Warm water and maybe a soft brush is a good place to start.  Maybe add a little mild dish soap to the mix if some of the spots are stubborn.  The cloth would be some sort of polyester (dacron) with a stiffish finish calendered in for stability and to seal the fibers.  In my experience, anything short of mildew should come out with a bit of gentle scrubbing.  Be careful around the threads; you don't want to damage those.

.....Michael

FWIW: I recently bought a Skerry that needs lots of TLC, and the sail, a Fowler, also was left outside under trees for a few months, by the kids of the person I purchased it from.

I cleaned a trash can, put the sail in and filled it with water. I added OXYclean powder to it. It took three days of soaking and stirring once in a while, but all the mildew and brown and dark spots came out. The sail is white, again and no damage to threads or otherwise. Good luck!

  I second the Oxyclean. I soak my sails at the end of every season.