Sudden Oak Death

By Thembi Borras

Sudden Oak Death (SOD) is a forest disease caused by a water mold fungus (Phytophthora ramorum). This fungus will cause SOD in hosts including tanoak, coast live oak and California black oak and a foliar/twig disease in other hosts including California bay laurel.

The most useful diagnostic symptom for Phytophthora ramorum in mature oaks and tanoak is cankers on the trunk from which dark black to red or amber sap exudes. Diagnosis of the disease is not always easy, for example, a tanoak tree may be infected but the symptoms do not show. The sudden browning of the tree crown, for which the "sudden" part of "sudden oak death" was derived, may occur several years after the onset of infection and not all trees end in a sudden browning, some have gradual leaf loss. Black charcoal bubbles, a fungus know as Hypoxylon decays sapwood and may move into a tree weakened by Phytophthora ramorum, but presence of Hypoxylon does not mean the tree has Phytophthora ramorum.

A common diagnostic symptom of Phytophthora ramorum in California bay laurel is dead areas on the leaves, where water collects. No bay laurel trees have reportedly died from this fungus. However, bay laurels are thought to be very important in spreading the disease as the fungus readily produces spores on moist bay leaves. The vectors of Phytophthora ramorum are wind and water.

By answering the following questions you can get an idea if a tree you suspect, is infected with Phytophthora ramorum: Is the tree a host species? Is it located in an infected area? Does it have the symptoms? The only way to be certain that a plant has Phytophthora ramorum is to have a tissue sample laboratory-tested. If you determine a tree is infected, what should you do? It depends, see " A Homeowner's Guide to Sudden Oak Death" at the website http://www.suddenoakdeath.org/. The fungus can be killed by burning infected wood or composting it very well. Moving infected wood can spread the fungus especially if conditions are wet. Remember, when leaving an infected area; disinfect your shoes or other wet muddy transport mechanisms.

The website http://www.suddenoakdeath.org/ is an excellent resource as are the County Agricultural Departments.