Nice looking apple tree. But that does not appear to be FB as FB (here) will turn the leaves from green to black without a intermediate color (as is the yellowish/orange in your pic). My apple trees rarely get FB and it's very light damage when it occurs on the apples, but FB will devastate my pears if not caught in time with heavy pruning to control it. Wish I could be of more help but maybe it's a disease or other problem local to your area?

Firebllght is also highly governed by a suitable climate and temperature and typically shows up much earlier in the season....springtime, just as the trees leaf out. If it was FB, it would have shown up months ago, not now.

Wickson is technically a crabapple and is moderately resistant to almost all common apple/crabapple diseases with the exception of rust.

That's what I thought. However, any ideas what this might be? Only one branch slowly dying and the rest of the tree perfectly healthy? Couldn't be a gopher, could it?

Yes, I have. I noticed a dark area there, almost as if it had been "oiled" or something. The area extends a bit up and down the trunk. What might that mean?

Not sure, but it wouldn't be typical of fireblight.

"Oiled"? Could be sap oozing from a borer hole or other damage to the trunk.