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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:
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.
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.
A fresh, whole plant sample is ideal for any test in a plant disease diagnostic lab.
Getting the Samples
Sending the Samples
Click the link at the bottom of the page for additional information about what happens at the lab once a plant sample is submitted.
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.
How to Submit a Sample at Oregon State University Plant Clinic.