Procedures for Late Blight Samples

The University of Idaho will diagnose both leave and tuber samples for late blight at no charge. Please follow the following guidelines for collecting and submitting samples.

Leaf and Stem Samples

Things you need: Zip-lock or plastic bags, paper towels, distilled or well water (tap water may contain chlorine that will inhibit growth of the fungus), labels, lead pencils, 5% Clorox.


  1. 1. Potato leaf (the entire leaf and not the leaflet) suspected with late blight should be removed from plants. Collect leaves that show dark brown or black lesions, sometimes with a yellow margin or water-soaked halo. Select, if possible, leaves that show white fungal growth on the underside of leaves.

    Note: When looking for symptoms do not mistake symptoms that look like late blight. Early blight usually has concentric rings in the affected area, while mold has lumps of cottony growth or scelorita, and bacterial soft rot may appear watersoaked, wet and slimy.

  2. What is an adequate sample? In case of a spore-bearing fungus, such as Phytophthora infestans, positive identification is possible from even a very small piece of plant material. In a potato field collect 3 or 4 leaves (not leaflets) per location, and a total of 4 locations in the fields.

  3. Place entire leaf or leaves from each location in plastic bags. If numerous leaves are placed in a plastic bag (with no holes) then no further moisture should be provided. If only two or three leaves are placed in the bag then place a moistened (not dripping wet) paper towel in the plastic bag with the leaf material. Do not mat the leaves between the paper towels. When sealing the bag do not squeeze the air out. By leaving air in the bag you help keep the leaves from becoming matted together. Leaves packed in this manner will hold in good condition at moderate temperatures for 2-3 days. Samples collected in this manner can be placed in a refrigerator prior to mailing.

  4. Place a label (written using a lead pencil) inside the bag with the following information:
    Name of grower or farm:
    Field location:
    Variety:
    Date collected:
    Phone number, fax number, or e-mail address:

  5. It is recommended that you deliver the sample in person (keeping the sample in a cooler or at least out of direct sunlight is helpful) to the nearest University extension office, or mail overnight. Avoid weekend mailing.

  6. If you plan to walk into other potato fields immediately, disinfect your boots with 5% Clorox or use disposible protectant covers to avoid spreading the fungus.


Tuber Samples

  1. Collect a representative sample of tubers suspected to have late blight from several areas of the storage (minimum of 10 tubers, maximum of 20). Collect tubers that are showing signs of decay but not those where the decay has progressed to the extent that more than 50% of the tuber has rotted. Tubers that are severely decayed often have secondary rot organisms that can "mask" the symptoms of late blight on tubers and greatly complicate diagnosis.

  2. Carefully wrap the tubers in dry paper towels and place into a paper bag. The paper towels should help absorb some of the moisture and prevent the paper bag from falling apart. We also suggest that the paper bag should then be placed into a leak-proof container such as a plastic bucket or plastic sweater box. Never enclose the tubers in plastic.

  3. Place a label (written using a lead pencil) inside the bag with the following information:
    Name of grower or farm:
    Field location:
    Variety:
    Date collected:
    Phone number, fax number, or e-mail address:

  4. It is recommended that you deliver the sample in person to the nearest University extension office, or mail overnight. Avoid weekend mailing.



Submit samples as soon as possible to the plant disease laboratory closest to you.

  1. Parma Research and Extension Center
    29603 U of I Lane, Parma, ID 83660
    Phone: 208-722-6701
    Attention: Krishna Mohan or Vincent Bijman

  2. Kimberly Research and Extension Center
    3793 North 3600 East, Kimberly, ID 83341
    Phone: 208-423-4691
    Attention:Bob Forster, Carl Strausbaugh, or Nora Olson

  3. Idaho Falls Research and Extension Center
    1776 Science Ctr. Dr., Idaho Falls, ID 83402
    Phone: 208-529-8376
    Attention: Phil Nolte, Melinda Bateman, or Melissa Bertram

If possible, submit samples within 24 hours of collection. If samples must be held overnight, place them in a cool, dry location. Please call before submitting samples.



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If you have any questions or comments, please send e-mail to jhughes@uidaho.edu
All contents copyright 1996. Dept. of PSES, University of Idaho. All rights reserved.
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