Accurate disease diagnosis can be difficult because the symptoms caused by Phytophthora ramorum are very similar to those caused by other fungi, insects, or adverse environmental conditions. The only way to confirm a P. ramorum or Sudden Oak Death infection is to take a sample and analyze the affected plant tissue in a laboratory.
There are two published diagnostic guides (Wildland Diagnostic Guide and Nursery Diagnostic Guide) to help you in assessing the likelihood of a P. ramorum infection on a plant and whether to send a sample to the laboratory for a confirmed diagnosis. The following question key can also be used to assist you, as well as the Misdiagnosis page.
Step One: Where Do You Live?
Is the tree in a county affected by P. ramorum ?
Infested Counties in California
|Contra Costa||Monterey||Santa Cruz|
Counties not known to be currently infested, but are of concern include: Del Norte, San Luis Obispo and San Benito.
Step Two: Identify Type of Plant Affected
The types of oaks affected are:
California black oak (Q. kelloggii)
canyon live oak (Q. chrysolepis)
coast live oak (Q. agrifolia)
Shreve oak (Q. parvula var. Shrevei)
tanoak (Lithocarpus densiflorus)
For help identifying oaks, visit the website of Steven Swain (UC Cooperative Extension, Marin County) on Oak Diagnosis, and the UC Davis Oak Woodland Management website. Also see the Host Plant List to see other types of plants that can be affected by P. ramorum.
Step Three: Check for Symptoms
On an oak:
- Bleeding from the bark separate from wounds or cracks.
- Tree is dead – make sure it is not just defoliated.
On foliar hosts:
- Are there California bay laurels or other foliar hosts nearby with dead tips or twig die back?
- In general, are there dead spots on the leaf, irregular in shape and large in relation to the size of the leaf, sometimes killing petiole and twig (varies by species).
- Check the plant symptoms photos to help identify the disease.