First report of powdery mildew caused by Podosphaera xanthii on Luffa cylindrica in Korea

Luffa cylindrica M. Roem. (syn. L. aegyptiaca Mill., Cucurbitaceae), called sponge gourd or Egyptian cucumber, is an annual climbing plant used as a biotechnological tool for a variety of applications (Saeed and Iqbal 2013). Young fruits are eaten in Southeast Asia. In October 2014, several plants exhibiting symptoms of powdery mildew were observed in a garden in Seoul (37°39′29.7″ N; 127°01′20.0″ E), Korea. Powdery mildew colonies were circular to irregular, forming white patches on both sides of the leaves. As the disease progressed, white mycelial growth covered entire leaves, causing premature senescence. Powdery mildew of sponge gourd had been collected in Jeju, Busan, Osan, and Anyang from 2005 to 2013. Four voucher specimens were deposited in the Korea University Herbarium (Accession Nos. KUS-F21623, F24810, F25120, and F28287). Morphological characteristics based on a representative specimen (KUS-F28426, Seoul, 2014) were as follows. Conidiophores (n = 20) were straight, 120 to 220 μm long (mean = 145.7 μm; SD ± 25.9 μm), 10 to 12 μm wide (mean = 10.2 μm; SD ± 0.5 μm), and produced 3 to 5 immature conidia in chains with a crenate outline. Foot-cells of conidiophores (n = 20) were cylindric and 50 to 75 μm long (mean = 56.7 μm; SD ± 7.5 μm). Conidia (n = 20) were ellipsoid-ovoid to barrel-shaped, measured 30 to 40 μm long (mean = 34.0 μm; SD ± 2.2 μm), 18 to 23 μm wide (mean = 19.2 μm; SD ± 1.3 μm), with a length/width ratio of 1.4 to 2.1, and had distinct fibrosin bodies. Simple to forked germ tubes were produced from the lateral position of conidia. Primary conidia were apically conical, basally truncate, and generally smaller than the secondary conidia. Chasmothecia were not observed. These structures are typical of the powdery mildew Euoidium anamorph of the genus Podosphaera. The measurements and morphological characteristics were consistent with those of P. xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012), a well-known cucurbitaceous powdery mildew. To confirm the identity of the causal agent, the complete internal transcribed spacer (ITS) region of rDNA of isolate KUS-F28426 was amplified with the primers ITS1/ITS4 and sequenced. The resulting 574-bp sequence was deposited in GenBank (Accession No. KR425670). A GenBank BLAST search of the Korean isolate showed >99% similarity with the sequences of P. xanthii isolates originated from various cucurbitaceous hosts (e.g., KP120970, KP120971, KP120972, etc.). Pathogenicity was confirmed by pressing a diseased leaf onto young leaves of three sponge gourd plants. Three noninoculated plants were used as controls. Inoculated leaves developed symptoms after 5 days, whereas the control plants remained symptomless. Powdery mildew of L. cylindrica caused by Podosphaera species has been recorded in China, India, Malaysia, Myanmar, Egypt, Libya, and Romania (Farr and Rossman 2015). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on L. cylindrica in Korea. Powdery mildew may be a threat to the continued production of L. cylindrica in Korea.