I live in Vermont near Lake Champlain Zone 4. I have had a Golden Ninebark probably Dart's Gold as it is very vigorous and has to be pruned to keep out of ,my walkway. For the last two summers it does well until about August and then gets black spots on the leaves, which then get very yellow and fall off. I have pruned it back and treated with baking soda, Murphy Oil Soap mixture and it doesn't look good at all. Have you any suggestions or is this shrub proned to fungus. I have other red leaf ninebark and they have no problems at all. Any suggestion would be appreciated.