Bulletin #2446, Potato Leak (PDF) By Steven B. Johnson, Extension Crops Specialist, University of Maine Cooperative Extension For information about UMaine Extension programs and resources, visit extension.umaine.edu. Find more of our publications and books at extension.umaine.edu/publications/. Potato leak is caused by several species of the soil-borne fungus-like organism, or oomycete, Pythium, with P. ultimum being