Following the 5" of rain this week we noticed these stained areas on the practice green after blowing leaves the following morning. The rain caused some of the organic compounds in the leaves to stain the turf. It's not harmful in anyway but an interesting phenomenon.
Cleaning up and mulching leaves on the golf course is the most time consuming task we deal with. You'll notice we mulch more leaves than not. This is simply because we have nowhere to put them where they can decompose without creating an odor, just like grass clippings. Our campus is very dense and we want to be good neighbors. This is the reason why you see us spending so much effort mulching the leaves with our rough mowers.