astilbe

I have a Thunbergii Straussenfeder astilbe that is about 3 years old. It is planted in a good location and the first year it did well. The last two years the blossoms just start and then about an inch away the stem seems to rot and they die. I think there is something on the plant as some leaves are curled and have little black spots on them. My question is are their specific pests and diseases to astilbes? I have older ones that have no problem. Thanks
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