Confirming the Phytophthora Diagnosis

Once you make a preliminary diagnosis of Phytophthora, laboratory tests will be needed to confirm it. These tests may be done on site, if your nursery has the resources, or you can send samples to a public or private lab. (If the species of Phytophthora is important, only certain labs can do the tests.)

Growers can buy and use diagnostic test kits (also called ELISA kits) in the field. Kits all work on the same principle: they detect proteins that Phytophthora produces.

Only plant tissue that has been colonized by Phytophthora will give a positive reaction. Discolored or recently killed tissue from leaf spots or root or crown rots works nicely.

A negative reaction can occur for two reasons:

  • Phytophthora did not cause the problem and was not in the tissue sample
  • Phytophthora did cause the problem but was not in the tissue sample. This is known as a “false negative.” For example, the plant has Phytophthora root rot, but the test sample was taken from the leaves.

Testing for Phytophthora,a positive reaction from a test kit is accurate about 85% of the time. Kits miss Phytophthora about 4% of the time and give a “false positive” reading about 11% of the time.

Test results, combined with other information, will help determine the type of disease management plan needed.

Kit Testing: Getting the Right Sample

Getting the proper plant sample is vital to the success of any test. Here are steps to taking the proper sample for each type of disease.

The Correct Sample for Leaf Spots
Cut out a small piece of the discolored leaf tissue, including the margin of the leaf spot. Do not include much of the green part of the leaf.

The Correct Sample for Shoot Diebacks
Use the portion of the stem closest to the advancing margin of discolored tissue. Use mostly discolored and recently killed tissue. Do not include much of the healthy-looking, green part of the stem.

 

The Correct Sample for Root Crown Rots on Woody Plants
The best place to sample is from the cambium, the layer just under the bark. With a pocketknife, peel off the bark near the root crown and look for an area with healthy, whitish tissue next to diseased, brown tissue. Use the brownish tissue for your sample. Be careful not to include too much bark or uninfected wood in your sample; both can dilute the sample and interfere with a positive reaction.

The Correct Sample for Root Rots
Remove as much soil or potting medium as possible from the roots. Identify the advancing margin of infection. For the sample, remove brown and discolored roots near the advancing margin. If the root system is very small, as with seedlings, use the entire system.

Laboratory Testing: Getting the Right Sample

A fresh, whole plant sample is ideal for any test in a plant disease diagnostic lab.

Getting the Samples

  • Collect samples early in the week.
  • Select several samples showing various stages of disease, especially early symptoms.
  • Get all parts of the plants, including roots if practical.
  • For field-grown plants, enclose roots and soil in a plastic bag by themselves. Use another plastic bag to enclose the rest of the plant.
  • For container plants, bag the entire plant. If it is not practical to send the entire plant, bag several affected portions making sure to include the transition zone between diseased and healthy tissue on stems and branches.

How to collect a disease plant specimen

"How to collect a disease plant specimen" by J. Pscheidt and L. Long, OSU Extension

Sending the Samples

  • Keep plants cool and moist before shipping. Samples that become too warm or dry out are harder to diagnose accurately.
  • Pack samples in a sturdy container, to prevent crushing in transit.
  • Include inside the container a waterproof, sealed bag with a Plant Disease Form which will have all of your preliminary diagnostic information.
  • Ship early in the week, as soon as possible after collecting.

Click the link at the bottom of the page for additional information about what happens at the lab once a plant sample is submitted.

What to do next

Although these various test results are helpful to determine if a plant is infected with Phytophthora they should not be used exclusively. Symptoms, patterns in the field or container yard and time development of the damage should all be integrated together with test results to make the Phytophthora diagnosis.

It is highly important to understand if your plant problem is actually caused by Phytophthora or not. If the plant problem is not caused by Phytophthora then a Phytophthora specific management plan is not likely to work and may lead to more plant loss. Once you are sure a plant problem is caused by Phytophthora you can begin to craft a successful management plan to control the disease. The next module will focus on management of Phytophthora diseases.

For Additional Information visit

How to Submit a Sample at Oregon State University Plant Clinic.