This is a leaf spot – most leaf spots are caused by fungi, but a very few cases are bacterial. Without culturing in a lab it’s impossible to know which fungus (or less likely, bacteria) you have but the treatment would be the same.
1) Be sure this plant isn’t getting hit with water frequently. Frequent splashing of foliage with water from hand watering, an irrigation system, or drips from leaks/air conditioning/gutters is the most common thing that promotes fungal leaf spots. Water gardens deeply less often – if Mother Nature hasn’t delivered an inch as measured in a rain gauge per week, water long enough to soak the area deeply (equivalent to 1″ in the gauge) once a week.
2. Now that you have a problem, cut all the damaged foliage to the ground and throw it in the garbage to remove as many spores as possible.
3. After clearing out the damage, spray the area with the fungicide of your choice. If you prefer organic solutions, there are still several products available – ask at your local garden center. Treat the area once this fall and spray as the plants start to break dormancy next spring, and repeat according to the label on the product you buy. Be sure to water deeply less often next year.
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