I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
It is probably the disease called rust fungus. You can shear the plants back and then rake up and remove all the debris. Then as they regrow fresh new foliage some periodic sprays with products containing chlorothalonil will be helpful in preventing it.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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