COMPARISON OF THIABENDAZOLE-SENSITIVE AND THIABENDAZOLE-RESISTANT HELMINTHOSPORIUM-SOLANI ISOLATES FROM NEW-YORK

A recent increase in the severity of silver scurf of potato has caused significant economic losses in many potato-producing areas in the Uni ted States. The increase in disease severity appears to be due, at lea st in part, to the development of thiabendazole (TBZ) resistance in th e Helminthosporium solani population. We evaluated the TBZ sensitivity , temperature response, virulence, and genotypic variability of H. sol ani isolates from upstate New York (UNY) counties and Suffolk County, New York, (SNY) on Long Island. The growth of TBZ-resistant isolates w as inhibited by 50% at a TBZ concentration of 25 mug/ml. Half of the 1 4 potato tuber samples collected during 1988-1990 in New York (NY) wer e infected with H. solani isolates that were resistant to TBZ at 25 mu g/ml. When multiple isolates were obtained from the same tuber lot, al l isolates had the same TBZ sensitivity, with the exception of one sam ple. The average growth rate of TBZ-resistant isolates was 75-94% of t he sensitive isolates on an unamended medium, depending on the tempera ture. Spore production had a bimodal temperature response with peaks a t 9 and 21 C, and did not differ between sensitive and resistant isola tes. Virulence on mature tubers, as measured by the area covered with sporulating lesions, did not differ among isolates regardless of TBZ s ensitivity or geographic origin. Estimated optimum temperature for rad ial growth (OG) ranged from 21.7 to 24.9 C. TBZ-sensitive and -resista nt 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 fro m 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 ori gin.