Answer: Based on your description it sounds like the plants are suffering the fungal infection, black spot. Some varieties are more prone to it than others, so it may be that you are in for an ongoing battle. The problem is worst in wet seasons, in a spot with poor air circulation, and in cases where the plants are watered from overhead. Pick off the infected leaves and destroy them to limit reinfection, dispose of all prunings and dead wood similarly; also do a very thorough cleanup in the fall. Apply a thin, fresh layer of mulch on top of the old periodically to limit the spores splashing back up onto the plant.
Finally, you may find a spray to be useful. You can treat it with a baking
soda solution. To make the solution, mix 1 tablespoon of
baking soda and 2 1/2 tablespoons of ultra-fine horticultural
oil spray with a gallon of water. The oil is slightly fungicidal
and acts as a spreader-sticker, helping the baking soda coat
the leaf and cling to the surface longer. The solution is a
contact protectant so it stops fungi from attacking plant
tissue, but doesn't cure the problem... Apply it as soon as
the symptoms appear, repeating about every 2 weeks. Be
sure to get under the leaves too.
Another treatment you could try (separately, not in combination) is to use a neem oil-based spray which some gardeners reports seems to help with black spot in addition to repelling pests.
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