Mechanisms Leading to Thiophanate Methyl-resistant Colletotrichum cereale Isolates Collected from Creeping Bentgrass Putting Greens.

Colletotrichum cereale T.F. Manns in A. Shelby & T.F. Manns, the causal organism of anthracnose, has become a problematic pathogen to manage on creeping bentgrass (Agrostis stolonifera L.) putting greens.  Thiophanate methyl (TM) is a broad spectrum, single-site mode-of-action fungicide that has a high propensity to form TM-resistance within certain turfgrass pathogens.  The C. cereale isolates infecting creeping bentgrass in the southern US had not been examined to determine their sensitivity to TM.  The objectives of this study were to determine if C. cereale isolates collected from the southern US were resistant to TM using in vitro bioassays and to identify amino acid point mutations that lead to TM resistance.  In vitro bioassays were performed in final concentrations of 0, 0.039, 0.156, 0.625, 2.5, and 10 ppm TM.  All the C. cereale isolates collected from creeping bentgrass putting greens were resistant to TM at the discriminatory concentration of 10 ppm TM.  A representative sample of C. cereale isolates was used to examine possible mutations in the beta-tubulin 2 gene.  Amino acid sequences were obtained from PCR-product of six C. cereale isolates.  Five of the resistant isolates had an amino acid mutation from glutamic acid to alanine at position 198.  The resistant isolate, FL A2-1, had an amino acid mutation from phenylalanine to tyrosine at position 200.  A separate study performed by researchers at the University of California Riverside concluded that all TM-resistant C. cereale isolates infecting annual bluegrass in California had an amino acid mutation from glutamic acid to lysine at position 198.