Gray blight infection follows an injury, such as winter injury. A spot, which is at first white with a dark brown margin, forms on the leaf. Protect plants from winter injury. No chemical control is listed.
Dieback is also called Botryosphaeria canker. The leaves are attacked near the tips or at the margins; then spots form on the entire leaf. Leaf stalks and twigs are eventually infected. Prune off infected parts. No chemical control is listed.
Leaves infected with Phytophthora dieback turn brown, roll, then drop off. The stem shrivels and a canker forms, eventually girdling the stem. Prune out diseased branch tips. No chemical control is listed.
Powdery mildews of different genera form a white coating on the leaves. Use sprays of benomyl or sulfur.
Leaf spots may be caused by any number of fungi. Use a spray of basic copper sulfate.