Foliar Late Blight

Leaf infections show areas of dead or dying tissue surrounded by a pale halo. Lesions are not delimited by leaf veins. Also, note the whitish sporulation of the pathogen around the dead tissue.

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. Manage volunteer potatoes and cull piles.
  3. Plant healthy seed.
  4. Use a seed treatment containing mancozeb or other preventive fungicide.
  5. Treat with foliar fungicides according to University recommendations.
  6. Monitor fields carefully for late blight infections, especially early in season.
  7. Avoid planting potatoes in ground that is expected to be excessively wet, such as pivot centers and pivot overlaps.

Further Reading:

Late Blight Information from University of Idaho

Pacific Northwest Pest Management Handbooks

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