First report on DMI fungicide resistance in Gymnosporangium asiaticum, the causal agent of Japanese pear rust, in Japan

Japanese pear rust, caused by Gymnosporangium asiaticum Miyabe ex Yamada, is one of the most important fungal plant diseases affecting Japanese pear cultivation. Although sterol demethylation inhibitor (DMI) fungicides have controlled this disease effectively, outbreaks have been frequent in some areas of Japan since 2008. We, therefore, evaluated fungal populations from Fukuoka Prefecture for DMI fungicide sensitivity in vivo. Aeciospores on diseased pear leaves were collected in 2013 from four orchards in Fukuoka Prefecture and one control orchard in Ibaraki Prefecture. Juniperus chinensis var. kaizuka Hort. test plants were sprayed with difenoconazole and fenarimol at recommended rates, and the plants were inoculated with aeciospores. Difenoconazole and fenarimol effectively controlled the population of rust fungus from Ibaraki Prefecture. In contrast, fenarimol and difenoconazole were found to be ineffective against fungal populations from Fukuoka Prefecture. Subsequently, telia were collected from diseased juniper twigs in five areas in Fukuoka Prefecture and one control area in Ibaraki Prefecture in 2013 and 2014. Similarly, test plants of Japanese pear ‘Kousui’ were treated with difenoconazole or fenarimol and inoculated with basidiospores taken from those telia. Both difenoconazole and fenarimol were largely ineffective against the populations from Fukuoka Prefecture. In addition, field trials conducted in an experimental orchard in 2014 and 2015 showed that difenoconazole and fenarimol were not effective in controlling this disease. These results indicate the occurrence of Japanese pear rust resistant to DMI fungicides in Fukuoka Prefecture.