Why is this tree shedding diseased-looking leaves?
A common problem in many landscapes is the competition of trees and lawn for the same light and resources. In many home landscapes, what began as a lush lawn with small trees has become a thin and weedy lawn shaded by many large trees. By knowing what both trees and lawns require for site and maintenance, you can determine if you can have a lawn in the shade of trees and, if so, how.
Both trees and lawns are not only attractive in landscapes, but serve many functions. Both trees and lawns produce oxygen, cool the air, stabilize dust, trap air pollutants, and help control erosion. In addition, trees provide shade for buildings in summer and windbreaks, they provide habitat and food for wildlife, and harbor many insects that birds need for food. Lawns provide a groundcover for lower traffic areas, a place to walk and play and entertain.
Both trees and turf grasses compete for water, nutrients, and light. By reducing this competition, you may be able to have both. Turf grasses, like many trees, also need good soil drainage to grow successfully.
Sometimes the fertilizer and weed control products (herbicides) used for lawns can affect the vigor of landscape trees. Check what products are being used on the lawn. Also some trees can be affected by the excess water a lawn requires.
Here are some publications you will find useful:
Hope this helps!