The difference of sensitivity of Colletotrichum horii causing persimmon anthracnose, which were obtained from infected persimom in the main cultivation areas of Sangju and Sancheong in Gyeongbuk, Yeongdong and Okcheon in Chungbuk, Changwon in Gyeongnam, and Yeongam, Naju and Suncheon in Jeonnam, to DMI fungicides as tebuconazole, difenoconazole, and prochloraz was investigated by using an agar dilution method. C. horii was divided into two subgroups based on 0.07 ㎍/㎖ of EC50 value for tebuconazole and difenoconazole. However, in the case of prochloraz, it was not confirmed to be divided into two subgroups as the two fungicides. While the mean EC50 values of the sensitive group to tebuconazole, difenoconazole and prochloraz were 0.0098 ㎍/㎖. 0.0063 ㎍/㎖, and 0.0016 ㎍/㎖ respectively, those of the resistant group were 0.855 ㎍/㎖, 0.608 ㎍/㎖, and 0.018 ㎍/㎖ respectively. The resistant group showing EC50 values about 100 times higher than the sensitive group was identified. The isolates of C. horii belonging to the resistance subgroup were found to be collected from sweet persimmon farming area.
To illustrate the resistance mechanism, the mutation site of nucleotide and the expression level of CYP51 gene, a target site of DMI fungicide, were investigated. Although there was no difference in the CYP51 gene between the sensitive and resistant isolates of C. horii, the difference in the expression level of CYP51 gene was found between the sensitive and resistant isolates. Especially, when treated with 0.5 ㎍/㎖ of tebuconazole for 2.5 hrs, the amount of CYP51 gene expression of resistant isolate was 70 times higher than in the untreated control. The results of this study suggest that the resistance mechanism of C. horii is closely related to the overexpression of CYP51 gene.
The characteristics of two groups of C. horii showed that mycelial growth and spore formation in sensitive group were superior to the resistant isolates. In order to investigate the competitiveness of the isolates of each group to DMI fungicide treatment, 15GKNJ3 and 15GKYD4, which were resistant and sensitive to DMI fungicide were selected and tested. Each suspension of the sensitive and the resistant isolate adjusted to the same spore density was mixed in equal amounts, and then inoculated on PDA with or without tebuconazole. After incubation for 1 days at 25℃, mycelial colonies, which grew on PDA with or without the indicated concentration of tebuconazole, were selected and inoculated onto 0.5 μg/㎖ tebuconazole PDA to evaluate the rate of sensitive and resistant isolates. As a result, only the sensitive isolates were detected after 3 inoculation on PDA and 0.0005 μg/㎖ tebuconazole PDA, and the ratio of sensitive isolates on the 0.05 μg/㎖ tebuconazole PDA increased as the isolates were continuously inoculated. However, only resistant isolates on 0.05 μg/㎖ tebuconazole PDA were detected at the first generation. Based on the above results, it is expected that and the proportion of resistant isolates of C. horii in fields will be reduced, if the use of DMI fungicides will be reduced.