I'm not certain what is happening to the trees, but it looks like it may be drought stress or root stress. Are they irrigated? Have you had adequate rainfall this summer? Has the root zone been disturbed? Or have any herbicides been sprayed nearby? Have you inspected the limbs and twigs for any potential insect pests that might be zapping them of water? It might be worth checking in with your local Extension Office and bringing them a sample. http://web.extension.illinois.edu/cook/