Arugula or rocket (Eruca vesicaria ssp. sativa) is a leafy vegetable for salad use worldwide. In March 2009, plants showing typical symptoms of downy mildew were found in plastic greenhouses in Yongin, Korea. Severely affected plantings were unmarketable and had to be abandoned before harvest. Infection resulted in chlorotic leaves with a white fungal-like growth developing on the lower surfaces typical of downy mildew. A sample was deposited in the herbarium of Korea University (KUS-F24009). Conidiophores were hyaline, 200–600 × 10–17 μm, straight, monopodially branched in 7–9 orders, and emergent from stomata. Ultimate branchlets were mostly in pairs, flexuous to curved, 15–30(−40) μm long and had obtuse or subtruncate tips. Conidia were subglobose, hyaline, and measured 20·0–23·8 × 17·5–21·5 μm (length/width ratio = 1·06–1·15). These characteristics unequivocally indicate the genus Hyaloperonospora.
The amplification and sequencing of the ITS rDNA were performed with procedures outlined by Cooke et al. (2000), and the sequence of the region was deposited in GenBank (Accession No. GQ919015). Comparison with the sequences available in the GenBank database revealed that the ITS sequence is identical to sequence EU049271 from Hyaloperonospora sp. on Eruca sativa in Spain, but is significantly different from the sequence AY210987 of H. parasitica on Capsella bursa-pastoris.
Based on morphological and molecular data, the arugula/rocket pathogen is clearly distinct from H. parasitica, to which it was previously attributed in most studies. The Korean accession is identical to the unnamed Hyaloperonospora sp. reported from arugula in Spain (Göker et al., 2009). There is no previous record of a downy mildew on arugula in Korea, although it has been previously recorded from Argentina, Iran, Pakistan, Spain, and the USA (Farr & Rossman, 2009; Göker et al., 2009). The first occurrence of downy mildew on arugula in East Asia was recently recorded in Japan (Sato et al., 2004). It suggests that the pathogen is capable of rapid spread into Asian countries. Since cultivation of arugula was recently started on a commercial scale in Korea, the downy mildew poses a serious threat to production of this leafy crop.