Understanding and Managing Sudden Oak Death in California

3.2.4. Use of Chemical Protectants to Prevent Infection

Although Phytophthora species are no longer considered to be fungi, chemicals that are used against diseases caused by Phytophthora are still classified as fungicides.  Most fungicides do not kill fungi directly; they inhibit fungal growth (see Sidebar 3-2—Selecting and Using Fungicides).  Hence, fungicides generally work best when disease pressure has also been reduced to the maximum degree possible by using other disease management methods. 

Some fungicides have shown promise against SOD in greenhouse tests and assays on mature trees. However, field studies to determine if fungicides can reduce SOD incidence or severity in mature trees are still ongoing. Results available to date have been mixed and no studies have shown that oaks or tanoaks can be completely protected from SOD by fungicides alone.

Systemic fungicides (see Sidebar 3-2—Selecting and Using Fungicides) have the highest potential to be useful for SOD management because infections may occur over a long time period through the winter and spring.  To be effective, systemic fungicides need to be absorbed by living plant tissues and accumulate in areas that are subject to infection. Areas already affected by cankers will not effectively absorb or translocate fungicides, so applications will be most effective if applied before infections occur. Even if applied well before infection occurs, systemic fungicides may not provide useful suppression of SOD if poor uptake or translocation patterns result in low fungicide concentrations in the bark of the lower trunk.

Chemical application is relatively expensive and needs to be repeated on an ongoing basis to provide continued protection against SOD. Therefore, fungicides are best suited for use on limited numbers of high-value trees in areas where SOD risk is high. Consider the overall disease potential at the site, including the abundance of California bay, and the significance of at-risk trees when deciding whether chemical applications may be appropriate.

Fungicide use considerations differ for tanoak, which can produce P. ramorum spores, and oaks, which are normally infected by spores produced on California bay.