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First Report of Powdery Mildew Caused by Neoerysiphe nevoi on Hypochaeris radicata in Korea

Hong, S. H., Lee, Y. H., Choi, Y. J., Shin, H. D.
Plant disease 2019 v.103 no.2 pp. 369
DNA primers, Hypochaeris radicata, Taraxacum, anamorphs, appressoria, conidia, conidiophores, flowers, fungi, germ tube, herbaria, host plants, internal transcribed spacers, introduced plants, invasive species, leaves, mycelium, noxious weeds, pathogenicity, polymerase chain reaction, powdery mildew, ribosomal DNA, stems, type collections, Australia, Europe, Korean Peninsula, New Zealand, North America, South America
Hypochaeris radicata L., known as catsear or false dandelion, is a perennial, low-lying edible herb belonging to the tribe Cichorieae of the family Asteraceae. The plant is native to Europe but has also been introduced to the Americas, East Asia, Australia, and New Zealand, where it can be an invasive weed. Since 2009, this plant has been designated as one of 14 “harmful nonindigenous plants” by the Korean Ministry of Environment. In June 2015, dozens of catsear plants growing wild in Jeju (33°29′09″N, 126°41′04″E), Korea, were found infected with a powdery mildew. Symptoms first appeared as white colonies, which subsequently developed into abundant hyphal growth on flower stalks but not on leaves. Similar symptoms and signs with typical powdery mildew colonies were also found at two localities in Jeju (33°28′32″N, 126°29′34″E and 33°25′22″N, 126°31′19″E) in 2018. Six voucher specimens have been deposited in the Korea University Herbarium (KUS-F28631, F28654, F29459, F29758, F30486, and F30725). Appressoria on the mycelium were mostly lobed and occasionally nipple shaped. Conidiophores were 110 to 180 × 10 to 12 μm and produced two to four immature conidia in chains with a crenate outline, followed by one to three cells. Foot cells of conidiophores were straight, cylindrical, and 32 to 46 μm long. Conidia were hyaline, ellipsoid to cylindric oval, measured 22 to 30 × 14 to 16 μm with length/width ratio of 1.5 to 2.1, and lacked distinct fibrosin bodies. Germ tubes were produced on the perihilar position of conidia. No chasmothecia were found during our field foray. The structures described above were typical of powdery mildew Striatoidium anamorph of the genus Neoerysiphe. The morphological characteristics were compatible with those of N. nevoi Heluta & Takam. (Braun and Cook 2012). To confirm the identification, molecular analysis of internal transcribed spacer (ITS) regions of ribosomal DNA (rDNA) sequence was conducted. The complete ITS regions of rDNA of KUS-F28631 and F30725 were amplified using primer pairs ITS5/P3 as described by Takamatsu et al. (2009) and were sequenced directly. The resulting sequences were deposited in GenBank (accession nos. MH685457 and MH685458). A GenBank BLAST search using the Korean isolates showed >99% similarity with those of N. nevoi isolates on many Cichorieae host plants (e.g., AB498973, AB498968, AB478969, AB498974, etc.). Pathogenicity was confirmed through inoculation by touching a diseased stem onto stems of three healthy, potted catsear plants. Three noninoculated plants served as controls. Inoculated plants developed signs and symptoms after 8 days, whereas the control plants remained healthy. The fungus present on the inoculated plants was identical morphologically to that originally observed on diseased plants. N. nevoi has been recorded on various genera belonging to the tribe Cichorieae but not on Hypochaeris (Braun and Cook 2012; Farr and Rossman 2018). To our knowledge, this is the first report of powdery mildew caused by N. nevoi on H. radicata in Korea. Our field observations suggest that the powdery mildew could be a limiting factor to suppress the expansion of this noxious weed in Korea.