First report of powdery mildew caused by podosphaera xanthii on ramie in Korea

Ramie (Boehmeria nivea (L.) Gaud.) (family Urticaceae) is a perennial herbaceous plant native to southeast Asia. It is widely cultivated throughout southeast Asian and Pacific Rim countries, especially China and India (Angelini and Tavarini 2013). In Korea, ramie is traditionally used to produce fibers for summer clothes, but also for its fresh leaves as a main ingredient of Moshiddeok, a green Korean rice cake, and Moshiguksu, a green Korean noodle (Park et al. 2014). In autumn 2015, ramie plants growing in shaded areas in Naju (35°02′49.7″ N; 126°42′39.3″ E), Korea, were observed to be infected with a powdery mildew with approximately 5% disease incidence. Powdery mildew colonies were circular to irregular or thinly effuse on both sides of the leaves and stems. Leaf withering or premature senescence, typical symptoms of powdery mildews, were not observed. A voucher specimen was deposited in the Korea University Herbarium (KUS). Appressoria on the hyphae were nipple-shaped or nearly absent. Conidiophores were straight, 90 to 200 × 11 to 13 μm, and produced 2 to 4 immature conidia in chains with a crenate outline. Foot-cells of conidiophores were cylindric and 40 to 70 μm long. Conidia were ellipsoid-ovoid to barrel-shaped, 25 to 38 × 16 to 23 μm with a length/width ratio of 1.5 to 2.0, and had distinct fibrosin bodies. Primary conidia were apically conical or rounded, mostly truncate at base, and generally smaller than secondary conidia. No chasmothecia were observed during the growing season. These structures are typical of the powdery mildew Euoidium anamorph of the genus Podosphaera. The morphological characteristics and measurements were consistent with those of P. xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012). To confirm the identification, the complete internal transcribed spacer (ITS) region of rDNA for an isolate KUS-F28930 was amplified with the primers ITS1/ITS4 and sequenced. The resulting 475-bp sequence was deposited in GenBank (Accession No. KU312040). A BLAST search of the Korean isolate showed 100% identity (475/475 bp) with the sequence of a P. xanthii isolate on Boehmeria nipononivea from Japan (AB026139). Pathogenicity was confirmed by dusting the conidia from a diseased leaf onto young leaves of three healthy ramie plants. Three noninoculated plants were used as control. Inoculated leaves developed signs similar to those observed on diseased leaves after 5 days, whereas the control plants remained healthy. Powdery mildew of Boehmeria spp. caused by a species of Podosphaera has been recorded in China, Japan, and Taiwan (Farr and Rossman 2015). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on ramie in Korea. The incidence and economic cost of powdery mildew on ramie plants seem to be currently negligible, but the occurrence of this disease may threaten production of this crop in Korea, especially the availability of fresh leaves for moshiddeok and moshiguksu.