Sudden Oak Death: The Facts As We Know Them

by Pavel Svihra and Nicole K. Palkovsky, Universtiy of California Cooperative Extension, and Andrew J. Storer, University of California, Berkeley

Sudden Oak Death has killed large numbers of coast live oaks and tanoaks in some of central California’s coastal counties. The name Sudden Oak Death is used because of the rapid color change of leaves from green to brown. Recently an unknown species of Phytophthora fungus was discovered as an underlying cause of Sudden Oak Death. A tree may be infected with Phytophthora for a number of months or years before exhibiting this sudden change in foliage. Basic biological research about this previously undescribed fungus is underway to develop an adaptive, integrated pest management program that will change as new information becomes available. In this Pest Alert we address some of the questions commonly asked by homeowners an landscape professionals. Responses are based on our current understanding of the science explaining Sudden Oak Death, and the information garnered from other disease complexes that involve Phytophthora species

What is Sudden Oak Death?

Sudden Oak Death is a disease complex involving a primary fungal pathogen as well as secondary agents including beetles and other fungi. Usually the first symptom of Sudden Oak Death is ‘bleeding’ or ‘seeping’ of a dark viscous substance produced by the tree in response to the infection by Phytophthora fungus. This fungus infects and destroys the inner bark in the lower trunk of susceptible trees. Bark and ambrosia beetles attack infected trees by tunneling into the bark and wood to excavate egg galleries. Tunnelling activity of beetles and fungal growth may result in reduced water availability to the leaves and accellerate tree death. While the course of the disease remains unpredictable and is sometimes variable, death of the tree is almost certain.

Is Sudden Oak Death the only cause of oak mortality?

No. Many other pathogens can also kill oaks. In particular, the Phytophthora root rot fungus (Phytophthora cinnamomi) and oak root fungus (Armillaria mellea) are common in landscape and garden settings. In addition to these and other pests and pathogens, improper cultural practices such as soil compaction, root pruning, over-watering and herbecide use may contribute to the death of oak trees.

How do you find out whether this new pathogen is on your property?

If any susceptible oak species grows on your property, look for the following symptoms:

Bleeding near trunk base

Bleeding near trunk base

Fine, beetle boring dust

Fine, beetle boring dust

Fungal Domes

This photo shows the growth of Hypoxylon fungus fruiting bodies in the vicinity of bleeding and beetle attacks

Fungal Domes

The beetle boring dust and Hypoxylon fruiting bodies are secondary symptoms that may occur on trees without Sudden Oak Death. Laboratory culturing is the only way to confirm whether a symptomatic oak is infected with the Phytophthora that causes Sudden Oak Death. Information about confirming the presence of the new Phytophthora can be obtained from your local Univerisity of California Cooperative Extension or County Agricultural Commissioner’s office.

Are all oak species susceptible to Sudden Oak Death?

At this time three tree species have been found to be infected with the new Phytophthora species: two oak species, California live oak (Quercus agrifolia) and California black oak (Quercus kelloggii); and one species closely related to oaks, tanoak (Lithocarpus densiflorus). Other oaks, such as valley oak (Quercus lobata), blue oak (Quercus douglasii) and many introduced ornamental oaks, have not yet shown symptoms of Sudden Oak Death.

How does Sudden Oak Death spread?

It is not currently known how the Phytophthora fungus that causes Sudden Oak Death spreads from an infected to a healthy tree. Most of the related species of fungi are spread by soil, water, and infected plant material. A few species are also known to be airborne. As with all pathogens, a susceptible host and favorable climatic conditions are necessary for infection to occur.

What can you do for oaks that do not have symptoms of Sudden Oak Death?

Focus on maintaining oak health through proper cultural practices. Avoid disturbance of the root zone, prevent frequent irrigation, and minimize injuries to the stem and lower limbs. Prune coast live oak and black oaks during the dry summer months when beetles and causal fungus are least active. Limit pruning to dead, dying and structurally unsound branches.

What can you do if trees are infected?

Monitor oaks in urban settings (private homes, gardens, parks and managed landscapes) for the bleeding symptom year round. If the bleeding symptom is detected, consult with a certified arborist, pest control advisor or horticulturist to find out whether the cause is the new Phytophthora species. If the new Phytophthora species is confirmed in a tree in an urban setting, application of insecticides and fungicides registered for commodities (woody ornamentals or hardwood forests) or site may be recommended by a certified arborist or pesticide applicator. Insecticides target beetles, but not the underlying causal fungus and should not be applied from November to mid-March when beetles are inactive.

What should you do when diseased trees are removed?

Try to avoid cutting down dead oaks from mid-October to the end of April. When oaks are cut, chip the branches and split the wood, and leave it on the property. Promote drying of the wood where possible by placing it in a sunny location. To reduce the risk of unnecessary contamination of equipment, do not grind the stump. Tightly covering the stump with clear plastic may help to kill the fungus and reduce infestation and emergence of insects.

Can the fungus be eradicated?

No. Eradication of these types of forest pathogens is biologically and physically impossible.

How can you prevent the spread?

Information from other Phytophthora species indicates that long distance spread occurs via the transportation of firewood and planting material. To help reduce the spread of the new Phytophthora species, avoid transporting infected or potentially infected oak material and soil that is potentially contaminated away from areas where the fungus is known to occur. Do not transport freshly cut firewood or soil out of your neighborhood. These pathogens may also be spread by contaminated equipment (saws, shovels, vehicles, soil, etc.) and in surface and ground water.

Where can you get more information?

Contact University of California Cooperative Extension county offices in Marin (415) 499-4204 and Sonoma (707) 565-2621.

The following websites contain up-to-date information about Sudden Oak Death and links to other relevant sites:

Symptoms & Solutions

Thank you for your great work—as always. Marta, Menlo Park

TCIA Accredited Certified Treecare Safety Professional Quality - Integrity - Confidence ISA Board-Certified Master Arborist International Society of Arboriculture