Tuber Late Blight

Tuber infection begins superficially, but can invade entire tuber. Sporulation can occur on cut or uncut tubers.

Causal Agent:

Phytophthora infestans


Pathogen of potato and a few related plants; infection encouraged by humid and wet conditions.


Sporangia move in the wind; zoospores in water.

Fungicide Resistance:

P. infestans is well-known to become resistant to site-specific fungicides used against it. Fungicides should be rotated frequently to prevent resistance.


  1. Prevention is key.
  2. Harvest during dry weather.
  3. Tuber temperatures going into storage should be less than 68 F.
  4. Mancozeb and metiram fungicides on the soil surface late season may help prevent tuber infection.
  5. Foliar applications of phosphorous acid at harvest and in storage can reduce late blight tuber rot.
  6. Late blight infection often leads to other kinds of tuber rots in storage — it is best to NOT STORE late blight infected potatoes, and there are no chemical treatments that will cure an infected pile of potatoes.

Further Reading:

Late Blight Information from University of Idaho

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