I have the same thing, its no big deal. The skinny part might yield grade B.
Pull yellow leafs off, remove dead branches if they die.
I bet its caterpillars drilling into the stem. Was what for me, its weird but it explains why its just one branch and not the plant.
if root rot then its entire plant. Pillers can take out just one branch.
Iv had 1/3 a plant die and I just cut it off (one branch) big deal.
It could be water related, way to hot this year, the root mass has some issue and you water AGAIN and AGAIN and it root rots on one part.
You need to scope for russets like your vet has not a loop.
Its way to late for avid,
Its most likely to late for anything.
Its not every plant is it?
Do you use Safer BT spray for caterpillars?
photo does not look like russets, it looks like the plant is eating the nitrogen and your humping with chems and its some what to much.
Id try molasses flushing,
Me I just stopped caring and I lost 5 biggins this year.
I just dont care because my bigger ones are TWICE as large as last year.
The heat sucks!
These are strange times my friend.