My first question: Do you use a weed killer on your lawn?  Herbicides that are aimed at broadleaf weeds harm any broadleaf (including trees) that take up the herbicide.  Remember that tree roots go out 50% beyond the canopy of the tree, so there are willow roots under the lawn near the base of the tree.

If you are not using a broadleaf herbicide, then I suspect that the issue is lack of water.  There were some dry spells last summer, and we are certainly in a very hot dry spell currently.  Even with sufficient water, some plants are showing stress because they cannot get water up to the leaves fast enough for the leaves to transpire. 

This is a difficult problem right now, because you really should be watering about 1" each time with a sprinkler that will cover the entire root system.  In this kind of drought, this deep watering should be done once a week.  See https://extension.umn.edu/planting-and-growing-guides/watering-established-trees-and-shrubs#when-to-water-trees-and-shrubs-1261760 for more info on watering.

Use a can or other means of seeing how much water your sprinkler puts out.  15 min may be only soaking the top couple of inches, encouraging shallow roots.  1” min. at each watering.  Normally 1x/week is sufficient, but this heat may require more frequent watering.