First Report of Powdery Mildew Caused by Podosphaera xanthii on Ixeridium dentatum in Korea

Ixeridium dentatum (Thunb. ex Mori) Tzvelev, known as toothed ixeridium, belonging to the family Asteraceae, is native to Korea and East Asia. In Korea, it has been used as a traditional herbal medicine for the treatment of hepatitis, indigestion, pneumonia, tumors, and hyperlipidemia. I. dentatum is commercially cultivated in some areas (over 100 ha) of Korea. In October 2017, symptoms of powdery mildew were observed on over 60% of the leaves and stems of I. dentatum plants in a garden at Chonnam National University located in Gwangju, Korea. Disease symptoms included white powdery colonies on the upper leaf surfaces and stems with or without necrosis. Appressoria of the fungus were indistinct to slightly nipple shaped. Conidiophores were cylindrical, 90.3 to 190.5 × 10.5 to 12.5 μm. Foot cells of conidiophores were 45.2 to 65.1 × 10.4 to 12.5 μm, cylindrical, or slightly constricted at the base, followed by zero to three shorter cells, forming conidia in chains. Conidia were ellipsoid to oval, 28.8 to 38.4 × 16.5 to 21.3 μm. Chasmothecia were not observed. Morphologically, the isolate was consistent with the description of Podosphaera xanthii (Castagne) U. Braun & N. Shishkoff (Braun and Cook 2012; Lee 2013; Meeboon and Takamatsu 2015). Genomic DNA was extracted directly from the mycelia and conidia using a gDNA prep kit (Solgent, Daejeon, Korea). The internal transcribed spacer (ITS) region of the rDNA of two isolates was amplified using the primers ITS1 and ITS4 as described by Lee (2015). A BLASTn search of the ITS sequences against the NCBI database indicated that the isolates EML-IDPW1726 and EML-IDPW1727 (GenBank accession nos. MG719995 and MG719996, respectively) matched P. xanthii (GenBank accession no. KY388505), with 100% (444/444 bp) sequence homology. Phylogenetic analyses based on the ITS rDNA indicated that EML-IDPW1726 and EML-IDPW1727 isolates were identical to P. xanthii. Therefore, the causal pathogen was identified as P. xanthii based on the morphology of its imperfect stage and molecular analyses. The pathogenicity of the biotrophic fungus was demonstrated by gently pressing the infected leaves onto healthy leaves of I. dentatum (Lee 2013). Plants were maintained in a greenhouse at 23 ± 2°C. Seven days after inoculation, symptoms similar to those observed under natural conditions developed on the inoculated leaves of I. dentatum plants. The fungus on inoculated leaves was morphologically identical to that first observed. Powdery mildew of I. dentatum caused by Golovinomyces cichoracearum and P. fuliginea has also been reported previously from Japan (Farr and Rossman 2017). P. xanthii (synonyms: P. fusca, Sphaerotheca fuliginea, and S. fusca) has been reported to cause powdery mildew on Ixeridium spp. (synonyms: Ixeris, Paraixeris, Youngia, Chondrilla, Prenanthes, and Lactuca [The Plant List 2013]) under the name of Sphaerotheca humuli var. fuliginea. To our knowledge, this is the first record of powdery mildew caused by P. xanthii on I. dentatum in Korea. The occurrence of powdery mildew on I. dentatum could pose a serious threat to the health of this plant, resulting in death and severe yield reduction.