i'm going to take a wild stab and say it's powdery mildew, probably due to the extreme humidity and moisture we've been having the last few days.

this is what it would look like after the mildew passed, so if you didn't see any white fluffy stuff on your plant, then that wouldn't be it. (i.e. nevermind =) )

I think it looks more like blackspot or another spotting fugus. Sorry, shouldn't be present next year. I think S.Head is right of the cause.

Satellitehead and Girlgroupgirl, thanks so much for your answers. It happened so suddenly it had me concerned but that makes sense about the humidity and moisture.

Thanks again,
Franny