I am going to put my toe into the contentious – and locally important – issue of what to do with sickly Arbutus trees that have die-back: in some cases entire stands of the trees are killed. The tree is truly gorgeous and an icon of the coastal areas from California up to BC.
Why would I put my toe in? I am looking for qualified and accurate information, not myth or speculation. After a quick bit of research it seemed to me that the picture is somewhat incomplete and of seemingly of low priority to the research funding bodies.
This is my current take on it: Arbutus die-offs affects stands of weakened trees. In the popular online gardening forums people refer to a virus affecting their trees: this you will not find in the academic and government sources in the US or BC that I looked at (the latest being a 2013 Pacific NW plant disease management handbook by Oregon State University). However, I would not be that surprised to find that a virus could be weakening trees since they are difficult to detect, and poorly understood. The actual symptoms of dieback are mainly caused by a number of fungal diseases (“twig dieback” and “cankers”), although the tree may have been made susceptible to such ailments as a result of site disturbance, pollution, extreme weather, root damage, another disease, and so on.
The actual cause – changes in soil moisture, virus, pollution, etc. – matters if you are looking for a cure. Anyhow, sometimes the tree recovers from new shoots from lower down – there are Arbutus trees that have clearly formed a circular ring of new trunks from an older tree that has died back to the ground – sometimes not. There seems to be some confusion as to whether the tree should be left alone or pruned. Quite a number of sources say that one should not prune, as any cut is an entry point to infection. On the other hand, leaving diseased material around the tree acts a a source of pathogens that can infect new tree wounds. I have chosen the most recent peer-reviewed publication that I looked at:
What To Do (from one source, cited below) –
Arbutus Dieback: Prune out and destroy dead branches
Arbutus Canker: As above, but try to prune 1 ft or more below the cankered area. Try to keep shade plants in the immediate area of the tree’s trunk to avoid sudden sunburn, or remove those shade plants during the fall or winter.
Pscheidt, J.W., and Ocamb, C.M., senior editors. 2013. Pacific Northwest Plant Disease Management Handbook [online]. Corvallis, OR: Oregon State University.http://pnwhandbooks.org/plantdisease (accessed 31 July 2013).
Do not go climb a tree with a chainsaw and seek help when appropriate. Also, clean all tools with a disinfectant. Should you want to prune the trees it makes sense to do it in winter while many organisms are dormant. If you have any other suggestions, please contact me at email@example.com
Then there is the practical cultural change that a gardener can undertake to try to help preserve Arbutus without having to go into detailed diagnosis of tree diseases: Let the light in and improve air circulation, which previously was enabled by brush fires (no longer allowed in residential areas or forests). While you are at it, try not to disturb the roots, and avoid making changes to the watershed. Also, there are some leaf spots caused by fungal diseases that are common, but as long as they are on older leaves about to be shed – and there is a new leaf following – then this should not be a concern. Just worry if no new leaves are appearing.
Good luck, and let there be many more years of painters gazing awestruck, easel on lap, and rapturously sketching away…
For further information and an excellent start that includes a good bibliography, see:http://www.fs.fed.us/database/feis/plants/tree/arbmen/all.html
For fungal diseases, see,