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Washington State University
WSU Puyallup Ornamental Plant Pathology

Testing potted plants, soil, and water for Phytophthora

Root disease symptoms consist of wilting, yellowing foliage.

Phytophthora can be present in potted plants and spread to the landscape when these plants are installed. Pathogenic soilborne Phytophthora species damage the fine roots of the plant before moving into the woody roots. Symptoms of wilting followed by death of the plant during a dry period will not become apparent until much of the root system is destroyed, sometimes several years after planting.

You can use one of the techniques below to test soil, potting media, and water for Phytophthora. You will need healthy rhododendron leaves that are free of any symptoms such as leaf spots. The best varieties of rhododendron are those that do not have any surface features such as indumentum (fuzz) or scales. You will also need Phytophthora immunostrips or a similar test kit.

Technique for testing potted plants for Phytophthora spp.

Soil testing

Water testing

If the test strip reads positive for Phytophthora, quarantine the plant and observe for symptom development. A positive test may indicate the presence of any Phytophthora species. This is something to consider when testing surface water or field soil, as many of the Phytophthoras present in these environments are not aggressive pathogens.