The G143A Mutation Confers Azoxystrobin Resistance to Soybean Cercospora Leaf Blight in Bolivia

During the summer of 2017, 38 Cercospora spp. isolates were collected from soybean leaves displaying symptoms of Cercospora leaf blight (CLB) from commercial soybean fields at three locations in Santa Cruz, Bolivia. Portions of cytochrome b (N = 38) and calmodulin (N = 37) were amplified, sequenced, edited, and assembled. Two representative isolates from each lineage (with distinct calmodulin haplotypes, if present) were selected for sequencing of two additional loci, histone 3 and elongation factor 1-alpha, for species identification. Based on the corresponding multilocus phylogenetic analysis, the 37 isolates belong to three species: C. nicotianae (N = 14), C. sp. “P” (N = 16), and C. kikuchii (N = 7). All 38 isolates (100%) possessed the G143A mutation in cytochrome b, and none carried the F129L mutation. Results of phenotypic assays on a subset of 15 isolates (five from each species) supported a high level of resistance to azoxystrobin. This is the first report of the G143A mutation in Cercospora species associated with CLB in South America and the first report of C. nicotianae associated with CLB. Future monitoring for G143A mutants of Cercospora spp. in South America will be necessary to assess whether strobilurin resistance is widespread.