Sechium edule (Jacq.) Sw. (Cucurbitaceae, chayote, mirliton) is native to Mexico and Central America. Several trials have recently been conducted to determine the ability of chayote cultivars to grow under the climatic and soil conditions of South Korea. In April 2013, chayote plants were observed showing typical symptoms of powdery mildew in a glasshouse in Jeju City, Korea. Powdery mildew colonies were circular to irregular, forming white patches on both sides of the leaves. As the disease progressed, entire leaves were covered with white mycelium, followed by leaf withering and premature senescence. The same symptoms were also found on chayote plants in a polyethylene-film-covered greenhouse in Iksan City, Korea, in 2014. Voucher specimens were deposited in the Korea University Herbarium (KUS-F27289, F27422, F28186). Hyphae were flexuous to straight, branched, septate, and 5 to 7 μm wide. Appressoria on the mycelium were nipple-shaped or nearly absent. Conidiophores were straight, 150 to 240 × 10 to 12 μm and produced three to seven immature conidia in chains with a crenate outline. Foot-cells of conidiophores were straight, cylindric, and 52 to 85 μm long. Conidia were hyaline, ellipsoid-ovoid to barrel-shaped, measured 27 to 36 × 16 to 23 μm with a length/width ratio of 1.3 to 2.0, and had distinct fibrosin bodies. Simple to forked germ tubes were produced from the lateral position of conidia. No chasmothecia were found. These structures are typical of the powdery mildew Euoidium anamorph of the genus Podosphaera. Dimensions of foot-cells and conidia were within the ranges provided for P. xanthii (Castagne) U. Braun & Shishkoff, and the length/width ratio of conidia, appressorial characteristics, and conidial germination patterns also conformed to the standard description (2). To confirm the identification, the complete internal transcribed spacer (ITS) region of rDNA of isolate KUS-F27289 was amplified with primers ITS1 and ITS4 and sequenced directly. The resulting 473-bp sequence was deposited in GenBank (Accession No. KM657960). A GenBank BLAST search of the Korean isolate showed 99% similarity with P. xanthii isolates from cucurbitaceous hosts (e.g., AB774155 to AB774158, AB040321, JQ340082, etc.). Pathogenicity was confirmed through inoculation tests by gently pressing a diseased leaf onto young leaves of three asymptomatic, potted chayote plants. Three non-inoculated plants were used as controls. Plants were maintained in a greenhouse at 24 to 34°C. Inoculated leaves started to develop symptoms after 5 days, whereas the control plants remained symptomless. The pathogenicity test was carried out twice with similar results. Powdery mildews of chayote caused by Podosphaera species have been reported in Australia, South Africa, Portugal, India, China, and the United States (1,3,4). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on chayote in Korea. Since chayote production was only recently started on a commercial scale in Korea, powdery mildew infections may pose a serious threat to the safe production of this vegetable.
References: (1) P. Baiswar et al. Australas. Plant Dis. Notes 3:160, 2008. (2) U. Braun and R. T. A. Cook. Taxonomic Manual of the Erysiphales (Powdery Mildews), CBS Biodiversity Series No. 11. CBS, Utrecht, 2012. (3) D. F. Farr and A. Y. Rossman. Fungal Databases. Syst. Mycol. Microbiol. Lab. Online publication, ARS, USDA, Retrieved October 4, 2014. (4) R. Singh et al. Plant Dis. 93:1348, 2009.