I would suggest that you cut off the foliage and discard it in the trash and let the plant generate new foliage. Make sure you clean up any foliage on the ground that may be left over from last year. Or call the university's extension service and inquire (see link).

Here is a link that might be useful: Extension service link

One of my BESusan plants had the same. I sprayed it with fungal spray, but I'm now thinking I should cut it back.