The fungus, Pythium ultimum, lives in soil and may cause leak in all potato growing areas in Canada. The disease is particularly troublesome in immature potatoes harvested under warm, moist soil conditions. Leak is a disease of wounded tubers. Symptoms include light to dark brown "watery" lesions on the tuber surface particularly around a cut or bruised area. Water may drip or run freely from diseased tubers, particularly in plastic retail packages kept at high temperatures. Diseased tuber tissue is granular, very watery and may range in colour from cream to black as it is exposed to the air.
A slight pressure on a rotted tuber causes large quantities of juice to run from breaks in the skin. The juice is relatively clear and almost odourless, unlike rots caused by bacteria. There is little or no spread of disease from rotted tubers lying on healthy tubers in a pile.
Leak may, at times, be confused with the smoky grey to black discolouration that occurs in tuber tissue that has been chilled or kept at near-freezing conditions. There may also be some confusion between leak symptoms and black-heart symptoms which are due to a lack of oxygen.