Characteristics of Meteorological Conditions during a Severe Outbreak of Onion Downy Mildew and Metalaxyl Sensitivity of Peronospora destructor in Saga, Japan, in 2016

In 2016, an onion downy mildew epidemic caused by Peronospora destructor severely damaged the commercial fields in Saga Prefecture, western Japan. To identify factors underlying outbreak, we investigated symptoms of secondary infections and examined P. destructor’s sensitivity to metalaxyl, most effective traditional fungicide used against this pathogen, 2016–2018. Disease developed late March which was earlier than symptom development 2017 2018. Furthermore, there were synchronous repeated disease favourable meteorological conditions for infection early April resulting polycyclic epidemics. field trials from 2016 2018, efficacy chlorothalonil + metalaxyl-M application ranged 18–45%, as calculated comparing severity at final stage each treatment that untreated plots. On basis metalaxyl observed concentration, reduced germ-tube elongation 50%, exceeded 200 µg ai/mL certain strains. Our observations indicate these characteristic major contributing severe outbreak 2016. The emergence less-metalaxyl-sensitive strains may be additional predisposing factor.