Studies on Occurrence and Infection Time of Downy Mildew of Onion (Allium cepa) in Korea

The onion (Allium Cepa) together with pepper and garlic is classified into seasoning vegetable and it has been regarded as one of important economic crops in Korea. Thus, its cultivation area has gradually increased from approximately 12,352 to 20,965 ha since 2003. The onion is generally seeded into seedbeds directly between August and September and its seedlings are transplanted into fields between October and November. The harvest is completed between April and June in the next year. Global climate changes, such as the increased temperature and increased frequency of rainfall has caused escalating outbreak of disease in onion. Especially, downy mildew has got worsen along with increased repeated cultivation area. Downy mildew could be categorized into two distinct infection stages depending on infection period. The first stage appears at late autumn or early spring (Oct. - Mar.) and the second stage is observed between April and June. An experiment was conducted to investigate damage aspect by downy mildew for some chief producing districts at onion in Korea in 2012 from 2014. In results, 26.4 % and 83.6 % of rate of damage fields were observed in total rice paddy and dry field, respectively. The average rate infected plants in each paddy and field where the least infection was occurred was 2.2 % and 8.9 %, respectively. Overall, average rate infected plants in dry field was significantly higher than that in rice paddy. The earlier the transplant was done the higher least infection rate was observed as follows; 76% at late October, 48.7% at early November, and 9.7% at mid-late November. Frequency of downy mildew appeared in early March and was maximized in mid-April on early maturing cultivar at 72%. Meanwhile, that in mid-late maturing cultivar was 42.7%. The main reason of higher frequency in early maturing cultivar would be due to the earlier transplant and thereby faster growth.