http://aces.nmsu.edu/pubs/_circulars/circ549.html#C
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Read the above article, which includes items about bacterial and fungal leaf spot. They talk about yellowing around the lesions, but I don't see that in your photo. However, it could just be the stage the problem is in.Â
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The area I live in had a bad bout with a fungus a couple years ago. It wasn't plant-specific - all kinds of plants had this black stuff that started on the leaves that spread to the stems. The pic makes me think of that. Regardless, a good fungicide might help, and if the leaves are regularly wet, you might consider using a watering method that keeps water at the ground so the leaves stay dry.