Originally Posted by bigdog

It is a leaf spot fungus, probably Cercospora. You can spray with a fungicide. I use a copper fungicide spray to treat it if the fungus has really spread a lot. Removing old infested leaves and then treating the plant with fungicide should get rid of it. The fungus will lie around in organic matter, so make sure you clean all organic matter around it and dispose of it. However, if it's not too unsightly, don't fret it. It won't kill your plants.

Frank