First confirmed report of downy mildew caused by Hyaloperonospora parasitica on broccoli in Korea

S.Y. Hong¹, H.J. Jee², Y.J. Choi³ and H.D. Shin^3*

¹ Jeju Agricultural Research and Extension Service, Jeju 697-701, Korea
² Organic Farming Technology Division, National Institute of Agricultural Science and Technology, Rural Development Administration, Suwon 441-707, Korea
³ Division of Environmental Science and Ecological Engineering, Korea University, Seoul 136-701, Korea

^*hdshin@korea.ac.kr

 Accepted for publication 22/11/07

In January 2007, broccoli plants (Brassica oleracea var. italica) showing symptoms of downy mildew were found in Jeju, the chief producing area in Korea. Broccoli growers report that downy mildew is the only disease of economic importance in their greenhouses, occasionally resulting in 100% loss of yield. Initial symptoms were irregular yellow-green spots on upper leaf surfaces and white fluffy fungal growth underneath. Floral heads developed pale brown or greyish discoloration (Fig. 1). Systemic infections caused internal dark grey to black spots and streaks in stems and floret branches (Fig. 2). These internal symptoms are found only in broccoli and cauliflower, not in other brassicaceous crops. A sample was deposited in the herbarium of Korea University (acc. no. KUS-F22524). The conidiophores were hyaline, 310-520 × 10-20 μm, monopodially branched 4-7 times (Fig. 3). The ultimate branchlets were usually strongly curved, 20-42 μm long, 2-3 μm wide at the base (Fig. 4). Conidia were hyaline and measured 23.3-31.2 × 20-27.5 μm (length/width = 1.0-1.22) (Fig. 5). This fungus was concordant with known characteristics of Hyaloperonospora parasitica (syn. Peronospora parasitica, Constantinescu & Fatehi, 2002).

Figure 1: Downy mildew symptoms on broccoli floral head infected by Hyaloperonospora parasitica.	Figure 2: Internal black streaks in the floret branch of broccoli infected by Hyaloperonospora parasitica.

The amplification and sequencing of the ITS rDNA were performed with procedures outlined by Cooke et al. (2000), and the sequence deposited in GenBank (acc. no. EU137726). Comparison of the ITS sequences in the GenBank database revealed that it was identical to H. parasitica found on Brassica campestris (AY210985, AY210986), and shows only one base pair substitution with one on B. napus subsp. napus (AY531407, AY531409).

Figures 3-5: Hyaloperonospora parasitica. 3. Conidiophore (bar = 100 μm);
4. Ultimate branchlets (bar = 50 μm); 5. Conidia (bar = 20 μm).

Hyaloperonospora parasitica (Peronospora parasitica) has a world-wide distribution and is an important pathogen of numerous crop hosts. In Korea, H. parasitica has previously been recorded on various brassicaceous plants including Brassica rapa subsp. pekinensis and B. juncea (Cho & Shin, 2004). Due to the recent taxonomic revision of the Peronospora, this is the first confirmed record of this pathogen from broccoli. Downy mildew of broccoli has previously been reported in Australia, Brazil, Canada, and USA. To our knowledge, this is only the second record of a downy mildew on broccoli in East Asia, the first being from Japan in 1991 (Satou et al., 1991). Commercial cultivation of broccoli in East Asia started in Japan in the 1980s and has expanded to Korea and China in the 2000s. Broccoli downy mildew appears to be spreading in Asian countries.

References

Cho WD, Shin HD, eds, 2004. List of Plant Diseases in Korea. Seoul, Korea: Korean Society of Plant Pathology.

Constantinescu O, Fatehi J, 2002. Peronospora-like fungi (Chromista, Peronosporales) parasitic on Brassicaceae and related hosts. Nova Hedwigia 74, 291-338. 

Cooke DEL, Drenth A, Duncan JM, Wagels G, Brasier CM, 2000. A molecular phylogeny of Phytophthora and related Oomycetes. Fungal Genetics and Biology 30, 17-32.

Satou M, Hagiwara Y, Ishii M, Fukumoto F, 1991. Downy mildew of broccoli caused by Peronospora brassicae. Proceedings of the Kansai Plant Protection Society 33, 67-68 (in Japanese).