Baseline sensitivity of Colletotrichum acutatum isolates from Brazilian strawberry fields to azoxystrobin, difenoconazole, and thiophanate-methyl

Abstract

Anthracnose fruit rot, caused by Colletotrichum acutatum, is an important disease affecting strawberry fields in Brazil where fungicide applications are frequently needed for disease control. Isolates of C. acutatum were collected in conventional and organic strawberry fields in the states of Sao Paulo and Espirito Santo, Brazil, from 2013 to 2015. Sensitivity to azoxystrobin, difenoconazole and thiophanate-methyl was evaluated based on mycelial growth, spore germination, detached fruit assays and molecular characterization of genes targeted by these fungicides. The effective concentration needed to reduce mycelial growth by 50% (EC50) was determined for 78 isolates. Mean EC50 values for isolates collected in organic fields were 0.44 and 0.10 μg/ml, and in conventional areas were 0.62 and 0.09 μg/ml for azoxystrobin and difenoconazole, respectively. Mean EC50 values, determined using spore germination test, for 43 isolates were 0.04 and 0.13 μg/ml for isolates from organic and conventional fields, respectively, for azoxystrobin. Azoxystrobin- and difenoconazole-resistant isolates were not observed. Populations of C. acutatum showed insensitivity rather than resistance to thiophanate-methyl and EC50 values could not be determined. Molecular analyses of the cytb, cyp51 and β-tub genes did not reveal any of the most common point mutations associated with fungicide resistance.