Ash Leaf Rust, Leaves, petioles and green twigs become infected during May and June. When infection is severe premature defoliation can occur. The spores retreat to marshlands during the winter and then infect the trees in early spring. Planting such trees near infected marshlands is not suggested. The disease is rarely lethal though consecutive severe infection, especially on young trees can lead to increased stress and eventually death. Especially in young less established urban trees.
What you need to know, Aside from proper plant selection for your specific micro climate there is little that can be done to protect your Ash trees from this disease.
How to Treat your Infected Ash, First one must consider the condition and location of the tree planting an alternative species may be the best course of action. When the tree is still young there are a few things you can do.
Spray the tree with an approved fungicide during early spring.
Mulch to control soil temperatures and moisture which will help strength the tree.
Deep root water during drought periods.