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

Consortium Members and Partners


Insects, Diseases, Nematodes, & Beneficial Organisms

Find out more information about insects, diseases, nematodes, and beneficial organisms for Potatoes.

About The Northwest Potato Research Consortium

In February 2012 the state potato commissions in Washington, Idaho, and Oregon officially launched a new cooperative effort in research. The aim of this initiative is to increase cooperation and efficiency of the research programs funded by the three potato commissions that total about $1.5 million annually. It will also work toward comprehensive research results reporting process that aims to get useful information to the growers and industry members who need it. This website is a big part of that effort. Research results, integrated pest management guidance, and production information of many kinds will be presented here. For feedback or suggestions on this site, please contact Andy Jensen, Manager of the Consortium.

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