First Report of Cercospora malayensis Causing Leaf Spot on Okra in Korea

Abstract

In Korea, okra (Abelmoschus esculentus [L.] Moench) is cultivated on small family farms and experimental plots (0.5 ha). In August 2014 and 2015, okra plants of cultivar ‘Green Hope’, grown in the farms of Iksan, Korea, were observed for leaf spot symptoms. Leaf spots were circular to irregular-shaped, initially brown, and subsequently turned reddish-brown with a yellowish margin. On leaves, stromata were brown, small, and composed of a few swollen hyphal cells. The conidiophores were fasciculate, olivaceous to brown, paler toward the apex, straight to minimally curved, geniculate, 30 to 250 µm long, 3.5 to 5.0 µm wide, one- to five-septate, and with conspicuous conidial scars. Conidia (n = 50) were hyaline, acicular, subacute to obtuse at the apex, truncate to obconically truncate at the base, 2- to 20-septate, 34 to 280 × 3.4 to 6.5 µm, and with a thickened, darkened hilum at the base. The morphological characteristics were consistent with Cercospora malayensis F. Stevens & Solheim (Chupp 1953). To obtain a pure conidial suspension, conidia were collected from 50 infected leaves using sterile forceps, placed in an Eppendorf tube containing sterile water, streaked onto the surface of 2% water agar plates supplemented with 100 mg/liter of streptomycin sulfate, and transferred onto potato dextrose agar (PDA). After 7 days of incubation, fungal colonies (10 isolates) appeared off-white and flat with aerial mycelia. Voucher specimens were housed in the Korea University herbarium (KUS-F28701), and one isolate was deposited in the Korean Agricultural Culture Collection (KACC47769). Additional fungal confirmation was conducted using the internal transcribed spacer (ITS), translation elongation factor 1-α (EF), and actin (ACT) gene sequences (Groenewald et al. 2013). A BLASTn search using ITS and ACT sequences showed >99% identity with C. malayensis ITS and ACT sequences (KM489070 and KY082664), whereas EF sequences showed 97.8% identity with C. malayensis EF gene (KY082663). The ITS, EF, and ACT sequences of the isolate were deposited in GenBank (MH129519, MH129517, and MH129515). Five 4-month-old okra plants (Green Hope) were spray-inoculated with C. malayensis (KACC47769) hyphal suspensions obtained from 2-week-old colonies grown on PDA at 25°C and transferred to a greenhouse (25°C) with a 12-h photoperiod. Five control plants were inoculated with sterile water. Symptoms observed on the C. malayensis inoculated plants were 100% identical with infected field conditions. C. malayensis was reisolated from symptomatic leaf tissues, and the identity was confirmed by microscopic studies. The pathogenicity test was repeated twice. Okra leaf spots associated with C. malayensis have previously been reported from several countries but not Korea (Farr and Rossman 2019). To our knowledge, this is the first report of C. malayensis on okra in Korea. Because okra hectares may increase, the disease poses a threat to okra cultivation in Korea.