Occurrence of QoI-resistant strains of Cladosporium carpophilum causing Japanese apricot scab in Wakayama Prefecture

Abstract

Japanese apricot scab is an agriculturally important fungal disease caused by Cladosporium carpophilum, which is presented on fruits as black spots of approximately 1–3 mm. A quinone-outside inhibitor (QoI) fungicide, kresoxim-methyl, has been used to effectively control this disease since 2005. Another QoI fungicide, azoxystrobin, has been in use since 2011. However, resistance to these fungicides was suspected in some Japanese apricot orchards in Wakayama Prefecture in 2012. Therefore, we investigated the susceptibility of C. carpophilum isolates from 19 orchards to kresoxim-methyl and azoxystrobin. First, a culture test was performed where the 41 isolates tested with kresoxim-methyl were divided into nine isolates with an EC₅₀ value of 0.02 ppm to 0.81 ppm and 32 isolates with an EC₅₀ value greater than 100 ppm. The 22 isolates tested with azoxystrobin were divided into five isolates with an EC₅₀ value of 0.09 ppm to 1.99 ppm and 17 isolates with an EC₅₀ value of 69.2 ppm or more. Thus, the frequency distribution of EC₅₀ for kresoxim-methyl and azoxystrobin had two distinct peaks, considered to be high susceptibility and low susceptibility, respectively. Thereafter, the inhibitory effects of three isolates with low susceptibility in the culture test and one control isolate with high susceptibility were investigated using a bioassay. The protective value for the control isolate was 100 for the two fungicides, whereas, for the three isolates with low susceptibility, the protective values for kresoxim-methyl and azoxystrobin were 0–41.6 and 0–58.8, respectively. Thus, we observed a measurable decrease in the protective values. These results strongly indicate the development of Japanese apricot scab resistance to QoI fungicides in Wakayama Prefecture.