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Oak tree problems

By John Fulton

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[August 17, 2011]  While we normally look forward to the change of tree foliage in the fall, this year the premature leaf drop and discoloration may cause us to lose some of our fall enjoyment. The reasons for the leaf problems are many and have been with us since early spring in many cases.

On oaks, particularly pin and red oaks, we could be experiencing some major problems such as oak wilt and bacterial leaf scorch. Other problems such as anthracnose (and other leaf spot fungi), oak tatters and water damage are not to be overlooked, but they usually don't signal the end of the tree is in sight.

Oak wilt has been a nonevent in our area, but the possibility does exist that it will rear its ugly head. It is very similar to verticillium wilt that we find in many of our shade trees and has a streaking of the sapwood as a telltale sign. There is no cure for oak wilt.

Bacterial leaf scorch is probably the main culprit in many rapidly declining oaks. The bacteria cause the ends and margins of the leaf to dry and turn brown. Some areas of the country have reported temporary results from antibiotic injections into the tree, but Midwest states have not reported any success. Essentially there is no cure for BLS in oaks, with tree death often coming within six or seven years of infection.

Amillaria root rot is another serious disease of oaks that has become more prominent. This is caused by a fungus that invades below ground. As is the case with many diseases, trees that are stressed by flooding, drought or mechanical injury are predisposed to getting the disease. Mushrooms at the base of the tree or structures like shoestrings growing just under the bark of the tree are symptoms of this disease. There is no real cure for this disease at the present time either.

The leaf spot fungi problems are less important but may be prevented with protective fungicides applied to leaves throughout the spring and early summer. Figure on at least three applications about 14 days apart at a minimum. It is also important to get good coverage of all leaves. These diseases weaken the tree by not allowing it to make as much food for the year. Weakening for a year or two doesn't make much difference, but over a long period of time we can get other problems on the weakened tree.

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These same fungi are affecting most of our good-quality shade and fruit trees. Anthracnose is the major fungus for shade trees; apple scab is the culprit for apples and crab apples; and there are more specific fungi that affect ash and other trees.

We'll have to live with what we have for this year, but a preventive program may be in order in future years. We have actually had outbreaks of the leaf spot fungus group each of the last 20 years. The little bit of weakening each year eventually catches up with us.

For now the adage of "Keep trees in good growing condition" holds true. Water with an inch of water per week when Mother Nature doesn't provide it, and fertilize with a lawn rate (to provide a pound of nitrogen, phosphorus and potassium per 1,000 square feet), and that will go a long way in helping the tree overcome weakness.

[By JOHN FULTON, University of Illinois Extension]

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