POTATO DISEASES: Pythium
There are many Pythium species with varying degrees of host specificity and pathogenicity. Diseases can be expressed as seed decay, pre- or post-emergence damping off and infection of the roots or stems of young plants.
The tuber and seed rot phase of leak is caused by a soil borne fungus, Pythium ultimum.
PYTHIUM SEED ROT SYMPTOMS
Pythium potato rot disease is prevalent in seed raising beds with excess moisture. Infected seeds and seedling roots become water-soaked and quickly decompose. Seedlings quickly display symptoms soon after emergence.
The pythium potato disease is characterized by the extremely wat
ery nature of the infected potatoes seed piece.
Seed that is planted in warm, moist soil will become infected, turn yellowish to brown, and then black. When the potato seed piece is uncovered, it is very watery (but not slimy). In most cases the water is clear, but may also be yellowish or brown. There is no distinct foul odor
PYTHIUM TUBER ROT SYMPTOMS
Damping off of seedlings is expressed as cotyledon and leaf chlorosis then a watery rot appears in the taproot and hypocotyls at or near the soil line. When the roots decompose, the stele is left intact to leave only a white strand, which is followed by seedling death.
Tubers become infected only through wounds, bruises, or insect injuries. Infections do not usually become clearly visible until the tubers are placed in storage.
After two to four weeks in storage, infected tubers become watery and soft. When the tuber is squeezed, clear, yellowish water will be expressed.
The rotted, central portion of the tuber is dark-brown to black.
A sharp line sets off the healthy external tissue from the infected internal tissue. Unless bacterial soft rot has set in, the tuber will not be slimy or foul smelling.
Leak disease is not transmitted from tuber to tuber in storage.
