There are three major groups of diseases that cause problems for Pacific madrone (Arbutus menziesii): foliage diseases, branch dieback and trunk canker diseases, and root diseases. Insects generally are not a serious threat to madrone health.
More than half a dozen fungal organisms can cause leaf spots on madrone. Young leaves are infected by air-borne spores during wet weather, often in the spring. Trees growing in creek bottoms or valleys, and in the forest understory are most likely to be infected. Infection results in circular to irregular spots about 1/4 to 1/2 inch in diameter. A related disease produces irregularly shaped, black tarlike spots on the undersides of leaves. These diseases may result in unsightly foliage but are seldom serious. They may lead to an increase in tree stress, however.
What can you do? Pruning dead branches and raking and destroying fallen leaves before the fall rains may help dampen the spread of spores and infection of new leaves.
Twig dieback and trunk cankers
These diseases are caused by fungi that kill a patch of cambium (the growth layer inside the bark). The bark initially looks discolored, then peels off, revealing blackened, cracked wood that almost looks like it’s been burned. Madrone twig dieback progresses from branch tips and works downward. It is associated with drought. Madrone canker occurs on the main trunk or major branches. It usually develops after bark injury. Cankers may spiral around the trunk or branch, girdling and killing it. Vigorous trees form a ridge a callus tissue around the margins of the canker, limiting its spread. Rapidly spreading cankers lack calluses. Often, both twig dieback and trunk cankers are present on the same tree. Some trees affected by these diseases may decline rapidly and die in a year or two. More vigorous trees may persist with multiple cankers for years, perhaps decades.
Deep waterings may help prevent twig dieback (but see the cautions below). Minimizing bark injury is the key to prevention of cankers. Since madrone bark is so thin, it is easily damaged by sunscald that may occur when the trunk is suddenly exposed to the sun, or by mechanical wounding.
Once the diseases are established, there’s not much you can do. No fungicides are known to be effective. However, if caught early, pruning and burning individual cankered branches seems worthwhile. If the canker is partway up a branch, cut it a foot or so below the visible canker margin, since the fungus may have spread into the wood.
Madrone root diseases
These diseases affects the roots and tree trunk. Cankers (areas of diseased tissue) are usually near the base of the tree but may occur further up the trunk. The infected bark is brown and the sapwood may also be discolored. Loss of foliage, and small, curled leaves are common symptoms. Infected trees often die, sometimes rapidly. Moist soil conditions favor the fungus; hence over-watered trees or trees growing in poorly drained soils are most susceptible.
The main way to avoid root disease problems to avoid overwatering. For yard trees, frequent waterings (every day or two) on flat or poorly drained ground, and/or heavy clay soils are a recipe for trouble. While some individual madrone trees may do OK in these situations, the risk of developing root disease is much higher. On the other hand, deep waterings every two weeks or so underneath the drip line but well away from the trunk may promote tree vigor during extended summer dry spells.
Some concluding thoughts...
After looking at a lot of sick madrone trees, I’ve concluded there are two common situations where it experiences the most problems. One is when it is growing in a dense forest where it is shaded. These shady environments are more humid, and thus more disease-prone. But more importantly, madrone is shade intolerant. If the tree’s canopy is not receiving full sunlight, it will decline and eventually die. That’s the natural process is a forest with Douglas-fir and other conifers that can grow in the shade of madrone and eventually overtop it. If you have large, “character” trees in such a situation, gradually releasing them by thinning underneath and around them will extend their lifespan.
The other common situation is when people invade an area where madrone are growing. We put in driveways, trenches, homesites. We run over the ground with heavy equipment, backfill over the existing soil surface, then install lawns and water them daily, all of which damage or kill roots. Dense stands of trees are suddenly exposed to full sun, leading to sunscald and bark injury. These impacts may be unavoidable. But the stress on trees is cumulative, and it shouldn’t be a surprise when they decline in health.