This spring, about 500 volunteers joined in the effort at 23 areas in coastal California to map Sudden Oak Death (SOD). Napa had a good-sized group of “citizen scientists” who joined in to search for the tree-killing disease, surveying over 700 trees in Napa County.

The results were just released by the UC Berkeley Forest Pathology and Mycology Lab, which leads the annual effort. They show an ominous increase in the disease. It is appearing in new areas and rebounding in areas where it retreated in drier years.

The disease organism, Phytophthora ramorum, is dependent on plentiful moisture on foliar hosts, like California bay laurels. Ironically, when we get much-needed rainfall, the disease gets more opportunities to spread. The most common mode of infection is water dripping from infected bay leaves onto oak tree trunks.

Locally, coast live oaks are by far the most numerous susceptible trees, followed by black oaks, canyon live oaks, and tanoaks, which are less common in Napa County.

Valley oaks and blue oaks are not susceptible to the disease. Bay laurels carry it in the leaves but do not die of the disease.

On the statewide scale, results show SOD now detected on “multiple trees” in San Luis Obispo for the first time south of Monterey County. There was also a first-time detection on Mount Diablo in Contra Costa County, said a press release from Katie Harrell, communications director for the California Forest Pest Council, on behalf of the UC Berkeley Lab.

Those new finds were on California bay laurel trees, but the disease is not yet evident as dying oak trees.

New finds were also confirmed in Mendocino, Piedmont (Alameda County), Golden Gate Park and the Botanical Garden at Strybing Arboretum.

The finds are of serious concern, but since the SOD Blitz program has no regulatory power the UC Berkeley lab “will be working closely with the California Department of Food and Agriculture and the USDA Animal and Plant Health Inspection Service (APHIS) to validate the data for regulatory use”, the press release said.

Results are shown in detail on the 2106 SOD Blitz results map at this website:

Positive finds are marked with a red tree icon on the 2016 SOD Blitz Results Google Earth map; negatives with a green tree icon.

Clicking on the icon opens a text box showing the initials of the collector, GPS coordinates and other data.

I examined the Napa results. Admittedly most curious about my own samples, I first checked the area I surveyed: the Napa River Trail between Lincoln Avenue and Trancas. All of those samples were negative, even though the bay leaf samples I collected showed foliar symptoms perfectly consistent with SOD infection.

It is possible that some other organism caused the symptoms. It is also possible the SOD organism caused the symptoms but was no longer detectable.

Then, zeroing in on red tree icons, and using the Google Earth “ruler” tool I noted eight positives, some in new areas in Napa County:

— In a creek near Silverado Trail in the Stags Leap region.

— .8 miles east of Conn Valley Road, east of St. Helena.

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— .3 miles northeast of the end of Redwood Road.

— .65 miles north of the bend in Redwood Road, adjacent to Browns Valley Road, where it turns northward at the foot of the hills.

— Near McCormick Lane and the Robert Ferguson Observatory on Bald Mountain.

— Three locations in Westwood Hills Park.

The new findings have also been uploaded to the free smartphone app: SOD Map Mobile. It shows your current location with red pins marking positive finds and green pins indicating negatives.

Touching the “Risk” icon reveals the relative risk of infection where you are standing at the time.

If you are concerned about the potential for infection in coast live oaks and other susceptible trees under your management, be sure to assess the risk before undertaking any disease prevention and management work.

And check out this website for a wealth of information:

Bill Pramuk is a registered consulting arborist. Visit his website, www.billpramuk.com, email questions to info@billpramuk.com or call him at 707-226-2884.

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