Characterization of <i>Fusarium</i> Species Composition, Trichothecene Genotype, and In Planta and In Vitro Fungicide Sensitivity of Isolates from Wheat and Corn in Michigan, United States

Mycotoxin contamination of wheat and corn grain from <i>Fusaria</i> is a major agricultural concern. To characterize the population of <i>Fusarium</i> in Michigan, 569 isolates were collected, and species composition, <i>TRI</i> genotype, in vitro fungicide sensitivity, and fungicide field efficacy were determined. In wheat, the <i>Fusarium sambucinum</i> species complex comprised 90% of isolates, of which 82.5% of all isolates were <i>F. graminearum</i>. In corn, the <i>F. sambucinum</i> species complex comprised 40%, with 37% identified as <i>F. graminearum</i>, whereas species from the <i>F. fujikuroi</i> species complex comprised 50%. Within the <i>F. fujikuroi</i> complex, <i>F. awaxy</i> (4.6%) was found and had not previously been identified in the United States. Across <i>F. graminearum</i> isolates, <i>TRI</i> genotypes were found at the following percentages: 92% 15-acetyldeoxynivalenol (15-ADON), 6% 3-ADON, 1.6% NX-2, and no NIV. In vitro mycelial growth sensitivity assays to triazole fungicides demonstrated that <i>Fusaria</i> were more sensitive to metconazole than to tebuconazole or prothioconazole. Species-specific differences in sensitivity were identified across the fungicides tested, with <i>F. tricinctum</i> species complex members significantly less sensitive than <i>F. graminearum</i> isolates, and the <i>F. fujikuroi</i> species complex was significantly more sensitive. Within <i>F. graminearum</i>, 10 isolates had median effective concentration (EC₅₀) values 10-fold greater than sensitive isolates. A subset of these <i>F. graminearum</i> isolates were chosen to investigate if reduced sensitivity in vitro would lead to practical resistance in vivo. Field plots were inoculated with spore suspensions; however, no differences in the relative fungicide efficacy were found, signaling that no demethylation inhibitor fungicide resistance was identified in our collected isolates, despite differences in vitro. Although currently, there may not be practical resistance, monitoring should continue, as there is variation in in vitro sensitivities present within and among species.