Goatsbeard (Aruncus) is not particularly prone to diseases other than fungal leaf spots or anthracnose fungus, which looks a little like rust. All of these would be possible with the wet weather we have been experiencing.
Can you send a photo of what you are seeing?
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This looks like a leaf spot disease possibly anthracnose. This can be a factor in wet weather. The plants look like they may be new. The new growth looks okay.
You will have to decide if you want to protect the new foliage. If so, you can try cutting the plant back and spray the new leaves with a fungicide as they come out. Follow label directions.
Or just prune back the affected leaves and stems and monitor the growth of the plant. Be sure to discard affected foliage in the trash. Here is more on anthracnose