It looks like powdery mildew. The leaves also appear chlorotic, which means there's something going on with the soil, possibly a nutrient deficiency. Since the tree is stressed, it is more susceptible to attacks from pests and diseases. The course of treatment is to identify and correct the imbalance in the soil, which could be that the soil is too alkaline (high pH), or there's poor drainage, or there's a nutrient that missing.

A certified arborist would be able to assess the site conditions and make a recommendation. Here's a site that can help you find a certified arborist in your area: https://www.treesaregood.org/findanarborist

It's never a bad idea to add a 2" layer of compost and 1" layer of mulch around the base of the tree (but not touching the trunk).