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

Abstract

Okra (Abelmoschus esculentus [L.] Moench) is a newly introduced vegetable in Korea. In August 2014, hundreds of ‘Red Hope’ okra plants developed symptoms of powdery mildew in a plastic greenhouse in Iksan (35°56′38″N, 126°59′37″E), Korea. Powdery mildew colonies first appeared as white patches on both sides of the lower leaves, later resulting in reduced growth and premature senescence of the leaves and affected approximately 30% of the plants. In September 2017, about 40 okra plants grown in a hobby garden also developed symptoms of powdery mildew in Mokpo (34°47′40″N, 126°22′25″E), Korea. Voucher specimens were deposited in the Korea University Herbarium (KUS-F27261, F27993, and F28188 from Iksan; F30094 from Mokpo). Appressoria on the mycelium were nipple-shaped or nearly absent. Conidiophores were straight, cylindric, 120 to 230 × 10 to 12 μm, and produced 3 to 5 immature conidia in chains with a crenate outline. Foot-cells of conidiophores were cylindric and 45 to 75 μm long. Conidia were ellipsoid-ovoid to barrel-shaped, measured 27 to 38 × 17 to 23 μm with a length/width ratio of 1.5 to 2.0, and had distinct fibrosin bodies. Germ tubes were produced from a lateral position on conidia. No chasmothecia were found during the growing seasons. These structures are typical of the genus Podosphaera (Braun and Cook 2012). To confirm the species identification, DNA of isolate KUS-F30094 was extracted from mycelia and PCR product was amplified using primers ITS5 and P3 (Takamatsu et al. 2009) for internal transcribed spacer (ITS) regions, and sequenced directly. The sequence obtained was deposited in GenBank (accession no. MG754404). Comparison with reference sequences in GenBank resulted in >99% similarity to ITS sequences of P. xanthii (e.g., AB525914, EF050036, KJ472787, and KR822240). The morphological characteristics and molecular analysis were consistent with those of P. xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012). Pathogenicity was confirmed by dusting conidia of a symptomatic leaf infected with isolate KUS-F27261 onto young leaves of three healthy, potted ‘Red Hope’ okra plants. Three noninoculated plants were used as controls. Inoculated plants and controls were maintained in a greenhouse at 24 to 30°C. Inoculated leaves developed symptoms after 5 days, whereas the control plants remained asymptomatic. The fungus present on the inoculated leaves was morphologically identical to that originally observed, fulfilling Koch’s postulates. Okra powdery mildew caused by P. xanthii (syn. P. fuliginea) has been identified in Australia, Brazil, Greece, India, Israel, Japan, Sudan, and Taiwan (Farr and Rossman 2018). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on okra in Korea. Okra is cultivated in commercial farms for its edible green seedpods. The occurrence of powdery mildew is a threat to the quality and marketability of okra, especially those organically grown where chemical control options are limited.