Sudden Oak Death, or SOD, is a tree disease caused by the plant pathogen Phytophthora ramorum. The disease kills some oak species and has had devastating effects on forests in California and Oregon.
If oaks dominate the site and are the preferred species, consider removing California bay laurels within 15 feet of the trunks of valued oaks, as California bay laurels greatly contribute to disease spread. Keep in mind that bays are important for many wildlife species, and should the oaks be lost, bay trees may be the only remaining mature trees. Combining bay removal with chemical treatments may be a viable option if the oaks are very high-value and removal of the California bay laurel will not diminish landscape value.
The California Department of Pesticide Regulation approved a special registration for Agri-Fos fungicide in October 2003. It is currently the only chemical treatment approved by the State for use against sudden oak death Phytophthora ramorum infections on oaks and tanoaks. The compound is best used as a preventative measure and is NOT A CURE, but it can help protect trees from getting infected, as well as suppress disease progression in very early infections.
The phosphonate compound may be injected or mixed with a surfactant and sprayed on the trunk for absorption through the bark. Both application methods take four to six weeks for the material to be assimilated by the plant. So, it is recommended that initial applications be applied either in the fall after temperatures drop (usually November to early December) or in the spring after new leaves emerge (late March to April).
In the first year, the treatments should be applied once in the fall and repeated again in the spring or vice versa. Every year thereafter, the treatment should be administered in the fall. (Note: if the first treatment is administered in the fall, and the second is administered in the spring, then the first follow-up annual treatment should begin in 1.5 years in the fall and then annually in the fall thereafter.)
Since sudden oak death treatments must be applied to healthy trees, and the pathogen’s distribution and activity is patchy and somewhat unpredictable, it is difficult to determine which trees need to be treated. Trees under consideration for treatment should be within a few miles of a known infestation.
Generally, you should treat healthy, high-value oak or tanoak trees within 150 feet of other infested plants. You may want to treat healthy, high-value oaks or tanoaks if they are surrounded by healthy California bay laurel and there are known infections between 150 feet and 1000 feet away. Treatment is not recommended in areas where infested plants are not already present.
Although these treatments are best used as a preventative approach, it may be possible to prolong the life of trees already infected by Phytophthora ramorum. Research results indicate that treatments are effective only if trees are treated within the first two months of infection. Treatment of trees having displayed symptoms for six months or longer is not recommended.
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