Hello Mary

This is powdery mildew on your leaves. Poor air circulation and leaves staying wet from rain or irrigation for long periods of time encourages the fungi to grow.

Improve air circulation around the tree by pruning or thinning trees and shrubs around it. ( Note, do not prune oaks or elms during the growing season, only in late fall to early winter to prevent disease). 
If the tree itself is very full, thin it to improve air circulation. The best time to thin is when the tree is dormant in late fall to early winter. Do not remove more than 20% of the foliage/branches in one season. For large trees we recommend you hire a tree service or arborist.

Thinning:

https://hort.ifas.ufl.edu/woody/thinning.shtml

https://aggie-horticulture.tamu.edu/earthkind/landscape/proper-pruning-techniques/

MSU Extension does not recommend retailers or brands. However, there are some organizations that can help you find  professionals-

Www.treesaregood.org

https://www.mnla.org/

Also be sure your irrigation is not hitting the leaves. If this is not possible be sure to run your irrigation early in the day so that leaves dry before sunset.

Once leaves have powdery mildew there is no treatment. Rake up the leaves this fall. Here is an article with more details on powdery mildew—

https://extension.umn.edu/plant-diseases/powdery-mildew-trees-and-shrubs#trees-and-shrubs-commonly-affected-by-powdery-mildew-1584111