Michelle,
This sounds to me like a fungus caused by either too much watering or too much rain. Fortunately, this will not kill your plant--- only make it very unsightly. I dealt with this last summer due to the overly rainy season in my area. Try removing the infected leaves and disposing of them. Try not to splash water onto the plant (this is how the fungus is spread) when watering. Water as close to the base as possible. The disease will disappear when the leaves die back in the winter, so try to remove all of the leaves away from the plant in the fall. I'm sorry I can't remember the name of this fungus. I'm sure someone else on this forum will have more details...

Jenny

I had a similar problem this year... not so bad... but my mopheads just didn't look as healthy and perky this year, and some of the individual petals turned brown. I too had lots of rain this year, and after a number of days of heavy rains, that's when the plant went "downhill".