How to Fix Cause of Rotten Subfloor?

Question

I'm currently renovating my kitchen, and I removed the tiles and mud bed to reveal a chunk of very rotten subfloor. This section has clearly been replaced before, since it is plywood, and the majority of my subfloor is planks. Replacing it should be easy enough. I'm just planning on replacing the plywood and adding a scab to the joist underneath (there is only a small section where the joist has also rotted, which is pictured), but while I'm here I would like to fix the cause of the leak.

The rotten section is directly under my french door and the deck is on the outside face, so I'm guessing one of these is to blame. Is there some sort of water proofing that I don't have? Thanks in advance!

View of the rotten plywood in question

Closeup of the worst section

The view of the rotten area from the crawl space

The view from outside, where the deck is attached to the house. It's kind of gross, but there is no visible damage from here

Answer 1

The cause of the leak is certainly at the threshold and it looks to me that there is no silicone under the door frame; the water damage looks linear across the entire thing and appears to be only below the threshold.

To repair it correctly, you'll need to remove the door (frame as well), cut out the damaged wood, replace the damaged wood with an appropriate product, flash it, apply silicone so water can't run in, set the door back in place, secure it, and silicone/caulk the door trim to the house.

Getting a storm door on the outside can add another layer to help mitigate rain infiltration.