Single branch discolouration and spotting?

Hey guys looking for some help identifying what this problem is. It's only on a single branch on one plant that started from the bottom and continues as it grows. Everything else looks good foliage is nice and green, just this one branch? ?The strain is 91 Krypt. Any advice would be appreciated, Thanks!

Needs some calcium by looks of it

Wouldn't that affect the whole plant though? or atleast leaves throughout the whole thing? i could see it being a deficency if that was the case but its only on this single branch. I have had it happened before to a plant all the way through to harvest with it just staying on the single branch.

Could be the beginning of root rot.