Secure subfloor underwall between joists
EDIT:

I let the photo lull me into a sense that the OP had more access to the joists than what is actually accessible. I did indicate what I think the answer is, in the final sentence of my final answer as an option...

You obviously can't leave plywood cantilevered out in space like that, and there's no way to butt one piece of plywood up against another in open air and have it support a load. The gap is also too big to sister the existing joist with a 2x4 and create a lip to support the plywood.

So I believe your only viable option is to use metal joist hangers and install short perpendicular joists between the two existing prefab joists. My first instinct was to suggest 2x6's for this, but I looked at an online joist load calculator and it exposed my tendency to overbuild. 2x4's are probably fine. (example:  http://www.awc.org/codes-standards/calculators-software/spancalc). I'd put them on standard 16" centers so you don't get any sag in the plywood subfloor. Then you could lay a strip of plywood down across those new joists, and the new joists will support everything.

You might have to work a little to tap/pound the ears on the joist hangers between the left joist and the subfloor above it, or find hangers designed to be mounted to a flat surface without a top ledge.




The only other options that would work would seem to involve demo work on the wall above the left joist.

ORIGINAL ANSWER:

If I'm seeing that picture correctly, you have full open access to everything from above and below.

You want to set the cutting depth on a circular saw so that it just barely cuts through the plywood, and do a rip cut from above, straight down the middle of the joist/truss on the left (do NOT notch into the top of that truss). Then tear up the plywood, and lay new plywood down spanning both joists.

Another option might be to create short joists between the existing ones, perpendicular to them.