I look around the city and surrounding areas and I am concerned about the number of oak trees that are showing signs of distress. I even see the problems with the native bur oaks growing across the road from where I live. As a business I diagnose tree problems. I have found that oaks can have more than one problem at a time, adding to the overall growth stress of the trees.
Every year the leaves of oaks get gall mites – and there are very many varieties of these leaf pests. The tree industry specialists consider these mites harmless to the oak but there is a variety found several hundred kilometres south of Winnipeg in the United States that causes an annoying itch in people. It is not known here. I believe that heavy populations of mites on the leaves do stress the leaves and impair the growth of new branches and twigs.
Oaks are killed by one insect in particular called the two-lined chestnut borer. It appears at the tops of the trees that have been stressed by human and environmental causes. Often a tree will have dead branches only at its highest points.
Disturbances to the roots of oaks can cause twigs and branches to die, also in the upper parts of the tree crowns. A fungal disease, usually caused by cytospora canker – a disease incidentally that is also responsible for the slow death of thousands of Colorado and white spruces in Manitoba – causes the leaves to become noticeably mottled along the branches revealing a mosaic of patchy colours. This disease is a major cause of dying oaks.
A common group of diseases in many tree species called anthracnose starts out infecting leaves but can spread to the twigs and branches. Oak anthracnose is yet another stressor on the tree that in combination with the other problems mentioned weakens the oak tree’s health, causing twigs and branches to die.
The worst cause of weak and dying oaks is people: people who add too much soil over the oak’s roots; people who dig through the oak roots to plant other trees, ornamental shrubs and flowers; people who place sheds, garages, storage bins, and driveway pads over the rooting areas of oaks. Compaction and disturbance stresses caused by these activities weaken oaks and make them subject to the problems I’ve mentioned.
If we are to help oak trees, we must begin by understanding why they are doing poorly. Get the problems properly diagnosed immediately. If possible, move vehicles and temporary driveway pads that are laid over the roots of nearby oaks. Have the tree’s rooting system properly fertilized in the spring and fall for at least three continuous years. Get dead branches pruned by a professional arborist. Our bur oak is the most sensitive to man-made disturbances of all Manitoba’s trees.
Michael Allen, consulting urban forester, tree diagnostician and certified arborist, owns of Viburnum Tree Experts.