Abstract
Japanese pear scab caused by Venturia nashicola is one of the most prevalent diseases of Japanese pear trees in Japan. We estimated the sensitivity of this fungus to sterol demethylation inhibitor （DMI） fungicides on the basis of the disease severity in young pear trees pretreated with DMI fungicides and inoculated with fungal populations collected from commercial orchards in Fukuoka Prefecture in 2015. The protective values, 26.4-100.0 for difenoconazole （active ingredient （a.i.）, 25 ppm）; 42.5-85.3, hexaconazole （a.i., 10 ppm）; 25.8- 44.2, fenbuconazole （a.i., 27.5 ppm）; and 30.6-49.9 fenarimol （a.i., 30 ppm）, were significantly reduced in comparison with those recorded in 2007（ P < 0.05, Welch test）. In an experimental orchard in Fukuoka Prefecture, the efficacy of difenoconazole（ protective value, 87.6 in cultivar ‘Kousui’ and 74.7 in cultivar ‘Housui’） was higher than that of hexaconazole （protective value, 73.3 in‘ Kousui’ and 59.3 in‘ Housui’） in 2016. In a sensitivity test on young pear trees, the efficacies of difenoconozole （protective values, 85.9） and hexaconazole （protective values, 42.6） against fungal population from that orchard were similar to those in the orchard test. Therefore, the resistance of Venturia nashicola to DMI fungicides has a broad range in Fukuoka Prefecture.
