First Report of Downy Mildew Caused by Plasmopara viticola on Vitis coignetiae in Korea

Vitis coignetiae Pulliat ex Planch., known as crimson grapevine, is a deciduous climbing vine native to the temperate climate zone of East Asia, including Korea and Japan. Edible fruits of crimson grapevine are used for wine production and medicinal purposes in Korea. In November 2018, downy mildew infections were observed on the vines growing at a farm of the Grape Research Institute of Chungcheongbuk-do Agricultural Research and Extension services (36°19′55″N, 127°45′39″E), Korea. Symptoms appeared as reddish or dark brownish, vein-limited, angular spots on the upper surface of leaves, with a white downy oomycete growth developed on the corresponding lower surface. A representative specimen was deposited in the Kunsan National University Herbarium (KSNUH376). Sporangiophores were hyaline, straight or slightly curved, (220–) 420 to 690 (–770) (n = 50), tree-like, and monopodially branched in three to six orders. Ultimate branchlets in pairs or three, straight to slightly curved, 2.5 to 7.0 μm long, 0.9 to 1.6 μm wide at the base (n = 50), with truncate or rarely swollen tips. Sporangia were hyaline, ovoidal or lemon-shaped, measured (17.7–) 20.9 to 24.4 (–26.9) × (12.7–) 13.9 to 16.6 (–18.3) μm, with a length/width ratio of (1.18–) 1.37 to 1.59 (–1.72) (n = 50). The morphological characteristics closely resembled those reported for Plasmopara viticola (Berk. & M.A. Curtis) Berl. & De Toni (Hall 1989). To confirm the morphological identification, genomic DNA was extracted from a diseased leaf of KSNUH376. Three DNA regions were sequenced: internal transcribed spacer I (ITS1) using primers DC6 and ITS2, large subunit (LSU) rDNA using primers LROR and LR6-O (Choi et al. 2011), and cytochrome c oxidase II (cox2) mtDNA using primers cox2F and cox2-RC4 (Choi et al. 2015). The resulting sequences were deposited in GenBank (accession nos. MK557845 for ITS1, MK517538 for LSU, and MK519373 for cox2). A BLASTn search revealed that the Korean sample is identical to the sequences of P. viticola parasitic to V. vinifera (DQ665668 for the ITS1, HM628762 for the LSU, and MH468768 and DQ365760 for the cox2). Pathogenicity was confirmed through inoculation by putting four discs (diameter 3 mm) of the infected leaves onto the lower leaf surface of five healthy V. coignetiae. They were incubated in a growth chamber at 25°C under a relative humidity of 80%, and sterilized dH₂O was sprayed daily on the inoculated leaves to induce zoospore formation and infection. Five noninoculated leaves served as controls. After 10 days, downy mildew symptoms developed on the lower surface of the inoculated leaves, whereas the control leaves remained symptomless. Based on morphological characteristics, molecular sequencing data, and the pathogenicity test, the pathogen was identified as P. viticola. Previously, P. viticola has been recorded on V. coignetiae in the United Kingdom and Japan (Farr and Rossman 2019) and under a laboratory condition by Jurges et al. (2009). To our knowledge, this is the first report of downy mildew caused by P. viticola on V. coignetiae in Korea. Our field observations suggest that downy mildew would be a potential threat to crimson grapevine cultivation.