Hello,

This is a common problem with this plant.  It can develop rust pathogens that spread to oats so is restricted in several states where it is considered invasive.  

Many sources recommend replacing plants that are susceptible to rust with resistant plants.  If you choose to treat the disease, there are are few things you should be aware of.  Rust control can be tricky because it can develop resistance to certain classes of compounds, so rotating fungicides is important.  Fungicides only prevent rust from developing.  This means you need to remove all foliage with rust before treating healthy foliage to prevent rust infection. 

The most effective fungicides have the active ingredient Myclobutanil or Azoxystrobin.  Chlorothalonil or Thiophanate methyl offer some protection, but not complete.

Always follow label instructions very carefully.  

I hope this answers your questions.  Thank you for contacting Ask Extension.

Ruth