For the eighth year in a row, Napa “citizen scientists” participated with the UC Berkeley Forest Pathology and Mycology Lab in an ongoing effort to track Sudden Oak Death (SOD), the exotic disease that has killed hundreds of thousands of oaks and tanoaks in California.
The Napa SOD Blitz was held on May 19, and the results came out this week. It looks like Napa is at low risk for the disease, depending on location in the county. Some areas provide more conducive conditions for the disease than others. And it looks like I have the dubious distinction of submitting the only positive sample for Napa this year.
The disease is caused by Phytophthora ramorum, a microscopic organism that spreads during periods of relatively warm, rainy weather in late spring in Northern California, says the lab’s website. Blitz volunteers take samples of symptomatic leaves from California bay laurels, the main vector for the disease.
In the previous year, Dr. Matteo Garbelotto, the Berkeley lab director, had correctly predicted increased infection rates because of persistent rains that winter. This year, the infection rate showed a sharp decline, probably reflecting the reduced rainfall in the winter and spring of 2017-2018 compared to the previous year.
In 2017, the volunteers collected 14,379 samples over all. In 2018, the total was 13,504. Looking at how the numbers break out by County, the greatest number of samples was taken in the San Francisco Peninsula West: 3,558. San Francisco and Marin followed at 2,982 and 1,073 respectively.
The lowest numbers of samples were here in Napa (53), the East Bay South (54), and the East Bay East (56).
As to infection rates, the over all rate of positives for the whole project this year was only 3.5 percent, down from 12.8 percent in 2017. Napa came in this year at 1.5 percent.
As to my own sampling, for the third year in a row I revisited an area I thought would be ripe for infection. And for the third year in a row, all the samples were negative for that area, the recreation trail along the Napa River from Lincoln Avenue to Trancas Street.
I collected the sole positive here in Napa from a cluster of California bay laurels off lower Wild Horse Valley Road, not far from previous-year positives on Green Valley Road and the Skyline Park region.
The low infection rate is good news for Napa, especially after all the fire destruction in October 2017, and the horrible outbreak of foliar diseases — anthracnose and powdery mildew this year. But sampling was limited by the small number of blitz volunteers.
Over the years, the positives in Napa have tended to be in remote, wooded areas like the southeast portion of the County and the western, mountains.
For California at large, looking at the Google Earth SOD map (available through the matteolab website), where the positives are shown as red tree icons, the disease appears concentrated approximately along the coast from San Luis Obispo at the southern limit and northward to about 15 miles north of Garberville.
Considering disease prevention and management, and before cutting down bay laurels, the biggest carriers of the disease, it is crucial to first assess risk. The free app “SOD Map Mobile” helps with that. Just download it to your mobile phone.
Second, check the graph provided on the matteolab website. It shows how large a buffer zone is recommended between infected bay laurels and susceptible oaks, depending on tree size.
For example, small oaks, up to about 8-inch trunk diameters are at low risk of infection and require no buffer zone. At the other extreme a 48-inch diameter oak gets a 65-foot buffer zone. The recommendations caution against removing bay laurels on stream banks and steep slopes.
The third step in prevention is a phosphonate application (AgriFos or Reliant plus Pentrabark as a bark spray in November or early spring, or by trunk injection every other year.
For details, visit the matteolab website.