Answer: Unfortunately, based on your description I am not certain what is affecting your plants. Weather and poor air circulation or constant dampness on the foliage could contribute to the problem spreading or continuing, certainly. It might well be a fungal infection and often once a fungal infection takes hold it is difficult to control it for the remainder of the season. However, it is also possible it is a bacterial or other disease problem -- the shrub dogwoods are potentially subject to a wide variety of foliage problems.
A thorough clean up of all infected foliage in the fall can often help limit reinfection the following year, so whether it is a fungal problem or something else I would recommend you do that. At this point I would strongly suggest you consult with your county extension to obtain a specific identification of the problem and based on knowing that, determine how to proceed. If it is something that requires a chemical control, they will have the most up to date information on what to use and how/when to use it. I'm sorry I can't be more specific for you and I hope it is nothing too serious.
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