Based on e-mails, comments from other arborists and my own observations, coast redwoods and their close relatives, the Giant Sequoias, are having a difficult time.
A local arborist recently asked me if I had noticed, as he has, coast redwoods with extensive loss of foliage. I recently received numerous calls — as I do every summer — from people concerned about their redwoods shedding needles, and an e-mail with photographs from a Napa resident concerned about a group of coast redwoods that looked nearly dead.
Then, an arborist on the American Society of Consulting Arborists online discussion group asked if others have noticed an increase in “redwood canker” this year in Contra Costa County. He saw a Sequoia sempervirens (coast redwood) in Concord that was 80 percent dead.
An arborist in Sonoma County responded that he has been seeing an increase in diseased redwoods for several years. His conclusion, with which I agree, is that there was a buildup of disease fungi in trees that have been predisposed to infection because of drought stress in the years before the good rainy season of 2009-2010. He attached a helpful article on the subject by consulting arborist and forester Bruce Hagen.
It is normal for coast redwoods to shed their older, inner needles in the summer. A coast redwood tree might live 500 to 2,000 years or more, remaining always-green, as the species’ name “sempervirens” denotes, but the needles in a flush of growth have a life span of only a few years. After the needles have matured and generated food for the tree for two or three years, they lose their green color as they give their remaining nutrients to the tree and cleanly separate from it. Stressed redwoods tend to shed more.
As stress increases, whether it arises from compacted soil, drought, poor drainage or various other causes, redwoods become susceptible to “opportunistic” disease. The three diseases mentioned in Hagen’s article have the tongue-twisting scientific names Botryosphaeria, Seiridium and Cytospora. All three are fungi that tend to spread in water and wind. These are not new diseases. They are just out there in the environment waiting for the opportunity to propagate in susceptible trees. Infections may result in foliar shedding, dead branches, dead tops and sap dripping on surfaces under the trees.
Coast redwoods thrive best where there are mild summers with persistent coastal fog, ample winter rainfall, deep well-drained soil and lots of undisturbed companion trees. A lack in any of those factors can induce stress.
In contrast, the Giant Sequoia, a.k.a. the Sierra redwood (Sequoiadendron giganteum) thrives where it gets a long, snowy and rainy winter and a short summer season. When they are grown in the low California valleys, with long, hot summers, they are stressed and typically infected and disfigured by Botryosphaeria. Perfect examples are those at Napa’s Franklin Station Post Office, Randolph at Second Street: scattered dead branches with retained dead foliage and spots of sap all over the sidewalk.
Hagen’s article and various other publications recommend managing these diseases by managing, as much as possible, the environment in which the trees are growing. Manageable factors may include appropriate and timely irrigation, proper use of mulch and pruning to remove dead dying and diseased branches.
Chemical control gets no credence in the Hagen article, but pest control companies may offer spray programs they claim may be of value. I checked with one and was told that pruning to remove dead and diseased branches is critical. After that, they recommend a fungicide spray program that requires thorough coverage of the tree. That is not easy to do for a 100 foot tall tree. I’ve heard that another pest control company is claiming that excellent control for Botryosphaeria in Giant Sequoia can be achieved through three consecutive years of systemic fungicide injections. So far, I have not found documentation to confirm or refute that claim.
This type of tree/disease relationship is a perfect example of the Plant Health Care model. Put simply, a healthful environment and appropriate climate produces healthy plants while stressful, inappropriate conditions induce stress and lead to disease and pest problems.