How do I stop water creeping in from the corner of my sliding door?

This looks like you need a gutter to divert the rain as that is what I was told rto do to mine this week . also you could put a cover over your patio to keep the water from splashing onto your patio floor and door frames . Gutters will help a lot .

Or small roof overhanging as a porch roof does

RED CIDER VINEGAR TO GET RID OF THE MOLD.

Spray Dettol Mould & Meldew spray on all the affected areas and leave for at least 4 hrs or a bit longer and you should see a big difference. No need for heavy scrubbing . Hope it helps.

Caulk and new weather stripping. Is that a step up on the other side of the door. It should be lower then the door, as the water will run in, instead of down.

Oh my. This looks like a major problem. From the picture it appears that the patio or a step is higher than the inside floor creating a trough for water to accumulate outside the door and seep inside. Until this is corrected .... well looks like a big project. Meanwhile try hydrogen peroxide on that mold and do your best to divert rain elsewhere. Good luck.

After removing mold and letting the frame dry real well, I would caulk it ,let dry and then paint all affected surfaces. A sheltering roof can be acrylic clear so not to block the light.

Water is so sneaky and it could be coming from many places and could be causing more damage than you can see. You may want to consider having someone look at this problem.

Thanks folks. Here are a few details to add clarity. The dark wood is actually pressure treated wood that was wet from rain last night. On the outside, there is an overhang roof with a gutter. There's a patio outside and it is lower; the pressure treated wood sits on the slab and the sliding door on top. I'm wondering if something is getting behind siding some how or like Jeanette said, there may be splashing occurring. But I would suspect the whole piece would get wet then? Maybe not.

Thanks for the ideas so far!

bleach it. spray bleach on it . works everytime

That is a familiar sight for sliding doors. Your weatherstrip is either worn or deteriorated to the point where it is no longer performing it's intended duty. That looks like black mold which needs to be eliminated before someone gets terribly sick from it. Try vinegar for a start on getting rid of the mold, let dry completly and then repaint. Get a neighbour to help you target the weatherstrip problem. Have them stand on one side of the door with a bright flashlight ( at night works best ) while you watch for the light to come through the weatherstripping. Check the top carefully. It is natural for a sliding door to wear downward because of the weight of the door sliding back and forth. If you can determine that the space between the weatherstrip and the door on top is a problem, check to see if you can adjust rollers that are in the bottom of the door. You will see holes on both sides with an adjusting screw. If there's caulking used on the installation, it could be the problem. You're very lucky to get 10 years out of good caulking so it it's close to that, peel it off and apply new caulking. Don't use a cheap painters latex caulk. Buy a better one that will probably say," Professional caulking".

Good luck! Hope this helps.

Liquid car wash does wonders. Try it!!

Is the track screwed down tightly? It looks like water has seeped under it from the outside. The mold and mildew you can treat. The important issues are to get it thoroughly dry by running fans or a dehumidifier directed towards the damp area. Then seal the track where it meets concrete or wood on the outside. Good that you have an overhang, was it covered with tar paper and shingles?