That sounds like a fungal problem. One approach is to reduce conditions that lead to the disease. Frequent wetting of the foliage and soil that stays too wet promote leaf spots. Fertilize the plants periodically with a light feeding and try to water them with a good soaking, but less often.
Another approach would be to treat the plants with a fungicide. Your County Extension Office (phone 904-387-8902) or a local garden center can assist you with choosing an appropriate product.
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