The grass under the tree starts the spring fine but as the fruit drops, the grass becomes damaged in splotches where the fruit has dropped. In the fall I reseed the affected area and cover with good soil. The grass does very well until the fruit drops.
Does a Purple Plum Fruit Tree Cause Damage to Grass in the Shaded Area?
Certified GKH Gardening Expert
This probably not caused so much by the fruit dropping but the canopy overhead filling out. In the spring, the canopy is rather sparse and some light is able to get through, and thus the grass is happy. Once the fruit drops, the tree diverts more energy to the canopy and the canopy grows thicker and more lush and so less light can get through. Then the grass dies because of the lack of light. It is a common occurrence with trees.
The normal solution is to do some targeted pruning on the tree to thin the canopy out some. Start by removing weak or crossing branches. This is normally enough to thin the canopy enough to allow the grass below to survive.