Walnut tree leaves
The leaves are being damaged on the underside by walnut blister mites, Aceria erineus.
This kind of mite is far smaller than the more common spider mites you may familiar with. They overwinter (survive the winter) beneath leaf bud scales. When the weather warms up, the mites feed among the leaf hairs (the patches of white) which form a shelter for the mites as they feed. Several generations occur during the summer, which attack new leavesas soon as they unfurl.
The damage is generally considered unsightly. Even so, the tree will survive. Any spray must be applied earlier in the season, as the leaves are just developing. Interview 3 f=different pest control companies to locate one which suits your needs. See "Selecting a pest control company " at http://npic.orst.edu/pest/selectpco.html