Oak tree in trouble
A very large (at least 70' tall) oak tree in my front yard lost a lot of leaves early this summer. I thought the damage was stable, but now I see that another large branch has had all its leaves turn brown. The tree is surrounded by lawn. Have fertilized the lawn regularly but not fed the tree specifically. Applied broadleaf weed-killer (dicamba) to the lawn twice in June. The unaffected leaves look fine, but the lowest branches are so high I can't get up close to any of the leaves. I'd hate to lose the tree. More photos available. Any suggestions? Many thanks!
Your tree is not well.
We can't say that there are not multiple factors going on, but dicamba is an herbicide that can kill plants when mis-applied over their root systems. The label warns of this as well as it's danger to water sources.
It looks as if most of the tree is dead or dying. If it's fall would threaten people or property we'd suggest removal.
Here is a publication which helps people decide when to remove a tree: https://extension.umd.edu/learn/how-do-you-decide-when-remove-tree