Dirty Seat

[GolfAddict]

I was thinking it looked like it might be a mold or mildew, like Gary mentioned. Could try spraying with diluted bleach.

[k0ua]

Wife says Vinegar, and Magic eraser

[Zebrafive]

SOS pad and SCRUB HARD!

[SHORT KID]

It was probably eaten up by UV's . On a restoration that I did, that was the problem. The JD sat outside for a whole summer in the sun and the seat was a mess, to the point it could not be cleaned. 75$ changed the whole appearance!!!!!! Good luck on cleaning it.

[topstoy]

You might try the awning cleaner used for RV awnings. Pretty amazing stuff, let it soak in for a bit. Also might try some Westlys Bleach White, normally for white wall tire clean up but I have used that for some amazing clean up as well.

[Rustywreck]

Some of the discoloration along the side looks like the foam underneath has become mold stained. As the vinyl ages it becomes porous and somewhat transparent, allowing moisture to reach the foam, which leads to mold. As the mold dries and dies it leaves a stain behind.
You could try using bleach on a small spot to see if it helps, but you may just have to live with it or replace it.

[Guesseral]

If all else fails spray some carb cleaner on a rag and try that, but wash with soap and water after.

[Don87]

I agree that it's mold/mildew. It's pretty bad also. You can try bleach, but more than likely you will need to replace it...............or paint it black

[6string]

The seat on my 5105 looks worse than that. It traps dirt in every little crack and crevice. I have used spray cleaners and foam upholstery cleaner and a nylon brush to clean it with.

[bhh]

BORAX will kill the mold. Works great on wood too.