Multiple resistance of Colletotrichum truncatum from soybean to QoI and MBC fungicides in Brazil

Colletotrichum truncatum, the most relevant fungal species associated with soybean anthracnose, is responsible for major losses in the crop. Chemical control via fungicide application is still the most effective strategy for the control of soybean foliar diseases. However, the increase in anthracnose incidence in some regions of Brazil indicates that current chemical control has not been effective against anthracnose. In this study, we assessed the fungicide sensitivity of C. truncatum lineages using isolates representing two important regions of soybean production in Brazil to the fungicides azoxystrobin, thiophanate-methyl, difenoconazole, and fludioxonil. We characterized the molecular resistance to quinone-outside inhibitors (QoI), methyl benzimidazole carbamates (MBC) and demethylation inhibitors (DMI) fungicide groups based on amino acid sequences of the cytochrome b (cytb), β-tubulin gene (β-tub), and P450 sterol 14a-demethylases (CYP51) genes. Multiple resistance of C. truncatum isolates to QoI and MBC was observed associated with mutation points in the β-tub (E198A and F200Y) and cytb (G143A). Alternatively, low EC₅₀ values were found for fludioxonil and difenoconazole indicating high efficacy. Analysis of C. truncatum genomes revealed two potential DMI targets, CYP51A and CYP51B, and higher genetic variability in the CYP51A gene. A slight correlation between genetic differentiation of C. truncatum populations and fungicide sensibility was found (Student’s t-test <0.001). To our knowledge, this is the first report of multiple resistance to QoI and MBC fungicides in C. truncatum in Brazil.