|RIVERA-VARAS, VIVIANA - North Dakota State University|
|DEL RIO MENDOZA, LUIS - North Dakota State University|
|KHAN, MOHAMED - North Dakota State University|
|SECOR, GARY - North Dakota State University|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2012
Publication Date: 12/1/2012
Citation: Bolton, M.D., Rivera-Varas, V., del Rio Mendoza, L.E., Khan, M., Secor, G.A. 2012. Efficacy of variable tetraconazole rates against Cercospora beticola isolates with differing in vitro sensitivities to DMI fungicides. Plant Disease. 96(12):1749-1756.
Interpretive Summary: Leaf spot, caused by the fungus Cercospora beticola, is a major disease of sugar beet. The disease is managed most effectively with the timely application of fungicides. A class of fungicides called sterol demethylation inhibitors or DMIs are widely used to control leaf spot. However, isolates of the fungus are known to become resistant to DMIs and therefore fungicide resistance management is a key component in disease management. To assess resistance is in field populations of C. beticola, more than 3,500 isolates were collected over four years from major sugarbeet growing regions of MN and ND and were assessed for DMI fungicide resistance using the EC50 technique. There was year to year variability for each DMI fungicide tested, but there was an increase in the percentage of isolates with high EC50 values. To help understand what a high EC50 means in terms of fungicide resistance in the field, we inoculated three isolates with low, medium, and high EC50 values and applied fungicide at the recommended field rate and dilutions of the field rate of Eminent. The resistant isolate caused more disease than isolates with medium or low resistance values at high fungicide rates, but all three isolates cause similar disease at low fungicide rates. We compared tetraconazole and Eminent and found that resistant isolates caused more disease on plants treated with tetraconazole than Eminent, but disease pressure was no different between plants inoculated with the sensitive isolate. The results of this suggest that monitoring fungicide resistance using the EC50 technique is valid and important, particularly
Technical Abstract: Leaf spot of sugar beet is caused by the fungus Cercospora beticola, and evaluating resistance to sterol demethylation inhibitor fungicides is major focus for leaf spot disease management. More than 3,500 isolates were collected over a four year period from the major sugar beet growing regions of MN and ND and assessed in vitro for sensitivity to three DMI fungicides. In general, isolate reaction was similar to the three fungicides tested. There was year-to-year variability in the average EC50 value for each fungicide, but the percentage of isolates with EC50 values > 1.0 µg/ml increased from 2007 to 2010. To explore the relationship between EC50 and loss of fungicide efficacy in planta, 12 isolates exhibiting a wide-range of sensitivities to tetraconazole were inoculated to sugar beet. No relationship was found between tetraconazole sensitivity and aggressiveness. Isolates with low, medium, or high levels of resistance to tetraconazole having similar aggressiveness were inoculated to sugar beet plants in the greenhouse and subsequently sprayed with various dilutions of Eminent, the commercial formulation of tetraconazole. The resistant isolate caused more disease than isolates with medium or low resistance values at high fungicide rates, but all three isolates cause similar disease at low fungicide rates. Resistant isolates caused more disease on plants treated with tetraconazole than Eminent, but disease pressure was not different between plants inoculated with the sensitive isolate.