I just did mine with a bleach/water solution 1/3 bleach and 2/3 water in a spray bottle......
I sprayed it on and used a bristle brush to work it in. Rolled it up and let set for about 10 minutes and unrolled and hosed it off and let it dry. No more stains and looks good as new!!
Oh and make sure you wear safety goggles, old clothes, shoes and a hat for obvious reasons!