first if it is mildew then it will be pale yellowish spots and off white beneath but if turning brown is there black or orange pustules underneath this is rust so remove the leaves next year should be ok, if mildew spray with mancozeb also spray under the leaf 3 or 4 times in a 10 day intervals

If mine get mildew, I cut them back. I did that earlier this year with 'Hocus-Pocus' which has dark leaves, and new fresh leaves and flowers appeared. If they get mildewed later in the season, cut them back and they should grow back in the spring as they're herbaceous.