Submitted to: International Institute For Beet Research Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: July 2, 1997
Publication Date: N/A
Technical Abstract: Triphenyltin hydroxide (TPTH) has been used extensively for the control of Cercospora leaf spot 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 intensive sampling throughout the region in 1995 and 1996. In 1995, 60% of the leaf spots in the southern most district were tolerant to 0.2 ppm TPTH and 42% tolerant to 1 ppm. 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 tolerant strains further north into districts where the disease is generally less severe and fungicide usage is less. For four of the seven factory districts the frequency of leaf spots tolerant to 0.2 ppm TPTH exceeded 35% and the frequency tolerant to 1 ppm was greater than 15% in 1996.