I have been told, by many, that BLS is primary culprit and that removal is likely path. My question for the experts is, these trees have never shown any leaf margin discoloration. Can it still be BLS without the discoloration ? I have been very on the lookout for this symptom. These photos are dated 10/22/17, but the trees have looked like since very early September.
Trees can thin and decline from multiple issues, including environmental stress, drought and root diseases.
Bacterial leaf scorch is common on pin oaks and you should see leaf symptoms by mid August to September each year. It is a slow decline, which can take 8-10 years to kill a tree. How does it look during the growing season?
If you are in doubt, you could hire a certified arborist to do an on-site evaluation of your trees to assess the situation for you. (You can search for one at www.treesaregood.org)
Here is our page on Bacterial Leaf Scorch: http://extension.umd.edu/hgic/bacterial-leaf-scorch-trees
This is our page on Oak Decline: http://extension.umd.edu/hgic/why-oak-trees-are-declining
And this page is helpful in assisting homeowners to know when it may be time to remove a tree:
Hi cm, Trees grow well during growing season, except for some branch die back. No BLS leaf margin symptoms ever show up on leaves. I have had 1 Arborist (Bartlett) and numerous other tree people tell me that it is probably BLS and trees are in decline (dying), but no one can answer why no leaf symptoms have ever appeared. I'm just trying to be 100% before I have them removed.
Our plant pathologist thinks you would indeed see leaf scorching, but maybe can't tell without binocular view?
Read through that last linked publication and see what you think.
It also wouldn't hurt to get a second opinion from another ISA (International Society of Arboriculture) certified arborist.