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First Report of Tatsoi Downy Mildew Caused by Hyaloperonospora brassicae in Korea

Tatsoi (Brassica narinosa L.H. Bailey), also called spinach mustard or spoon mustard, is cultivated for edible greens in Asia. In Korea, this plant has recently become popular as a sprout vegetable that is grown to harvestable size in 5 to 6 days. During April 2012, tatsoi seedlings showing typical symptoms of downy mildew were found in plastic greenhouses in Pyeongtaek City of Korea. Infection resulted in chlorotic areas on the leaves with a white mildew developing on the abaxial surface, and finally led to necrosis of the lesions. Affected sprouts were unmarketable and abandoned without harvesting. A sample was deposited in the Korea University herbarium (Accession No. KUS-F26445). Microscopic examination of fresh material was performed under a light microscope. Conidiophores emerging from stomata were hyaline, 270 to 550 × 10 to 25 μm, straight, and monopodially branched in six to eight orders. Ultimate branchlets were mostly in pairs, flexuous, and 15 to 25 μm long. Conidia were hyaline, subglobose, and 20.5 to 26.5 × 19.5 to 24.5 μm with a length/width ratio of 1.05 to 1.20. These characteristics unequivocally indicate the genus Hyaloperonospora (1). Previously H. parasitica (formerly under Peronospora) has been considered a causal agent of downy mildew on tatsoi (2,4), but the present Korean accession is morphologically distinct from the former species by possessing subglobose conidia with a low length/width ratio. To confirm this morphological difference, 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. JX401551). A comparison with the ITS sequences available in the GenBank database revealed that it was identical to Hyaloperonospora brassicae found on Brassica oleracea var. italica (EU137726), and showed only one base pair substitution compared to pathogens from B. rapa ssp. pekinensis (JF975613) and B. napus spp. napus (EU049248), but is significantly different from H. parasitica on Capsella bursa-pastoris (AY210988) with a base-pair dissimilarity of about 13%. Therefore, the pathogen found in Korea was confirmed to be H. brassicae. Pathogenicity was demonstrated by shaking diseased leaves onto the leaves of healthy tatsoi seedlings, incubating the plants in a dew chamber at 20°C for 24 h, and then maintaining them in a greenhouse (22 to 26°C). After 3 days, inoculated plants developed downy mildew symptoms from which identical fungi were observed, thus fulfilling Koch's postulates. Control plants treated with sterile water did not develop any symptoms or signs of downy mildew. This is the first report of a downy mildew on tatsoi in Korea, although it has been found in China (2) and Japan (4). To our knowledge, there is no record of tatsoi downy mildew outside East Asia (2,4).

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/ July 18, 2012. (3) M. Göker et al. Mycol. Res. 113:308, 2009. (4) M. Satou et al. Annu. Rep. Soc. Plant Protect. N. Jpn. 50:62, 1999.