PERSISTENCE AND EFFICACY OF THIABENDAZOLE ON POTATOES FOR CONTROL OF SILVER SCURF

Potato samples were taken randomly from four commercial storages 250-2 65 days after treatment with thiabendazole at the recommended rate of 40 g of ai/ton for control of silver scurf (Helminthosporium solani). The mean concentrations of surface residues varied greatly among the s amples, ranging from 1.02 +/- 1.46 to 28.6 +/- 5.4 ppm (n = 10). There was no direct correlation between the occurrence and the severity of the disease with the levels of surface residues on the potatoes. The t emperature and relative humidity of the storages appeared to be import ant. In potatoes treated with thiabendazole using proper technique, at 1/2, 1, 2, and 4 times the recommended rate and stored at 7-degrees-C with 95% relative humidity, the surface residue decreased by 57-67 % in about 84 days and persisted thereafter for 154 days (34 weeks total ). Colonies of H. solani, isolated from infected potatoes, consisted o f strains that were susceptible, tolerant, or even resistant to thiabe ndazole. The existence of mixed populations of the pathogen may be the cause of inconsistent control of the disease with this chemical in th e recent past.