My lawn started showing a semicircle dead spot two years ago and the death circle increased diameter season after season. The death circle is always surrounded by a strip of lawns that show stress first, and this stressed area is pushed further out and now there is a large piece of lawn is dead with moss left. Is it a fungus infection? How to treat it?
This does not look like a disease. The lawn is declining due to competition for moisture and nutrients from the tree roots and lack of sunlight. We notice that the roots around the tree are out of the ground. You may need to do some regrading around the tree. Add no more than several inches of soil around the root system and mulch around the tree trunk about several feet out. Make sure mulch is no thicker than several inches and keep away from the base of the trunk.
You will have to do some reseeding, grading, and adding organic matter. Begin by testing the soil. http://extension.umd.edu/hgic/soils/soil-testing
The best time to seed is late summer through mid October. The second best time is by mid April. Your seeding mixture should be a mix of tall fescue (full sun to part shade) and fine fescue such as a hard fescue. http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/non_HGIC_F...
Keep the lawn watered until established. If you seed this spring you will have to water during dry periods and you may need to overseed again this fall. http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG102-Rev-...
If you are not successful, mulch the area and plant some groundcovers and perennials. https://extension.umd.edu/hgic/flowers/groundcovers
Here is a publication on moss. Some homeowners cultivate moss and like it in their landscape. http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG100_Moss...