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This fungal problem results in rounded spots on the leaves. Although it's unsightly, leaf spot is only a significant problem on poorly growing or unhealthy plants, which might end up dropping a substantial number of leaves.
Solution
Organic
Promptly remove and destroy all infected leaves, both on the plant and on the ground. Don't put them on the compost heap. Also improve the growing conditions - add a mulch of well-rotted compost, and regularly feed and water. Keep deadheading.
Chemical
If the problem is severe, spray with mancozeb.
