena brasiliensis (KJ472787). Based on the morphological characteristics and molecular data, the fungus on R. communis was identified as P. xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012). Inoculation tests were conducted by pressing infected leaves onto 12 healthy leaves of three castor bean plants. Three noninoculated plants were used as controls. These plants were kept in a greenhouse at 24 to 30°C without artificial illumination. Typical powdery mildew symptoms developed on the inoculated plants after 5 days; however, the controls remained symptomless. The morphological characteristics of the pathogen on inoculated plants were in agreement with those on the diseased plants observed originally. There have been many records of powdery mildews belonging to Golovinomyces, Leveillula, Oidium, and Ovulariopsis on R. communis (Farr and Rossman 2017). However, no previous record of Podosphaera on R. communis has been found. To our knowledge, this is the first report of P. xanthii on R. communis globally as well as in Korea. Although identified from one locality, the natural occurrence of powdery mildew would be a new threat to this economically important crop in Korea.