Field experiment was conducted to evaluate the effects of fungicide spray intervals on the control of potato late blight (Phytophthora infestans (Mont.) de Bary) on three potato varieties at Umbulowacho and Hagereselam during the 2010 cropping season from August to December under rain fed condition with natural infection. The study consisted of two released potato varieties Gudene (CIP 386423.13) moderately resistance, Jallene (CIP-384321.19) moderately susceptible and one local variety(Agazer) highly susceptible to late blight, and three Ridomil-MZ 63.5% WP spray intervals 10, 20 and 30days at the rate of 3kg/ha in a randomized complete block design in split plot arrangement with three replications. Varieties were assigned to the main plots and spray intervals to the sub-plots. Disease development started earlier at Umbulowacho (53days after planting) than at Hagereselam (61days after planting). Generally, it was observed that the effects of application of Ridomil-MZ 63.5% at different spray intervals on the tested varieties were found to be highly significant (P>0.01) on total tuber yield, marketable tuber yields, total tuber number and marketable tuber numbers on both locations.