See:
Walnut (Juglans spp.) - Downy Leaf Spot
Cause Microstroma album a fungus that is reported to be common in Washington on Oregon oak (Q. garryana).
Symptoms The upper surface becomes yellowish and mottled. Angular white patches, fruiting stage of the fungus, appear on the lower surfaces of the leaves. These patches may grow together to form a large discolored area involving most of the leaf surface.
Cultural Seldom needed as the incidence is low.
References Shaw, C.G., and Harris, M.R. 1960. Important diseases and decays of trees native to Washington. Ag Extension Service, WSU. Extension Bulletin 540.