Genetic diversity of Stenocarpella maydis in the major corn production areas of the United States

The fungus Stenocarpella maydis (Berk.) B. Sutton, causal agent of Diplodia ear rot, is a prevalent corn (Zea mays L.) pathogen in the United States. Although S. maydis reduces grain quality, causes yield loss, and can produce mycotoxins in some countries, few studies have examined its biology and genetic diversity. We analyzed the genetic diversity of 174 S. maydis isolates sampled across the major corn production areas in the United States using nine different microsatellites. In all, 55 unique multilocus genotypes (MLG) were observed out of the 174 S. maydis isolates tested. After conducting a Bayesian clustering analysis by STRUCTURE, it was observed that the most probable number of genetic groups was two; however, no separation by their geographical location was identified. According to the minimum spanning network, the S. maydis population is linked across geographic regions of the United States but also contains private genotypes. Temporal diversity in the inoculum source was also observed at one location across 4 years. The haploid stage of S. maydis was confirmed and both mating type genes were amplified among selected isolates with unique MLG. We theorize that, although S. maydis is primarily an asexual fungus, sporadic cryptic recombination may occur, which could contribute to the genetic diversity observed in this study.