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Disease Note.

Cercospora beticola Strains from Sugar Beet Tolerant to Triphenyltin Hydroxide and Resistant to Thiophanate Methyl. W. M Bugbee, Research Plant Pathologist, USDA, ARS, Northern Crop Science Laboratory, 1307 18th St. N., Fargo, ND 58105-5677. Plant Dis. 80:103. Accepted for publication 6 November 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-80-0103B.

Strains of Cercospora beticola Sacc, which cause leaf spot on sugar beet (Beta vulgaris L.), developed resistance (growth rate similar on un-amended and fungicide amended media) to benzimidazole-type systemic fungicides in the 1970s (I). The replacement protective fungicide was triphenyltin hydroxide (TPTH). A decrease of disease control with TPTH in 1994 in Minnesota prompted a survey in 1995 to estimate the prevalence and distribution of possible tin-tolerant strains (growth rate greater than sensitive isolates on fungicide amended media, but less than on unamended media). Using a micropipette, conidia from single leaf spots were suspended in 3 µl of water and aliquoted to each of four culture dishes containing potato-dextrose agar amended with 0.2, 1.0 TPTH or 5 µg ml-1 of the systemic fungicide thiophanate methyl (TM), or unamended. Occasionally, colonies from conidia from (he same leaf spot grew on both 1 µg ml-1 TPTH and 5 µg ml-1 TM. Three to six hyphal-tip cultures were established from each of 12 colonies that exhibited resistance to TM and tolerance to TPTH. All of the hyphal-tip colonies (61) grew on 1 µg ml-1 TPTH with mean linear growth that was inhibited 64%, compared with the sensitive isolates, and on 5 µg ml-1 TM with mean linear growth that was inhibited 4%, compared with the sensitive isolates. Of 41 isolates that were sensitive to both fungicides, mean linear growth was inhibited 91% on 1 µg ml-1 TPTH and 100% on 5 µg ml-1 TM. Measures to control leaf spot with these fungicides should proceed with caution in light of this finding of multiple resistance in C. beticola.