First report of powdery mildew caused by erysiphe begoniicola on begonia in Korea

In early May 2015, several plants of begonia (Begonia × semperflorens-cultorum ‘Cherry Red’) exhibiting typical powdery mildew symptoms were sent from a local florist to HDS of Korea University for diagnosis. The plants were known to be pot-grown for 2 years in the greenhouse. Sign of powdery mildew infection started to appear from early April. When infected, thin white colonies appeared on both sides of leaves, stems, and inflorescence. Severe infections often caused discoloration of flower petals and premature defoliation, markedly decreasing the aesthetic value of glossy leaves and pink-colored petals. No chasmothecia were found. A voucher specimen was deposited in the Korea University Herbarium (KUS-F28603). The hyphal appressoria were well-developed and lobed. Conidiophores were straight, 65 to 105 × 7.5 to 9.5 µm, and composed of 3 to 4 cells. Foot-cells of conidiophores were straight at the base, and 25 to 38 µm long. Singly produced conidia were oblong-elliptical, 30 to 54 × 15 to 20 µm with a length/width ratio of 2.2 to 3.5, devoid of distinct fibrosin bodies, and showed angular/rectangular wrinkling of outer walls. Germ tubes were produced at the perihilar position of the conidia. The morphological characteristics and host genus were consistent with those of Erysiphe begoniicola U. Braun & S. Takam. (Braun and Cook 2012). Though E. begoniae R.Y. Zheng & G.Q. Chen was known to be associated with begonia powdery mildew in China, its conidia were smaller (23 to 38 × 13 to 16 µm) than those of E. begoniicola (Baiswar et al. 2008; Braun and Cook 2012). To confirm the species identity, internal transcribed spacer (ITS) regions of KUS-F28603 were amplified with primers ITS5/P3, and sequenced directly (Takamatsu et al. 2009). The resulting 573 bp sequence was deposited in the GenBank (accession no. KY747496). A GenBank BLASTn search conducted with the ITS sequence represented that the Korean isolate most closely matched those of E. pisi (97%). Since this was the first sequence submitted for E. begoniicola, comparable data were not available. Pathogenicity was confirmed through inoculation tests by gently pressing a diseased leaf onto young leaves of three asymptomatic, potted plants. Three noninoculated plants were used as controls. Plants were maintained in a greenhouse at 24 to 30°C. Inoculated leaves developed signs of powdery mildew after 5 days, whereas the control plants remained symptomless. The fungus present on the inoculated leaves was morphologically identical to that observed on the original infected leaves. Three species of powdery mildews, i.e., Golovinomyces orontii, E. begoniae, and E. begoniicola have been recorded on many species of Begonia (Braun and Cook 2012; Farr and Rossman 2017). G. orontii has Euoidium-type anamorph, and is thus clearly differentiated from Erysiphe spp. having Pseudoidium-type anamorph. To our knowledge, this is the first report of E. begoniicola on B. × semperflorens-cultorum in Korea. Although noticed in one place, the natural occurrence of powdery mildew would be a new threat to Begonia production in Korea.