Abstract

Triphenyltin hydroxide (TP'I H) has been used exten­ sively for control of Cercospora (Cercospora betiola) leaf spot of sugarbeet (Beta vulgaris) in Minnesota and North Dakota foll owing the development of benz­ imidazole resistant strains in t he early 19808. The discovery oftolerance to TPTH in 1994 prompted ex­ tensive 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 Ippm. By 1996 these frequencies had increased to 83 and 60%, respectively. More alarm­ ing than this increase in the southern district was the rapid increase in the occurrence of tolerance fu rther north where the disease is generally less severe and fungicide use is less. In four of the seven factory dis­ tricts the frequency of leaf spots tolerant to O.2ppm exceeded 35% and the frequency tolerant to 1 ppm was greater than 15%, in 1996. Resistance to thio­ phanate-methyl, a benzimidazole-type fungicide, per­ sisted in local populations even though TPTB has been the predominant fungicide for control of Cer­ cospora leaf spot for about 15 years.

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