I'm trying to figure out what is wrong with my green ash tree, how I can diagnose it, and if there is action I can take to save it. Our small backyard in SE Denver includes a green ash tree that is approximately 50 yrs old and has looked great since we moved in 5 years ago. The tree is forked with two main sections. About a month ago, I noticed the leaves at the top of Section A were wilting and falling, and this has progressed to half of the top of that side of the tree. Now there is wilting of leaves at the top of branches on Section B, on the same side of the tree where much of Section A has died back, but again, originating on a different branch of the tree. Lower branches in Section A still look normal, and there are no obvious other problems with the tree. Many branches have died back completely at this point in Section A, with some only having a few leaves left, making up a significant amount of the tree's foliage affected. The other half of the tree appears largely unaffected in any way, with the exception of a few branches that reach out into the same area as the top of Section A, and have started to wilt.

Although we replaced our weed/Kentucky blue grass lawn 4 years ago with DogTuff grass that has been thriving with little to no water, the tree is near the corner of our property surrounded by other lawns that are fully irrigated, and I don't expect this sudden dieback to be the result of lack of water. It also started around when we were still getting regular rain events. There have been no other changes in our backyard or with neighbors that I believe could have affected the tree (no pesticide/herbicide use, no construction/digging/landscaping), and no other trees in the area appear affected.

 There was a dead branch below the affected area that I've been meaning to cut for a few years, and now I'm wondering if this has allowed the introduction of pests/disease to the areas above it.

Should I cut this entire affected section of the tree away before things spread, wait until next year to see if it recovers, or take other or no action?