RESISTANCE TO THIABENDAZOLE IN FUSARIUM SPECIES AND HELMINTHOSPORIUM-SOLANI IN POTATO-TUBERS TREATED COMMERCIALLY IN EASTERN CANADA

During the 1992-1993 and 1994-1995 winter storage period for potatoes (Solanum tuberosum) in Quebec, New Brunswick, and Prince Edward Island , tubers were collected which had symptoms of fusarium tuber rot and s ilver scurf and wh ich had been treated commercially after harvest wit h thiabendazole. Resistance to thiabendazole was detected in isolates of Fusarium sambucinum and Helminthosporium solani but not in isolates of F. avenaceum and F. oxysporum. However, the majority of those farm s surveyed (64%) had adequate disease control with no pathogen isolate d from the diseased tubers. Incidence and EC50 values of resistant iso lates were lower than found elsewhere and the occurrence of fa rms wit h resistant isolates of F. sambucinum (18%) was greater than for H. so lani(7%). For H. solani, EC50 values of resistant isolates were substa ntially less than those found in Alberta. While the study investigated commercial operations employing a wide range of thiabendazole rates ( 6-42 g a.i. t(-1)), no specific trends were detected between the occur rence of resistant isolates and cultivar or thiabendazole application rate.