| dc.description.abstract | North Dakota and Minnesota produce 55% of USA sugarbeet production. Diseases caused by Rhizoctonia solani, Aphanomyces cochlioides, and Cercospora beticola are the major diseases affecting sugarbeet production in North Dakota and Minnesota. Growers mainly use partial resistant varieties and fungicides to manage diseases of sugarbeet. Sensitivity of R. solani and A. cochlioides to fungicides were evaluated in vitro using mycelium radial growth assay and by evaluating disease severity on inoculated plants treated with fungicides in the greenhouse. Phenotypic stability of tetraconazole-resistant isolates of C. beticola after exposure to different temperature regimes was evaluated. For R. solani, mean EC50 values for baseline isolates were 49.7, 97.1, 0.3, 0.2, and 0.9 μg ml-1 and for non-baseline isolates were 296.1, 341.7, 0.9, 0.2, and 0.6 μg ml-1 for azoxystrobin, trifloxystrobin, pyraclostrobin, penthiopyrad, and prothioconazole, respectively. The mean EC50 values of azoxystrobin, trifloxystrobin, and pyraclostrobin increased with a change factor of 6.0, 3.5, and 2.7, respectively. All fungicides at labeled rates effectively controlled R. solani in vivo. For A. cochlioides, tetraconazole, prothioconazole, and pyraclostrobin reduced mycelium radial growth in vitro with mean EC50 values of 3.5, 2.4, and 0.8 μg ml-1, respectively. However, these fungicides were not effective at controlling A. cochlioides in vivo. Sugarbeet plants up to three weeks old were found susceptible to A. cochlioides. Resistant isolates of C. beticola had no fitness penalty as measured by spore production, spore germination, mycelium radial growth, and disease severity after exposure to different temperature regimes. 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 with a factor of change of 38.6 and 32.8, respectively. This research indicated that R. solani sensitivity to the evaluated QoIs had decreased, but they were still effective at labeled rates under greenhouse conditions, and rotation of different fungicide classes could be a useful strategy to manage fungicide resistance. No fitness penalty was found after exposure of C. beticola isolates to cold treatments. However, C. beticola isolates resistant to tetraconazole became more sensitive to this fungicide after exposure to cold treatments. | en_US |
