“Autumn Purple” ash trees suffer from several problems. The emerald ash borer, an invading pest, kills infested ash trees within five years if not sooner. Other insect pests include lilac borers, carpenter worms, leaf miners, ash sawflies and ash leaf curl aphids. Nests constructed out of webbing appear in ash trees infested with webworms. These are especially visible in the fall. Plant diseases like fungal leaf spots, powdery mildew, rust, anthracnose, and cankers can cause serious problems for the “Autumn Purple” ash tree.

Your tree is likely suffering from anthracnose. The disease is worse in wet years and is partially controlled by gathering and disposing of diseased, fallen leaves. Anthracnose is also called leaf scorch and leafspot. Infected parts of the leaves turn brown, along the margins. Infected leaves fall prematurely. Rake up and destroy infected leaves. Fungal diseases can be controlled in the early stage by pruning affected branches and fertilizing the soil with a low-nitrogen, high-phosphorous blend to ensure the tree has the nutrient it needs to fight the diseases off.

Hope this helps!