The organism that causes Sudden Oak Death (SOD) disease is increasing in some areas, appearing in new areas, and contracting in others, like a living blob inhabiting our state.
That is the impression I got when I attended the 2014 SOD Blitz Results meeting in Santa Rosa last week when Dr. Matteo Garbelotto, director of the U.C. Berkeley Forest Pathology and Mycology Lab, presented the results to some of the 514 people who sampled 2,125 California bay laurel trees in 19 local SOD Blitzes in California this year.
Beginning in 2008, the Lab and local volunteers have teamed up every year to seek, map, and help people to manage Phytophthora ramorum, the organism that causes SOD. It has killed more than three million trees in 15 coastal California counties, from Monterey to Humboldt.
The disease kills coast live oak, California black oak, tanoak, Shreve oak, and canyon live oak, but the lab found that the best way to monitor for the disease is to test symptomatic leaves from California bay laurel trees.
The good news is that, in some areas, the drought is taking back what started to get established, Dr. Garbelotto said. The “biggest surprise” was in Big Sur, where the cool coastal climate strongly favors the disease. Of 200 trees sampled only 8 were positive.
Here in Napa, a small group of volunteers submitted 36 samples. All tested negative.
As to other parts of the state, there was a single find in the Sierra foothills. Fortunately, it was a single Rhododendron plant, which has been removed. There is a possible positive on the north side of Mt. Diablo, heavy increases in Sonoma County east of Petaluma, positives in the Alexander Valley area, and surprising new findings in Gilroy.
North Berkeley, not far from the lab, is “a haven” for Phytophthora ramorum, Garbelotto said, while San Luis Obispo had all negative results again this year.
Volunteers can check their own sample results by going to the matteo lab website:
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http://nature.berkeley.edu/garbelottowp/?page_id=1458 and looking at the Excel File. You will see your city, your initials and your results.
The same web page also provides a portal to the 2014 SOD Results Map. Positives appear as a red tree icon; negatives are green.
With respect to SOD management, Dr. Garberlotto recommends that property owners first determine the level of risk of infection. You can do this by downloading the SOD map at www.sodmap.org, or downloading the free app: Sodmap Mobile to your smartphone. If these maps show positive finds within 1,000 meters of your oaks, the risk is “Moderate.” Within 200 meters, the risk is “High.”
If the risk rating says “Insufficient Data”, you should monitor by participating in the next blitz in your area.
Risk is managed, in part, by removing the greatest source of infection: California bay laurel trees that are near oaks.
If the risk is “Moderate” or ‘High”, recommendations include doing any significant dead tree removal, pruning, grading, and removal of key bay laurels (not all bay laurels!) in late summer to fall, when infections are less likely. (Infections occur primarily in wet spring weather.)
Bay laurel management recommendations respect the importance of large specimen bays as well as watershed erosion concerns. They include removal of bays only up to 20 inches diameter that are not on steep slopes and not within 10 yards of a stream.
Protective phosphonate applications (AgriFos or Reliant) should be made between Halloween and Christmas. Oaks are actively absorbent in fall and they need time to build up their defenses before the spring infection period.
The ongoing Blitzes continue to provide valuable information on a large as well as local scale. They show us that the disease is still expanding even if it may have contracted locally during the drought.