First report of powdery mildew caused by golovinomyces orontii on lactuca sativa in Korea

During autumn 2014, powdery mildew with ∼5% disease incidence was found on leaf lettuce (Lactuca sativa L.) grown in polyethylene-film-covered greenhouses in Seoul, Korea. Infections caused leaf distortions, withering, and reduced growth. Mycelia were amphigenous, thinly effuse, initially forming indistinct white patches, finally covering whole leaf surfaces and stems. Appressoria on the mycelium were nipple-shaped or occasionally moderately lobed. Conidiophores were 100 to 260 × 10 to 12 μm, arising laterally from hyphal mother cells, and produced 2 to 6 immature conidia in chains with a sinuate outline, followed by 2 to 3 cells. Foot-cells of the conidiophores were 35 to 65 μm long, and characterized by having a distinctly curved base. Conidia were hyaline, ellipsoid to ovate, 30 to 43 × 13 to 18 μm with a length/width ratio of 1.7 to 3.2, and lacked distinct fibrosin bodies. Germ tubes were produced on the perihilar or lateral position of conidia. No chasmothecia were observed. Three voucher specimens were deposited in the Korea University Herbarium (Accession Nos. KUS-F28522, F28575, and F28576). The morphological characteristics were typical of the Euoidium anamorph of the genus Golovinomyces. The measurements and morphology were consistent with G. orontii (Castagne) V.P. Heluta (Braun and Cook 2012). To confirm the identification, the internal transcribed spacer (ITS) region of ribosomal DNA of isolate KUS-F28576 was amplified with universal primers ITS1/ITS4 and sequenced directly. The resulting 682-bp sequence was deposited in GenBank (Accession No. KT964904). A BLAST search of this sequence revealed >99% similarity with the ITS sequences of G. orontii isolates on L. sativa from Japan (AB769447) and L. serriola from Korea (KP260660). Pathogenicity was confirmed by gently dusting a diseased leaf onto leaves of five healthy potted leaf lettuces. Five noninoculated plants served as controls. Plants were maintained in a greenhouse at 24 to 30°C. Inoculated plants developed signs and symptoms after 4 days; control plants remained symptomless. The fungus from the inoculated plants was identical morphologically to that observed from initially diseased plants. Association of L. sativa with Golovinomyces powdery mildews has been known in many European countries, South Africa, the United States, Canada, Brazil, Chile, Argentina, Australia, Israel, and Japan (Farr and Rossman 2015). To our knowledge, this is the first report of powdery mildew caused by G. orontii on L. sativa in Korea. Previously, Podosphaera xanthii (syn. P. fusca) has been known to be associated with powdery mildew infections of leaf lettuce with economic losses in Korea (Jee et al. 2006; Shin et al. 2006). Therefore, lettuce powdery mildew disease in Korea could be caused by one or both of the two mildew fungi, although G. orontii is comparatively rare. This information may help lettuce growers and breeders in disease monitoring.