In April, for the fifth year in a row, Napa contributed to the SOD Blitz (Sudden Oak Death), a distinctive collaboration of volunteer “citizen scientists” and the UC Berkeley Forest Pathology and Mycology Lab, to map and learn about Sudden Oak Death disease. The results have just been announced. Napa showed an unexpected increase in SOD-positive infection samples.
Since it first started killing off tanoaks in Marin in 1995, this plant disease, caused by an exotic organism, Phytophthora ramorum, has spread and killed hundreds of thousands of trees. Starting at a few point sources of infected Rhododendron nursery stock planted in landscapes, the disease jumped into the adjacent woodlands. It now infests about 1,500 miles of primarily coastal portions of California and Oregon.
On our local level, the disease has been confirmed in many locations, primarily in dense woodlands.
The scientific name is very appropriate: “Phytophthora” means “plant killer” and “ramorum” means “branch” or “stem.” There are many species of Phytophthora and they are all plant killers that may live in water, soil, roots, or like this one, in stems, leaves and tree trunks.
Though usually referred to as fungi Phytophthoras are in a Kingdom, a super-group of life forms, called “Chromista,” separate from bacteria, protozoa, plants, animals, and fungi. This super-group includes various other water-dependent organisms, including seaweed.
The disease spreads most readily from infected leaves of more than 100 susceptible foliar hosts. California bay laurel, Umbellularia californica is probably the most significant source of infections because it thrives alongside coast live oak and drips infected rainwater onto the stems, trunks and root zones of the oaks.
When conditions are conducive, especially in late-spring rains, the organism releases zoospores from microscopic sac-like containers called sporangia. Strangely, though they are not animals, the spores swim like certain bacteria and sperm.
(Check out the You Tube video: http://www.youtube.com/watch?v=rTo6UwnZiQg) .
In a film of rainwater, they are able to enter tree trunks where they proliferate and kill the inner bark and cambial zone. Infected oaks may not show obvious symptoms of infection for a year or two but when they do it seems “sudden.” The leaves quickly turn brown and hang on the tree for while as the tree becomes infested with beetles and saprophytic fungi, dries out, and falls apart.
California bay laurels are the focus of the SOD Blitz effort. On the Blitz day last April, UC Berkeley lab Director Matteo Garbelotto instructed the Napa volunteers about the disease, how to recognize the symptoms in bay laurel leaves, and how to collect bay leaf samples and record the location. The volunteers returned the completed sample packets to our meeting place where Matteo picked them up and took them to the lab for analysis.
Results of this year’s Blitz are posted on the lab’s website at this link:
Our local volunteers sampled 90 trees with a result of 21 percent positive for the disease.
Clicking on the red tree icons on the map, I found that two volunteers (“SB” and “BL”) seem to have a knack for getting into the areas where the disease exists and getting good samples. These were from the forested hills west of Oak Knoll, Orchard Avenue and Rutherford.
I collected many samples that looked just right but turned out negative.
The public is invited to attend any of the training and results meetings as follows:
- Nov. 3 at 6 p.m. Sebastopol Center for the Arts (Veterans Hall), 282 S. High St. Sebastopol
- Nov. 4 at 6 p.m. Mulford Hall, UC Berkeley campus
- Nov. 13 at 6:30 p.m., Dominican University C. Science Center, room 102, 155 Palm Ave., San Rafael