Sensitivity of Cercospora spp. from soybean to quinone outside inhibitors and methyl benzimidazole carbamate fungicides in Brazil

Cercospora leaf blight and purple seed stain are caused by Cercospora kikuchii and other Cercospora spp. The fungus can infect leaves and seeds on soybean, and the disease is controlled with fungicides. In Brazil, the intensive use of fungicides on soybean to control other diseases such as soybean rust has exerted resistance selection pressure on all fungal pathogens that attack in the crop. In this study, we evaluated the sensitivity of 56 Cercospora spp. soybean isolates collected during 9 crop seasons in 9 Brazilian states to quinone outside inhibitors (QoI) and methyl benzimidazole carbamate (MBC) fungicides using mycelial growth inhibition in amended media with a discriminatory dose of 10 μg/mL. We also analyzed single polymorphisms in the target genes cytb and β-tubulin using target genotyping by sequencing by Illumina short reads. Genome-local association was used to correlate the point mutations found in the coding sequence with in vitro assays results. For cytb gene, it was observed only the G143A mutation in 73% of the isolates. The mutation G143A was present in 97% of the isolates classified as resistant at least for two QoI fungicides. The mutation E198A was present in 71% of the isolates being significantly associated with the reduction control to MBC fungicides. Cross-resistance was observed into QoI (azoxystrobin, picoxystrobin, and pyraclostrobin) and MBC (carbendazim, and thiophanate-methyl) active ingredients fungicides. About 97% of Cercospora spp. isolates analyzed possessed the double mutations G143A and E198A and resulting in less sensitive to QoI and MBC fungicides, respectively. All isolates showed homozygous variation in the alternative allele in G143A and E198A mutations. Our results suggest that resistance of the Cercospora spp. from soybean to QoI and MBC fungicides has occurred since the 2008 cropping season in Brazil associated with G143A and E198A mutations.