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Sensitivity to eight fungicide chemical groups of Colletotrichum fioriniae, the cause of anthracnose of pistachio in California

Paulo Lichtemberg: University of California - Davis

<div>In 2016 and 2017, pistachios of cv. Red Aleppo showed severe anthracnose symptoms in fruit, leaves, and rachises in a few orchards located in Glenn County, California. In total, 39 isolates identified as <em>Colletotrichum fioriniae </em>were assessed for multiple resistance to different fungicide groups such as the quinone outside inhibitors (QoIs) azoxystrobin and trifloxystrobin, the demethylation fungicide inhibitors (DMIs) metconazole and propiconazole, the succinate dehydrogenase inhibitors (SDHIs) fluopyram and penthiopyrad, and the anilino pyrimidines (APs) cyprodinil and pyrimethanil. Results indicated that DMI fungicides consistently inhibited mycelial growth at dosages such as 0.37 µg/ml (ranging from 0.10 to 0.71 µg/ml) for metconazole and 0.79 µg/ml (ranging from 0.34 to 1.41 µg/ml) for propiconazole. The EC<sub>50</sub> value for SDHI penthiopyrad was 0.09 µg/ml (ranging from 0.04 to 0.28 µg/ml) and no inhibitory effect was observed for fluopyram. The AP fungicides cyprodinil and pyrimethanil did not inhibit mycelial growth. The QoI sensitivity values for azoxystrobin ranged from 83.81 to 1,000.88 µg/ml (avg. 647.82 µg/ml) and for trifloxystrobin from 108.35 to 956.39 µg/ml (avg. 609.13 µg/ml). Molecular characterization of the <em>cyt b </em>gene revealed that all 37 isolates tested had the mutation G143A. A protective fungicide efficacy test performed after wounding detached pistachio leaves of cv. Red Aleppo revealed that metconazole and penthiopyrad can best control <em>C. fiorioniae </em>lesions. The least control was obtained with fluopyram and trifloxystrobin fungicides. These results will certainly support future studies on resistance and resistance management of pistachio anthracnose in California.</div>