This disease, caused by the fungus Glomerella cingulata, 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.
A fungal attack causes an outbreak of spots on the leaves. The dark marks are often surrounded by a pinkish-purple ring.
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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.