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Comparison of Thiabendazole-Sensitive and -Resistant Helminthosporium solani Isolates from New York. CARMEN L. MERIDA, Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5908. ROSEMARY LORIA, Department of Plant Pathology, Cornell University, Ithaca, NY 14853-5908. Plant Dis. 78:187-192. Accepted for publication 12 October 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0187.

A recent increase in the severity of silver scurf of potato has caused significant economic losses in many potato-producing areas in the United States. The increase in disease severity appears to be due, at least in part, to the development of thiabendazole (TBZ) resistance in the Helminthosporium solani population. We evaluated the TBZ sensitivity, temperature response, virulence, and genotypic variability of H. solani isolates from upstate New York (UNY) counties and Suffolk County, New York, (SNY) on Long Island. The growth of TBZ-resistant isolates was inhibited by 50% at a TBZ concentration of 25 ?g/ ml. Half of the 14 potato tuber samples collected during 1988-1990 in New York (NY) were infected with H. solani isolates that were resistant to TBZ at 25 ?g/ml. When multiple isolates were obtained from the same tuber lot, all isolates had the same TBZ sensitivity, with the exception of one sample. The average growth rate of TBZ-resistant isolates was 75-94% of the sensitive isolates on an unamended medium, depending on the temperature. Spore production had a bimodal temperature response with peaks at 9 and 21C, and did not differ between sensitive and resistant isolates. Virulence on mature tubers, as measured by the area covered with sporulating lesions, did not differ among isolates regardless of TBZ sensitivity or geographic origin. Estimated optimum temperature for radial growth (OG) ranged from 21.7 to 24.9C. TBZ-sensitive and -resistant isolates did not differ in OG. However, isolates collected from UNY and SNY did differ in OG. Restriction fragment length polymorphisms (RFLP), detected with two moderately to highly repetitive sequences from H. solani, indicated that TBZ-resistant H. solani isolates collected from NY are not clonal. Analysis of similarity coefficients indicated no aggregation of isolates based on TBZ sensitivity or geographic origin.

Keyword(s): benzimidazole fungicides