Eucalyptus leaf discs for baiting Phytophthora are placed onto a selective agar culture medium to test for the presence of the pathogen (the thread-like rings around the leaf discs). Photo by Dr. Robert Wick, University of Massachusetts.
Check whether your state university extension plant diagnostic lab can run the sample. For example, the University of Massachusetts Plant Diagnostic Clinic plates out water samples to test for Pythium and Phytophthora to the genus level.
You can also send a sample to the University of Guelph, which does a DNA fingerprint-type analysis for a range of pathogens down to the species level.
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