Answer: Based on your description I am not sure what you are seeing. The typical problem listed for astilbe is powdery mildew and it is not that. Cercospora leaf spot and some similar problems can also affect astilbe but I would not expect to see an elongated red area due to that. If it is isolated on a few stems or leaves I would remove them and put them in the trash and see if that stops it. If not, you may want to consult with your local professionally certified nurseryman to see if you can get a firm diagnosis. (Keep in mind also that it might be a nutrient issue for example and not a disease problem per se.)I'm sorry I can't be more specific for you long distance.
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