Birch tree decline may not just be old age
OLDER birch trees will naturally show signs of their advancing years as various branches start to die. If they die within the crown of the tree it is likely due to age. If the branches die from the top and gradually show a progressive pattern of dying branches year after year through the crown, then the cause is likely to be a result of internal wood feeding activity of the bronze birch borer – the grublike larvae of the adult which is identified by its bronze head and a grey-black body.
The first sign of this pest is the presence of dying branches near the top of the tree. Usually the leaves shrivel up and turn brown. With careful observation, especially with binoculars, the presence of ‘ripple or muscle’ bark can be seen in infested branches or the stems near the top of the tree. The rippling is internal callous wood created by the tree to limit the feeding area of the grubs. The grubs consume the living wood under the bark thereby girdling the affected area at the feeding zone. This living wood contains conducting columns of cells that transport water and nutrients to the leaves. Girdling cuts off this supply causing the leaves on the affected branches and stems to wither and die.
There is no approved insecticidal control for this pest once it’s in the tree. It is essential to remove the top dying branches when they first show up as soon as possible to stop the downward spread of this insect in the tree. In addition to the ripple appearance of the bark, there are horizontal ‘D’ shaped exit holes caused by the newly emerging adult beetles. All signs of the beetles’ presence on the tree’s stems and branches should be removed. It is usually necessary to cut into apparently living wood with healthy looking leaves. It is very important to hire a skilled arborist who can climb the tree to detect the beetle’s presence and be able to remove the infested branches and stems.
In younger white birch trees I have noticed a disease called crown gall. It also occurs in other trees as well. Swellings of the stems, branches often producing a distinct gall are signs of this disease. ‘Witches brooms’ or clusters of twigs from a common origin can also appear with this disease. Young trees are most susceptible to this disease as the fungus disease can girdle or destroy the living tissues under the bark of twigs and stems, consequently killing the tree. Older trees usually can withstand the girdling effects of this disease as it rarely does any significant injury to the tree. It is not uncommon for this disease to be associated with the bronze birch borer in the same tree.
Prevention of this disease is key to having a healthy tree. Never purchase a woody shrub or tree showing woody swellings or galls. Look very carefully at the bark of a birch that you want to purchase. The surface of the bark should be smooth and should not have any visible swellings or bumps. If a small gall develops on a previously uninfected tree, it can be carefully removed if it is on a twig or small branch. If it is on the trunk, it is essentially impossible to treat. The tree has to be removed once dead portions of the crown show up. If a gall is small enough, on a large stem or branch, it can sometimes be simply cut off with a very sharp knife. The exposed wood can then be sealed with an inert tree pruning paste or seal. Michael Allen M.Sc.F., RPF is a consulting urban forester, tree diagnostician and certified arborist. He owns Viburnum Tree Experts, a Manitoba company that provides objective assessments of the condition and the care required for trees and shrubs on home and business landscapes. He does make house and garden calls. He can be reached at 204-831-6503 or at viburnumtrees@ shaw.ca His web site is www.treeexperts.mb.ca
If the branches of a birch tree die from the top, the cause is likely to be a result of the internal wood feeding activity of
the bronze birch borer.