One of the many Napa property owners who lost trees in the fires of October 2017 was concerned about a coast live oak that died this summer. This one, with no signs of fire damage, dried up suddenly.
Many of his fire-damaged and killed trees have been removed. Some of the surviving oaks have loose bark plates on the charred lower trunks. Among his concerns were fuzzy brown caterpillars congregating there. Are they harming the oaks? What’s with the little green pellets accumulating there, and no signs of foliar damage in the trees?
Sorting things out, first I wanted to look into the caterpillar issue. I captured a couple and took them back to the office for a closer look. They resembled none of the common oak pests I know. I checked reference books and links and found no California native oak pests resembling these fuzzy brown little guys.
What I did find as a good match was the common “wooly bear.” I had not had occasion to think about them since I was a kid. We used to play with them. It turns out they are “generalists” that feed on herbaceous plants and trees. Since there was no sign of foliar damage in the trees, I assumed they have been feeding on wild plants around the oaks, congregating in safety under the loos bark plates with their greenish droppings accumulating there.
So, we scratched that off the list of things to deal with.
Coast live oaks are one of the most fire-resistant local tree species, but survival and recovery depends on several factors: Tree age, bark thickness, heat intensity and duration.
I have seen many variations: Where fuel is minimal, consisting of a normal layer of natural woodland leaf mulch and the trees are mature with thick bark, heat damage is generally minimal. Where more fuel is present, sometimes just a fence post that burned near the trunk, damage is more severe.
Now, about 1 ½ years afterward, some of the oaks that sprouted new growth have failed to thrive. Sprouts generated with the last of the stored energy became infected with powdery mildew or had insufficient vascular connection to the roots.
But the oak that died this summer had little or no evidence of charring. What might account for it drying up suddenly? The most tempting conclusion is Sudden Oak Death (SOD). It has killed many trees in Napa County, but infestation is not nearly as intense here as it is in our close neighbor, Sonoma County.
Checking the SOD Blitz results map on Google Earth and using its ruler tool, I measured the distance from this property, a few miles east of Napa, to the nearest confirmed infection. It is about 5 miles. That is a “low” risk distance. But the map is based on virtually random sampling by SOD Blitz volunteers. There could be more infection sites not yet scouted.
The disease tends to march slowly through woodlands via California bay laurel trees, but it has made some great leaps. New, distant infections often result from the movement of infected nursery stock, but Napa has had some remote infections where no nursery stock has been planted. We know the disease can also move in plant debris and mud from infested sites, but what else might account for long-distance movement of SOD?
As to confirming the presence of the disease, this tree shows the typical sudden dying of all of the leaves, seeping dark fluid on the bark, and lesions visible after shaving away the outer bark showing darkened patches of tissue surrounded by an irregular thin dark line.
It appears exactly right for SOD—Phytophthora ramorum – but other Phytophthora species, which can cause similar lesions, infect oaks as well: P. citricola, P. cinnamomi, P. pseudosyringae, P. cambivora and maybe others.
The California bay laurels nearby showed no typical symptoms of the disease, so foliar sampling of bay leaves in the next SOD Blitz (spring 2020) might not be helpful at this site.
In the meantime, the Berkeley lab has made direct sampling of symptomatic oaks available to registered participating arborists through the OakSTeP program.
If any interesting results come out, I will try to make them available.