If you are in high humidity area, it could be rust, which needs to be treated with (if I remember correctly) a copper spray, or an anti-fungal.
Otherwise, the most likely issue is underwatering and maybe too little fertilizer. Small dark spots and yellowing are typical of leaves in the Fall, but can also happen with a cool weather period which fool the plant into thinking that it is Fall, too much shade or too little sun, and underwatering. I have heard that severe overwatering (or excessive rain) can also cause the same behavior.