THE CONTROL OF SILVER SCURF AND DEVELOPMENT OF THIABENDAZOLE RESISTANCE IN HELMINTHOSPORIUM-SOLANI AS AFFECTED BY THE RATE OF FUNGICIDE APPLIED TO POTATO SEED TUBERS

Seed tubers were immersed in suspensions of thiabendazole (Storite) or thiabendazole plus imazalil (Extratect) at different concentrations a nd planted in a field experiment. During the following 2 years samples of the produce were planted after treatment with the same fungicide f ormulation used on the seed. Silver scurf on daughter tubers decreased as concentrations of fungicide increased and Extratect gave better di sease control than Storite applied at the same thiabendazole concentra tion. Isolates of H. solani resistant to thiabendazole were found on s eed tubers 6 weeks after treatment with Storite, and also on daughter tubers after the first application of the higher rates of Storite; the ir incidence increased with further annual treatments. In 3 years no t hiabendazole-resistant isolates were found after treatment with Extrat ect. Chlamydospores of H. solani developed when sensitive isolates wer e subcultured onto agar containing thiabendazole. Isolates sensitive t o thiabendazole were slightly more sensitive to imazalil than resistan t ones.