It is difficult to tell from the images alone what may be happening. Knowing how the trees were planted and cultural information may help. It could be stress, disease, or nutrient deficiencies. I suggest taking samples into the Oklahoma County OSU Extension Center for diagnosis and possibly to be sent off to the disease lab. Take cuttings of twigs/branches that are about 6 to 10 inches long that have healthy and affected leaves. I would wait until early next week instead of doing it today so the samples can be shipped, if necessary, and received quicker. Contact information for the Oklahoma County Extension Center can be found here -https://extension.okstate.edu/county/oklahoma/. Be prepared to provide additional information so they can properly provide a diagnosis.

One additionl thought, the canopies look pretty thick and it would probably help to do selective removal of some interior branches/twigs to thin out and open up the canopy. This would allow better air circulation and reduce the potential for fungal diseases. For information on pruning and training trees see our fact sheets - Training Young Shade and Ornamental Trees and Pruning Ornamental Trees, Shrubs, and Vines.

Please see directions and link to the extension center in response above.