In 2016, an onion downy mildew epidemic caused by Peronospora destructor severely damaged the commercial onion fields in Saga Prefecture, western Japan. To identify the factors underlying the outbreak, we investigated the symptoms of downy mildew caused by secondary infections and examined P. destructor’s sensitivity to metalaxyl, the most effective traditional fungicide used against this onion pathogen, in 2016–2018. Disease symptoms developed in late March 2016, which was earlier than symptom development in 2017 and 2018. Furthermore, there were synchronous repeated disease development and favourable meteorological conditions for infection in early and late April resulting in the development of polycyclic epidemics. In field trials from 2016 to 2018, the efficacy of chlorothalonil + metalaxyl-M application ranged 18–45%, as calculated by comparing disease severity at the final stage of each treatment to that in the untreated plots. On the basis of the metalaxyl sensitivity observed in 2016, the effective concentration, which reduced germ-tube elongation in P. destructor by 50%, exceeded 200 µg ai/mL for certain strains. Our observations indicate that these characteristic meteorological conditions were major factors contributing to the severe disease outbreak in 2016. The emergence of less-metalaxyl-sensitive P. destructor strains may be an additional predisposing factor.
Downy mildew of onions caused by Peronospora destructor is a significant disease that occurs in most onion-producing regions worldwide, causing losses in both yield and quality. During the 2015-2016 growing season, epidemics caused by this disease severely damaged commercial onion fields in western Japan (Fujiwara et al., 2021).Yarwood (1943) and Demizu (1963) reported that the first symptoms of onion downy mildew induced by primary infection are leaf chlorosis (i.e., pale green to yellowish leaves) and distortion (i.e. down-curled lower leaves). The pathogen forms sporangia on these symptomatic leaves under conditions of high humidity and moderate temperature, causing secondary infection (Demizu, 1963;Yarwood, 1943). In western Japan, onion seeds are primarily sown in nursery beds in autumn, seedlings are transplanted to the main fields
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