Pythium Leak

Occurs wherever potatoes are grown. This pathogen only enters through wounds. There is usually a distinct line between healthy and diseased tissue. Infected tissue is a smoky grey color. When squeezed, infected tubers produce a dark watery liquid. Following exposure to air, infected tissue changes from grey to brown, then black. In storage, infected tubers are sometimes reduced to empty shells.

Causal Agent:

Pythium ultimum and sometimes other Pythium species.


Wide host range including many crops. Infection of tubers occurs at wounds.


Infection can spread from tuber to tuber during harvest and handling. Infected seed can also spread the pathogen.

Fungicide Resistance:

Pythium has begun to demonstrate resistance to fungicides.


  1. Crop rotation and destruction of diseased tubers are important.
  2. Some fungicides applied at planting or during the growing season can reduce losses caused by Pythium leak.
  3. Harvest with pulp temperatures between 45 and 65 degrees F.
  4. Minimize mechanical injury to tubers during harvest and handling.
  5. In storage, encourage maximum suberization and wound periderm formation; do not allow free water on tubers.

Further Reading:

Pacific Northwest Pest Management Handbooks