Tubers are infected in the potato hill by late blight spores washed by water from infected foliage into the soil.
Tubers are also contaminated with spores of the fungus if they are dug while infected vines still have active lesions with viable spores.

External symptoms appear as slightly sunken brown to purplish areas
of variable size

The internal symptom is a tan to brown granular dry rot
There is no clear separation between the infected and the
healthy tissue
Internal rot develops rapidly
A tuber completely infected with late blight



Often soft rot bacteria invade infected tubers
Bacterial soft rot develops rapidly under warm
humid conditions
Late blight mycelium produced on pre cut seed
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T.G. Lynn
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