Last week, writing about the top 10 tree problems for 2015 I mentioned foamy canker disease in live oaks. It could turn out to be a major threat to oak woodlands. On the other hand, it might just be a threat to trees stressed by drought.

After that column ran, I received confirmation from the state Department of Food and Ag Lab that a sample I submitted from a dead oak in the Stags Leap area tested positive for the disease.

And since I made an error in the previous column in the scientific name of the disease, I want to revisit the subject.

First, the correct name for the fungus that causes foamy canker is Geosmithia pallida. The species I gave in error is the one that causes thousand canker disease in walnuts: Geosmithia morbida.

The new lab result is the third of which I know in Napa County: One last year, reported by Bartlett Tree Experts, from a private property in the Lake Hennessy area, one last year, which I obtained from a private property in Soda Canyon, and the third last week.

All three are from very dry sites on the east side of Napa Valley.

I have seen two other possible cases, not confirmed by lab analysis. These were also from dry, eastern parts of Napa.

To get an expert opinion, I contacted Matteo Garbelotto, director of the UC Berkeley Forest Pathology and Mycology Lab and asked him if this disease is “on his radar.”

Matteo responded “Based on Steve Swain’s observations, there are multiple cases of Foamy Canker, but I would wait to see if we get new reports after the drought has officially ended.”

Steve Swain is the UCCE Environmental Horticulture Advisor in Marin. His 2014 California Forest Health Highlights for the California Forest Pest Council says, “Recent finds in Marin, Sonoma, Napa, and El Dorado Counties have extended the disease range north. The primary host has been coast live oak, although other oak species have been impacted and pathogenicity tests are ongoing for various oak species.”

Get home and garden tips sent to your email inbox

The key is the fact that a native beetle, the Western oak bark beetle, spreads this disease, and is attracted to drought-stressed oaks. Several years of drought have facilitated the spread of the disease.

Western oak bark beetles are inactive in cold weather so, with the current return of seasonably chilly nights, we’re out of the woods, so to speak, for a few months. And with the encouraging signs of rain, it is possible that this new threat will not develop into a catastrophic loss of oaks.

The balance of December and the following winter months are a good time to turn our attention to dormant season pruning and tree risk management with respect to winter storms.

Next spring, we will see if “the drought has officially ended” and evaluate the need for action against this potentially devastating disease of California native oaks.

Bill Pramuk is a registered consulting arborist. Visit his website Email questions to, or call him at (707) 226-2884.