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Abstract
Ten geographically diverse Texas strains of the oak wilt fungus Ceratocystis fagacearum were tested in vitro for their sensitivity to five triazole fungicides based on accumulated linear growth, linear growth rates, and dry weight accumulation in response to fungicide concentrations of 0.1 to 600 parts per billion (ppb). None of the triazoles inhibited growth at 0.1 ppb, but four of the five fungicides were highly effective in totally inhibiting growth on homemade potato dextrose agar medium and in neopeptone broth cultures at minimum effective concentrations (MECs) in the 10–200 ppb range. Triadimefon did not prevent growth at concentrations up to 1100 ppb. The sensitivity of Texas strains to the triazoles was up to tenfold higher in aqueous broth culture than on solid medium. Exceptional strains exhibited tolerance to certain triazoles, requiring MECs up to 500 ppb or higher for total growth inhibition. The specific activity of individual triazoles was related to substituent R-groups attached to chiral carbons. The ketal triazoles, propiconazole and difenoconazole with the dioxolane ring, had the greatest effect on growth rates of the Texas strains. Myclobutanil with nitrilo and butyl R-groups and terbuconazole with dimethylethyl and hydroxyl R-groups had intermediate effects on growth rates, while triadimefon with a dimethyl butanone group had the least effect on growth rates of the fungus. Triazole sensitivity was not linked to specific mating incompatibility alleles. The triazole sensitivity of Texas strains of Ceratocystis fagacearum was much greater than the sensitivity of many other phytopathogenic fungi previously tested. The significance of these results relative to current fungicide application methods used for oak wilt control by the Texas Oak Wilt Suppression Project (TOWSP) are discussed.