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POSTERS: Fungicide and antibiotic resistance

Azoxystrobin sensitivity testing in Rhizoctonia solani AG2-2 populations affecting Michigan sugar beet
Jaime Willbur - Michigan State University. Linda Hanson- USDA ARS, Douglas Minier- Michigan State University, Chris Bloomingdale- Michigan State University

Rhizoctonia root and crown rot (RRCR) is caused by Rhizoctonia solani AG 2-2 and continues to be a major pest of sugar beets. In Michigan, azoxystrobin (Quadris) is widely applied to manage RRCR. Azoxystrobin, a quinone outside inhibitor, targets a single site to inhibit fungal respiration and so possesses a high risk of fungicide resistance development. Continued reliance on one to two applications of this product per season has prompted investigations of azoxystrobin sensitivity in Michigan R. solani populations. Isolates were collected from research and commercial fields in the Michigan sugar beet growing region. Two additional baseline isolates, collected prior to azoxystrobin use in sugar beet, were included for comparison. Isolates were screened in half-strength clarified V8 agar amended with salicylhydroxamic acid at 10 µg ml-1 and azoxystrobin at concentrations: 0, 0.01, 0.1, 1, 10, and 100 µg ml-1. After 96 hr, mycelial mats were removed from media, air dried for 48 hr, and mass was measured. Percent inhibition was calculated and effective concentrations for 50% inhibition of colony mass were determined using three-parameter logistic regression. In a preliminary screen, azoxystrobin insensitivity was not observed in Michigan R. solani populations, however, additional testing is ongoing. Evaluation of azoxystrobin efficacy, and potential alternatives, for RRCR management will be conducted at the Saginaw Valley Research and Extension Center in Frankenmuth, MI.