Pythium root rot

Pythium root rot. Pythium attacks juvenile tissues such as the root tip. After gaining entrance to the root the fungus may cause a rapid, black rot of the entire primary root and may even move up into the stem tissue. As the soil dries, new roots may be produced and the plant may recover or never show symptoms of disease. Under wet conditions brought about by poor soil drainage or excess irrigation, more and more roots are killed and the plant may wilt, stop growing, or even collapse and die. Bulbs of susceptible plants turn black, gradually desiccate, and form a hard mummy.

Pythium leak. 
Early Symptoms. Very moist, gray or brown lesions form around wounds or near stem end. Internal starch is breaking down thus cutting the tuber shows a grayish creamy inside which darkens to black upon exposure to air. [note - the blackened tissue looks like blackheart, a physiological disorder, but leak is watery.] A noticeable vinegar-like smell develops; for me this is a really distinguishing symptom.
Later Symptoms. A liquidizing of the tuber's inside is very noticeable and cavities may form. The skin will remain intact, a papery shell, unless ruptured ("shell rot"). Also very noticeable is a strong stench much like rotting fish. No mold is usually seen unlike Fusarium rots. [note - The black-bordered creamy cavities are similar to bacterial soft rot (blackleg), but leak is not slimy and it has that smell. In many cases, leak and bacterial soft rot will occur together since both are favored by the same conditions.]