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Fitness of tetraconazole-resistant isolates of Cercospora beticola after exposure to different temperature regimes
M. F. R. KHAN (1), S. Arabiat (2). (1) North Dakota State Univ & Univ of MN, Fargo, ND, U.S.A.; (2) North Dakota State University, Fargo, ND, U.S.A.

Cercospora leaf spot (CLS) caused by <i>Cercospora beticola</i> is the major foliar disease of sugar beet in North Dakota and Minnesota. Growers use fungicides to manage CLS and tetraconazole-resistant isolates of <i>C. beticola</i> have been reported. Phenotypic stability of tetraconazole-resistant isolates of <i>C. beticola</i> after exposure to different temperature regimes, -20ºC (for 4 weeks), 4ºC (for 4 weeks), 20ºC (for 4 weeks), -20ºC (for 2 weeks) to 4ºC (for 2 weeks), -20ºC (for 1 week) to 4ºC (for 1 week) to -20ºC (for 1 week) to 4ºC (for 1 week), and -20ºC (for 1 week) to 20ºC (for 1 week) to -20ºC (for 1 week) to 20ºC (for 1 week) was evaluated. Resistant isolates of <i>C. beticola</i> had no fitness penalty as measured by spore production, spore germination, mycelium radial growth, and disease severity. However, isolate 09-347 resistant to tetraconazole reverted to a moderate resistance level after exposure to -20ºC, and -20ºC to 4ºC to -20ºC to 4ºC. It may be important to use effective non-triazole fungicides in the first application to control overwintering population of <i>C. beticola</i> triazole-resistant isolates, and use triazoles later in the season so as to prolong the usefulness of this class of fungicides.

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