Efficacy of Variable Tetraconazole Rates Against Cercospora beticola Isolates with Differing In Vitro Sensitivities to DMI Fungicides.

Cercospora leaf spot (CLS) of sugar beet is caused by the fungus Cercospora beticola. CLS management practices include the application of the sterol demethylation inhibitor (DMI) fungicides tetraconazole, difenoconazole, and prothioconazole. Evaluating resistance to DMIs is a major focus for CLS fungicide resistance management. Isolates were collected in 1997 and 1998 (baseline sensitivity to tetraconazole, prothioconazole, or difenoconazole) and 2007 through 2010 from the major sugar-beet-growing regions of Minnesota and North Dakota and assessed for in vitro sensitivity to two or three DMI fungicides. Most (47%) isolates collected in 1997-98 exhibited 50% effective concentration (EC) values for tetraconazole of <0.01 μg ml, whereas no isolates could be found in this EC range in 2010. Since 2007, annual median and mean tetraconazole EC values have generally been increasing, and the frequency of isolates with EC values >0.11 μg ml increased from 2008 to 2010. In contrast, the frequency of isolates with EC values for prothioconazole of >1.0 μg ml has been decreasing since 2007. Annual median difenoconazole EC values appears to be stable, although annual mean EC values generally have been increasing for this fungicide. Although EC values are important for gauging fungicide sensitivity trends, a rigorous comparison of the relationship between in vitro EC values and loss of fungicide efficacy in planta has not been conducted for C. beticola. To explore this, 12 isolates exhibiting a wide range of tetraconazole EC values were inoculated to sugar beet but no tetraconazole was applied. No relationship was found between isolate EC value and disease severity. To assess whether EC values are related to fungicide efficacy in planta, sugar beet plants were sprayed with various dilutions of Eminent, the commercial formulation of tetraconazole, and subsequently inoculated with isolates that exhibited very low, medium, or high tetraconazole EC values. The high EC isolate caused significantly more disease than isolates with medium or very low EC values at the field application rate and most reduced rates. Because in vitro sensitivity testing is typically carried out with the active ingredient of the commercial fungicide, we investigated whether loss of disease control was the same for tetraconazole as for the commercial product Eminent. The high EC isolate caused more disease on plants treated with tetraconazole than Eminent but disease severity was not different between plants inoculated with the very low EC isolate.