|Lamey, H - NORTH DAKOTA STATE UNIV|
|Cattanach, A - NORTH DAKOTA STATE UNIV|
Submitted to: Journal of Sugarbeet Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 1, 1998
Publication Date: N/A
Interpretive Summary: Cercospora leaf spot is the most widespread and destructive foliar disease of sugarbeet. Control measures often combine the planting of moderately resistant hybrids with multiple fungicide applications. In the 1970's and 80's the leaf spot fungus developed resistance to the then prevalent fungicides (benzimidazoles), rendering them ineffective. Consequently, producers switched to triphenyltin fungicides, which have provided adequate control; however, in 1994, strains of Cercospora tolerant to these fungicides were discovered. One year later fungicide-tolerant strains of Cercospora had become predominant in the southern areas of Minnesota and North Dakota. More alarming than the rapid increase of fungicide tolerant Cercospora strains in the southern areas of the region was the rapid increase in the prevalence of tolerant strains further north where the disease is less severe and fungicide use is less. Information on the prevalence and distribution of fungicide tolerant strains presented in this paper will assist extension personnel and crop advisors in recommending appropriate control strategies.
Technical Abstract: Triphenyltin hydroxide (TPTH) has been used extensively for control of Cercospora (Cercospora beticola) leaf spot of sugarbeet (Beta vulgaris) in Minnesota and North Dakota following the development of benzimidazole resistant strains in the early 1980's. The discovery of tolerance to TPTH in 1994 prompted extensive sampling throughout the region in 1995 and 1996. In 1995, 60% of the leaf spots in the southern most district were tolerant to 0.2ppm TPTH and 42% tolerant to 1ppm. By 1996 these frequencies had increased to 83 and 60%, respectively. More alarming than this increase in the southern district was the rapid increase in the occurrence of tolerance further north where the disease is generally less severe and fungicide use is less. In four of the seven factory districts the frequency of leaf spots tolerant to 0.2ppm exceeded 35% and the frequency tolerant to 1ppm was greater than 15%, in 1996.