Gardening Questions Answered
Modesto Ash Spring Leaf Drop
The new leaves on my Modesto ash are dropping off in spring. There are irregular tan, brown, or white areas on the leaves, starting at the base of the leaf. Why? And will it kill the tree?
Modesto ash anthracnose is a fungus that infects the leaves and twigs. The fungal spores are spread by sprinkling and by splashing rain. They proliferate in moist spring weather. Anthracnose rarely kills the tree unless complete defoliation reoccurs over several years or there is severe branch dieback and large cankerous areas develop. The spores overwinter on twigs, so pruning out dead and infected twigs in winter will help with control as will thorough clean up of fallen branches and leaves. Fungicide sprays (lime sulfur and copper compounds) offer some control if properly applied on all new growth as buds open in spring. Repeat in 2 weeks if weather is wet. (Note: Anthracnose also affects American sycamore, California sycamore, London plane tree and Chinese elm. Spraying with fungicide on these trees has not proven effective. Anthracnose-resistant varieties of these trees are available.)
UC PUBLICATION 7420: Anthracnose