First Korean report of downy mildew on Coreopsis lanceolata caused by Plasmopara halstedii

Coreopsis lanceolata is a popular ornamental plant grown worldwide. In October 2008, plants showing typical symptoms of downy mildew disease were first collected in Pocheon County of Korea. Infected leaves turned yellow or pale green, with a whitish fungal-like downy growth developing on the abaxial leaf surface. As the disease progressed, the leaves wilted and curled (Figs. 1, 2). Detailed microscopic examinations of three representative samples (KUS-F23701, KUS-F23741, KUS-F23742) were made to identify the pathogen. Sporangiophores were hyaline, straight or rarely slightly curved, 350-500 x 7-13.5 µm, tree-like, and 6-8 times monopodially branched (Fig. 3). Ultimate branchlets were straight, 7-10 µm long, 2-3.5 µm wide at the base, and had truncate or round tips (Fig. 3). Sporangia were hyaline, oval to cylindrical, 19.1-26.2 x16.1-21.3 µm, length/width ratio = 1.12-1.30, tip and base round; a short conical pedicel mostly present (Fig. 3). These data are in full agreement with those given for Plasmopara halstedii (Shin & Choi, 2006).

To confirm the identification, the D1-D3 region of the 28S rDNA was amplified and sequenced, and the resulting sequence deposited in GenBank (Accession No. FJ638471). Comparison with the sequences available at GenBank revealed that the isolateshared 100% similarity withsequences EF553469 and AY035523 of Plasmopara halstedii on Helianthus annuus. Therefore, on the basis of morphology and sequence comparison the assumed pathogen was identified as P. halstedii.

Plasmopara halstedii has been commonly recorded on many asteraceous plants worldwide, but only reported on Coreopsis spp. in the USA (Farr and Rossman, 2009)and Poland (Skrzypczak, 2007). To our knowledge, this is the first report of downy mildew disease on Coreopsis spp. in Asia. As C. lanceolata is a popular garden and roadside plant in Korea, this disease has the potential to cause significant economic losses in nurseries and landscape businesses. In addition, the identical large subunit sequences of downy mildews accessions from sunflower and coreopsis indicate that infected plants might constitute a potential inoculum source for both hosts.