DEVELOPMENT OF RESISTANCE TO THIABENDAZOLE IN HELMINTHOSPORIUM-SOLANI(SILVER SCURF) AS A RESULT OF POTATO SEED TUBER TREATMENT

Applying thiabendazole to potato seed tubers affected with silver scur f caused by Helminthosporium solani sensitive to thiabendazole decreas ed the severity of disease on progeny tubers at harvest, but about 50% of the isolates from these were resistant to the fungicide. The disea se was not decreased when samples of the progeny tubers were treated w ith thiabendazole and planted in the following year, and the incidence of resistant isolates increased. Resistant isolates continued to be p resent when tubers were planted in the next 2 years without fungicide treatment. Treatment with a mixture of thiabendazole and imazalil also decreased the disease and fewer isolates were resistant than when tre ated with thiabendazole alone, although the proportion increased after treatment with the mixture in the following year. When seed tubers we re infected with thiabendazole-resistant H. solani, silver scurf on pr ogeny tubers was not affected by thiabendazole applied to the seed tub ers but was decreased by the mixture of thiabendazole and imazalil. Im azalil was equally effective against H. solani sensitive or resistant to thiabendazole. Some isolates of H. solani had grey aerial mycelium and of 516 of these isolates obtained in 4 years 29% were resistant to the fungicide. Other isolates produced small, black colonies and thei r frequency increased with thiabendazole treatment of seed tubers. Of 244 of these isolates, 62% were resistant.