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First Report of Resistance of Helminthosporium solani to Thiabendazole in the United States. C. L. Merida, Department of Plant Pathology, Cornell University, Ithaca 14853. R. Loria, Department of Plant Pathology, Cornell University, Ithaca 14853. Plant Dis. 74:614. Accepted for publication 8 May 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0614A.

Silver scurf of potato (Solanum tuberosum L.), caused by Helminthosporium solani Dur. & Mont., appears to be increasing in the northeastern United States. We suspected the increase could associated with resistance of H. solani to thiabendazole. This fungicide is used for oontrol of other diseases of potato tubers, and H. solani isolates with resistance to thiabendazole have been documented in the United Kingdom (1). Although silver scurf has generally not been a target for thiabendazole applications, this fungicide is effective against H. solani and docs control this disease on potato tubers in storage. H. solani was isolated from tubers with silver scurf collected from five locations in New York State. Eight single-spore isolates were grown on V-8 agar amended with 1, 3.2, 10, 31.6, or 100 mg/ L of thiabendazole, and radial growth was measured after 20 and 40 days at 21 C. Two of the eight isolates tested were resistant to the fungicide. Sensitive isolates did not grow at concentrations greater than 1 mg/L of thiabendazole. Resistant isolates grew at concentrations up to 31.2 mg/L of thiabendazole. The increase in silver scurf on potatoes in the northeastern United States may be due to the selection of thiabendazole·resistant isolates of H. solani.

Reference: (1) G. A. Hide et al. Plant Pathol. 37:377, 1988.