I have a bad-looking maple tree in my backyard. There has been a considerable amount of leaves falling to the ground. Some of the leaves have burnt brown areas. We have another maple 100 feet away and it is fine. Both maples are about 15 years old. What might be wrong?
The causes are fungal and bacterial diseases. Tar spot and petiole borer flourish in wet weather. Tar spot started in the spring with a tiny infection the size of a pinpoint that spread into a yellow spot as summer progressed. Although unsightly, it is a harmless, nuisance disease. Similarly, petiole borers won’t do any real damage to the tree, but the tiny insects that bore into and weaken the leaves make their presence known by the cosmetic changes to the leaves. Spraying will not help because the petiole borers are now inside the tree and impervious to surface applications.
It is advised that leaves be raked up and removed as often as possible and not run over with mulching lawn mowers. Allowing the leaves to remain under the trees and mowing over them can cause microbes to spread and the disease can return next year. Diseased debris can be burned, if local ordinances permit burning, or composted in a compost pile where internal temperatures reach 130 degrees. Trees should not be fertilized after August 1 because that encourages new growth that will be tender and vulnerable to colder temperatures.