They seem to turn brown an entire twig at a time like they do when there is cicada damage. It started at the bottom of the tree and is traveling up the entire 40 ft, tree. Other leaves are turning yellow like they do in autumn. I see no insect damage, I didn't find any twiggs or leaves "in transition" just seemingly healthy leaves on one twigg and on the neighboring twig, brown, dead leaves. Is this something to be concerned about? I'm afraid the entire tree will be dead by spring. Is there something we can do?The weather has been a bit dry this year and the tree is on a bit of a slope in rocky soil.