First Report of Downy Mildew Caused by Plasmopara viticola on Vitis fielfolia var. sinuata in Korea.

Vitis ficifolia var. sinuata (Regel) H. Hara, known as sinuata vine, is a deciduous shrub, native to East Asia in the temperate climate zone (China, Japan, and Korea). Sinuata grapes became popular for both direct consumption and winemaking. The stems, leaves, and fruits of the vine were also used in traditional medicine for fatigue, arthritis, and neuralgia, and to stop bleeding, increasing urinary elimination, and detoxifying (Kim et al. 2012). Typical symptoms of downy mildew were first detected on vines growing in the wild, around a pond in Gunsan (35°55′38″N; 126°45′21″E) in September 2015, and then in Jeju (33°28′07″N; 126°28′54″E) in October 2016, Korea, where temperatures ranged from 10 to 23°C. Infected leaves turned yellow or paler green than normal, and revealed vein-limited, angular spots on the upper surface, with a whitish fungal growth developing abaxially. As the disease progressed, leaves appeared wilted and turned reddish to brownish. A representative specimen was deposited in the Korea University Herbarium (KUS-F29604). Sporangiophores emerging through stomata were hyaline, straight or rarely slightly curved, (250–) 320 to 540 (–600) × (8–) 9.2 to 11.4 (–12) µm (n = 30), tree-like, and monopodially branched in 4 to 6 orders. Ultimate branchlets were straight, 2 to 6 µm long, 0.5 to 1.5 µm wide at the base (n = 50), with a truncate or somewhat swelling apex. Sporangia were hyaline, ovoidal to shortly ellipsoidal, with a protruding pedicel, and measured (16–) 18.2 to 22.8 (–25) × (12–) 13.5 to 15.5 (–17) µm, with a length/width ratio of (1.2–) 1.28 to 1.46 (–1.6) (n = 50). No resting structure was found. These features were compatible with those of Plasmopara viticola (Berk. & M.A. Curtis) Berl. & De Toni (Hall 1989). The cytochrome c oxidase II (cox2) of mtDNA of the herbarium specimen KUS-F29604 was amplified using oomycete-specific primer set, cox2F and cox2-RC4. The resulting 550-bp sequence was deposited in GenBank (accession no. KY962512). Comparison of the sequences available in the GenBank database revealed that the isolate is identical with two sequences (DQ365760, EF426553) of P. viticola parasitic to V. vinifera. Therefore, based on both morphology and sequence comparison, the collected plant pathogen was identified as P. viticola. Pathogenicity was confirmed by gently pressing the infected leaves onto 20 leaves of four healthy V. ficifolia plants and incubating in a plant growth chamber (22°C and relative humidity 80%). Ten noninoculated leaves of two plants served as controls. All inoculated leaves developed symptoms after 10 to 15 days, whereas the control leaves remained symptomless. Previously, P. viticola has only once been recorded on V. ficifolia in Japan (Farr and Rossman 2017). To our knowledge, this is the first report of downy mildew caused by P. viticola on V. ficifolia var. sinuata in Korea. Given the well-known genetic plasticity of P. viticola toward specific species of Vitis (Rouxel et al. 2013; Schröder et al. 2011) and the morphological and molecular identity of the downy mildew causal agents from V. vinifera and V. ficifolia var. sinuata, it is tempting to speculate that the pathogen might constitute a potential inoculum source for both plant species. Our field observations suggest that downy mildew would be a potential threat to the sinuata vine, which began to be cultivated for commercial purposes. © 2017, American Phytopathological Society. All rights reserved.