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First Report of Persian Buttercup Downy Mildew Caused by Peronospora sp. in Korea

Persian buttercup (Ranunculus asiaticus L.) is an ornamental plant cultivated mainly in the countries surrounding the Mediterranean Sea, and has recently become popular in Korea. During March and April 2012, Persian buttercups ‘Elegance’ showing symptoms of downy mildew were found in plastic greenhouses in Hwaseong City of Korea. Infection resulted in chlorotic leaves with a dark greyish and dense fungal-like growth on the lower surfaces, and finally led to necrosis of the lesions. A sample was deposited in the Korea University herbarium (KUS-F26431). Conidiophores emerging from stomata were hyaline, 250 to 550 × 7 to 15 μm, straight, and dichotomously branched in 6 to 8 orders. Ultimate branchlets were mostly in pairs, slightly curved, 5 to 15 μm long, and had obtuse tips. Conidia were brown, broadly ellipsoidal to subglobose or ellipsoidal, often pedicellated, and measured 24 to 33 × 20 to 27 μm with a length/width ratio of 1.15 to 1.30. Fourteen species of Peronospora have previously been described on the genus Ranunculus (2), of which P. ficariae was mostly considered the causal agent of downy mildew on Persian buttercup (1,3). The present Korean accession is morphologically distinct from P. ficariae on R. ficaria (a synonym of Ficaria verna) by somewhat larger conidia with often pedicel-like ends. The nuclear ribosomal LSU and ITS regions were PCR-amplified and sequenced as described in Göker et al. (4), and the resulting sequences deposited in GenBank (Accession Nos. KC111207 and JX465737, respectively). A comparison with the GenBank sequences revealed that the present Korean pathogen differed from P. ficariae on R. ficaria at 10 of 688 characters (about 1.5%) in LSU (AF119600) and 11 of 802 characters (about 1.4%) in ITS sequences (unpublished sequence). In addition, the ITS sequence exhibits a dissimilarity of 1.5 to 2.0% from three species of Peronospora parasitic on Ranunculus; P. alpicola on R. aconitifolius (AY198271), P. illyrica on R. illyricus (AY198268), and P. ranunculi on R. acris (AY198267) and R. recurvatus (AY198269). Based on morphological and molecular distinction between P. ficariae and the Korean pathogen, we provisionally indicate this pathogen as an undetermined species of Peronospora. Pathogenicity was demonstrated by shaking diseased leaves onto the leaves of healthy Persian buttercup ‘Elegance’, incubating the plants in a dew chamber at 20°C for 24 h, and then maintaining them in a greenhouse (20 to 24°C and relative humidity 60 to 80%). After 3 to 4 days, inoculated plants developed downy mildew symptoms, from which an identical fungus was observed, thus fulfilling Koch's postulates. Control plants treated with sterile water did not develop any symptoms of downy mildew. To our knowledge, this is the first report of a downy mildew on Persian buttercup in Asia, although this disease has been found in other continental countries, such as Italy (1), New Zealand, South Africa, and the United States (3). The presence of a downy mildew on Persian buttercup in Asia can be considered as a potentially new and serious threat to commercial production of this ornamental plant.

References: (1) E. Buonocore and R. Areddia. Informatore Fitopatologico 49:25, 1999. (2) O. Constantinescu. Thunbergia 15:1, 1991. (3) D. F. Farr and A. Y. Rossman. Fungal Databases, Syst. Mycol. Microbiol. Lab., Online publication, ARS, USDA, Retrieved August 4, 2012. (4) M. Göker et al. Mycol. Res. 113:308, 2009.