How to repair void in edge of subfloor from rot?
Yick, we had something similar but worse.  I:
Cut out whatever subfloor was gone.
The joists were stained and powdery looking in spots, so to prevent any mold colonies I painted those up to the edges of the remaining subfloor material with Kilz.  When that was dry, I screwed in pieces of matching plywood to replace the gaps, noting that I made sure to have all edges secured over the joists - no pieces with hanging edges short of the joists.
I then painted everything I replaced on the exposed subfloor with Kilz, going on the idea that if it seeped once, it could seep again someday.

(Replacing the entire floor - once I had 1/2" plywood down, replaced where needed, I went over that with 1/4", which made sure no repair seams showed through the flooring over time.)