Sudden Oak Death

Sudden Oak Death
Barrie D. Coate
Registered, Certified Arborist #586

Sudden Oak Death is one of many watermold diseases called Phytophthora ramorum. It may have been present in California since 1985 but not recognized.

Many of the visible symptoms are typical of other diseases. For example, a live oak in Los Gatos whose leaves suddenly turned brown and which had numerous black fruiting bodies of the Hypoxylon fungus, which is considered an important symptom of Sudden Oak Death, proved to have been killed by a massive infestation of Pacific Flathead borer insect larvae.

Tanbark oak (Lithocarpus densiflora) represent ninety percent of the trees symptomatic of the disease in the Santa Cruz Mountains. All of the tall, pyramidal dead trees adjacent to Highway 17 are tan oaks.
Clients who have sprayed the trunks of their trees with Astro systemic insecticide from eight feet above ground to the ground during the first weeks of April and the first week of August have not lost treated trees, even though nearby trees have died. This may be a coincidence, of course.

There has not been a new case of Sudden Oak Death reported on coast live oaks in Marin County in the last year. Since the symptoms appeared in Marin County before they appeared in the Santa Cruz Mountains, perhaps it will gradually stop appearing here. In my opinion, this is merely one of nature’s “housecleanings,” removing genetically weak or damaged trees and leaving the stronger ones to produce better progeny.
What about findings of Phytophthora ramorum spores on redwoods, maple, bay trees and other species? If you read information from authoritative sources carefully, none are claiming that this disease is infecting or killing those assorted plants, only that the spores are found on them. In other words, we should not assume that this disease is transferring to plants other than oaks until infections and death from this disease in those species is actually demonstrated.

What about infection of rhododendrons by this disease? Rhododendrons in Europe have been infected by a similar disease but are not dying.
What if you have a dying madrone tree? Madrones (Arbutus menziesii) in the Santa Cruz Mountains have been infected and very gradually killed by Botryospheria ribis, called madrone twig blight, for many years. Indeed, every madrone I have seen in these mountains that is fairly mature is infected. This is not Sudden Oak Death.

What to do? My advice is to spray coast live oak, black oak and unusually fine specimens of tan oak trunks with Astro in early April and early August. Also remove dead live oaks. Be patient while the natural processes work.