First report of powdery mildew caused by Leveillula taurica on Oxalis triangularis in Korea

In June 2016, a pot-grown false shamrock (Oxalis triangularis A.St.-Hil.) exhibiting typical powdery mildew symptoms was sent from a gardener in Mokpo (34°47′33″N; 126°22′57″E), Korea, to Korea University for diagnosis. Symptoms included chlorosis of the adaxial leaf surface, white fungal colonies on the abaxial leaf surface, and premature defoliation, markedly detracting from the aesthetic value of the plant. No chasmothecia were found. A voucher specimen was deposited in the Korea University Herbarium (KUS-F29220). Mycelium was internal and external. External mycelium was abundantly hypophyllous, very occasionally epiphyllous on young leaves, forming thin white patches or covering entire leaves. Hyphal appressoria were nipple-shaped to coral-like. Conidiophores arose from the internal mycelium through stomata, less frequently from external mycelium, straight, 150 to 300 × 5 to 7 µm, and composed of 3 to 4 cells. Singly produced conidia were dimorphic. Primary conidia (n = 30) were navicular to lanceolate, attenuated at the apex, rounded at the base, and 40 to 85 × 14 to 22 µm with a length/width ratio of 2.8 to 4.5. Secondary conidia (n = 30) were oblong-elliptical to long obovate, rounded at both ends, and 40 to 70 × 14 to 22 µm with a length/width ratio of 2.8 to 3.8. Conidia were devoid of fibrosin bodies and showed angular/rectangular wrinkling of outer walls. Germ tubes were produced at the perihilar position of the conidia. The morphological characteristics were consistent with those of Leveillula taurica (Lév.) G. Arnaud (Braun and Cook 2012). To confirm the identification, the internal transcribed spacer (ITS) regions of KUS-F29220 were amplified with primers ITS5/P3, and sequenced directly (Takamatsu et al. 2009). The resulting 791-bp sequence was deposited in GenBank (accession no. KY885104). A GenBank BLAST search of the Korean isolate showed >99% similarity with L. taurica in Catharanthus roseus from the United States (KF703447) and Cynanchum kaschgaricum from China (JN861731). Pathogenicity was confirmed by 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 7 days, whereas the control plants remained symptomless. The fungus on the inoculated leaves was morphologically identical to the fungus on the original infected leaves. Several species of Oxalis have been recorded as hosts of L. taurica from India, Italy, Morocco, Pakistan, Papua New Guinea, Singapore, and Turkey (Farr and Rossman 2017). In China, L. oxalidicola T.Z. Liu & U. Braun was reported on O. triangularis (Liu et al. 2009). Sequence data for O. triangularis is not available but morphological characteristics of our isolates are more consistently similar to L. taurica than L. oxalidicola; sequence data also supports L. taurica. Though limited in occurrence now, we consider this powdery mildew to be a threat to the ornamental value of O. triangularis.