Cook potato in water to cover until tender. Pour off liquid to measure 1 c, saving potato for other use. Let potato water cook to lukewarm. In a glass or ceramic bowl that has been scalded, place flour, yeast and sugar; add lukewarm potato water and stir in well. Cover with plastic wrap and pierce with fork to release gases. Place in a warm, draft-free location at an even 85F for 2 days; stir several times daily. (do not let sourdough starter rise above 95F because higher temp are favorable to less desireable microorganisms) Refrigerate until ready to use. Replenish with one c flour and 3/4 c water and let stand overnight or 12 hrs in a warm location before refrigerating again. When replenishing, add lukewarm water with flour. Starter should be at room temp when using in recipes, always after having stood 12 hrs from addition of replenishing flour and water. At least 1 c should remain to refrigerate.