Characterisation of sensitivity of Colletotrichum gloeosporioides and Colletotrichum capsici, causing pepper anthracnose, to picoxystrobin

In 2014, 45 isolates of Colletotrichum spp. from Jinzhong City (Shanxi Province, China) that had never used Qo inhibitor (QoI)-type fungicides were characterised for sensitivity to picoxystrobin through growth inhibition tests with 100 mu g/ml salicylhydroxamic acid. The isolates showed similar sensitivity, the EC50 values ranging from 0.021 to 0.197 mu g/ml. Baseline sensitivity was distributed as unimodal curves with mean EC50 values of 0.108 +/- 0.004 mu g/ml for picoxystrobin. In field experiments in 2015 and 2016, 57 isolates of Colletotrichum spp. from four geographical regions of China revealed a mean EC50 value of 1.074 +/- 3.287 mu g/ml, which was 9.94-fold higher than the baseline sensitivity. In these four regions, Qo inhibitors provided control efficacy of about 85%. The isolates with low, moderate, and high resistance to the QoI fungicide picoxystrobin accounted for 3.5%, 7.0%, and 5.3% of all 57 isolates, respectively. However, during continuous asexual culturing of the resistant isolates on fungicide-free medium for ten generations, picoxystrobin sensitivity of low and moderate resistant isolates was restored. Moreover, spore production, mycelial growth rates, and spore germination rates of the resistant isolates did not differ significantly from those of the sensitive isolates. Genetic analysis for mutations showed that picoxystrobin resistance was associated with a point mutation from GGT to GCT at codon 143 in the CytB gene (G143A) in high resistant mutants of C. gloeosporioides, but no point mutation was detected in low and moderate resistant isolates. Positive cross-resistance between picoxystrobin and QoIs (azoxystrobin and kresoxim-methyl) was observed; no cross-resistance between picoxystrobin and other type fungicides (carbendazim and prochloraz) was detected.