First Report of Downy Mildew of Spider Flower Caused by a Hyaloperonospora sp. in Korea.

Spider flower, Tarenaya hassleriana (Chodat) H. H. Iltis (synonym Cleome hassleriana; C. spinosa), which is native to South America, is now cultivated as an ornamental plant worldwide. In Korea, this plant has recently become popular in gardens and parks because of its beautiful flowers. During July 2010, plants showing typical symptoms of downy mildew were observed in public gardens along the lakeside in Ganghwa, South Korea. Infection resulted in chlorotic areas on the leaves with a white mildew developing on the abaxial surface and finally leading to necrosis of the lesions. Representative samples of infected leaves were deposited at the herbarium of Korea University, Seoul, Korea (KUS-F25091 and F25462). Microscopic examination of fresh material was performed under a light microscope. Conidiophores emerging from stomata were hyaline, 250 to 650 × 10 to 15 μm, straight, and monopodially branched in five to eight orders. Ultimate branchlets were mostly in pairs, flexuous to sigmoid, 15 to 30 μm long, and had obtuse or subtruncate tips. Conidia were hyaline, subglobose, and measured 23 to 26.5 × 21 to 24 μm with a length/width ratio (L/W) of 1.05 to 1.15. Up to now, the downy mildew pathogen of the spider flower has been considered to be Hyaloperonospora parasitica, Peronospora capparidis or P. cleomes, but the latter two names were considered as synonyms of the former (1). In the current study, the spider flower pathogen was morphologically distinct from H. parasitica; in the Korean specimen, conidia were subglobose with a low L/W value, while in H. parasitica sensu stricto, originated from Capsella bursa-pastoris, conidia were broadly ellipsoidal and measured 22.5 to 26.5 × 18 to 21.5 μm with a L/W ratio of 1.17 to 1.31 (1). To confirm this morphological difference, the amplification and sequencing of the internal transcribed spacer (ITS) region of rDNA of the Korean specimen were performed using procedures outlined by Göker et al. (3). The resulting 874-bp sequence of the region was deposited in GenBank (Accession No. JQ301468). A comparison with the ITS sequences available in the GenBank database revealed that the Korean accession exhibits a high dissimilarity of approximately 11% (99 of 882 characters are different) from that of H. parasitica from C. bursa-pastoris (AY210987). On the basis of morphological and molecular data, the spider flower pathogen found in Korea was clearly distinct from H. parasitica. Therefore, we provisionally indicate this pathogen as a Hyaloperonospora sp. To our knowledge, there is no previous record of a downy mildew on spider flower in Asia, although this disease has been previously recorded in Malawi, South Africa, Uganda, New Zealand, Poland, Romania, the United States, and Venezuela (2). The presence of a downy mildew on spider flower in Asia can be considered a potentially new and serious threat to this ornamental plant. References: (1) O. Constantinescu and J. Fatehi. Nova Hedwigia 74:291, 2002. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ , December 13, 2011. (3) M. Göker et al. Mycol. Res. 113:308, 2009.