First report of powdery mildew caused by Podosphaera xanthii on Torenia fournieri in Korea

Torenia fournieri E. Fourn., known as torenia or wishbone flower, is an annual plant in the Linderniaceae. In October 2014, typical powdery mildew symptoms were observed on torenia plants of an unknown cultivar in a private garden in Seongnam, Korea. Symptoms appeared as white colonies, which subsequently covered both sides of leaves and stems. Chasmothecia were abundantly formed by November. A voucher specimen (KUS-F28456) was deposited in the Korea University Herbarium. Appressoria on the mycelium were nipple-shaped or nearly absent. Conidiophores were straight, 100 to 200 × 10 to 11.5 μm and produced 2 to 5 immature conidia in chains with a crenate outline. Foot-cells of conidiophores were straight, cylindric, and 40 to 85 μm long. Conidia were hyaline, ellipsoid-ovoid to barrel-shaped, 24 to 36 × 16 to 22 μm with a length/width ratio of 1.4 to 2.1, and contained distinct fibrosin bodies. Dark brown chasmothecia were found partly embedded in the mycelial felt on leaves and stems, gregarious to scattered, dark brown, spherical, and 70 to 100 µm in diameter. Peridium cells of chasmothecia were irregularly polygonal, large, 15 to 35 µm wide. Appendages were mycelioid, 1- to 5-septate, and brown at the base and becoming paler upwards. Each chasmothecium contained a single ascus. Asci with a terminal oculus of 15 to 25 µm wide were sessile, broadly ellipsoid-ovoid to subglobose, 8-spored, and 62 to 95 × 50 to 65 μm. Ascospores were colorless, ellipsoidal, and 14 to 20 × 13 to 16 µm. The structures and measurements were consistent with those of Podosphaera xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012). The internal transcribed spacer (ITS) regions of rDNA of isolate KUS-F28456 were amplified using universal primer pair ITS1/ITS4 and sequenced (White et al. 1990). The obtained ITS sequence was deposited in GenBank (Accession No. KR822240). A GenBank BLAST search of the Korean isolates showed >99% similarity with those of many P. xanthii isolates (e.g., AB040349, AB046985, KC460208, etc.). Pathogenicity was confirmed by pressing a diseased leaf onto young leaves of three healthy, potted torenia plants. Three noninoculated plants served as controls. Plants were maintained in a greenhouse at 24 to 34°C. The first lesions developed on leaves of inoculated plants after 7 days, whereas the control plants remained symptomless. Podosphaera mildew associated with torenia has been recorded in Japan and Finland (Amano 1986, Farr and Rossman 2015). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on T. fournieri in Korea. Our field observations in several places of torenia plantings showed that torenia powdery mildew infection occurred in shady areas or in densely planted plots. Despite its low incidence, powdery mildew can detract from the aesthetic value of plants.