High Quinone Outside Inhibitor (QoI) Resistance Risk in Colletotrichum Species Causing Strawberry Crown Rot in Korea

Anthracnose crown rot caused by Colletotrichum spp. is one of the most important diseases of strawberry (Fragaria × ananassa) in Korea. Single spore isolates of 72 Colletotrichum species collected in 2018, 2020, and 2023 from different locations in Chungbuk, Chungnam, Gyeongbuk, Gyeongnam, and Jeolla regions were characterized using a multi-locus sequence typing (MLST) of four genetic loci (GAPDH, ACT, TUB2, and ITS). Isolates were identified as C. fructicola (n=64) and C. siamense (n=8). Quinone outside inhibitors (QoIs) active ingredient pyraclostrobin is utilized to manage anthracnose crown rot in the field. Sensitivity to pyraclostrobin were assessed on potato dextrose agar using a mycelial growth inhibition assay at 20, 4, 0.8, 0.16, and 0.032 μg/ml fungicide concentrations. Our results demonstrated that all the 72 Colletotrichum spp. isolates were not inhibited by pyraclostrobin at all the tested fungicide concentrations, with EC50 ranging between 13 and >100 μg/ml. Sequences of the partial cytochrome b (cyt b) gene flanking the mutation site revealed the presence of the G143A mutation in all the isolates. No other site modifications were detected in the cyt b gene. Cross resistance assays with 10 selected isolates involving EC50 values of pyraclostrobin, mandestrobin, trifloxystrobin, and kresoxim-methyl showed positive correlations among these fungicides. These findings suggest a high risk of resistance development associated with using QoI fungicides to control the anthracnose crown rot of strawberries in Korea.