By Robert Tigner
Yellow/orange, raised spots on leaves are signs of a fungal rust disease. We are seeing rust on ash, hawthorne, apple and crabapple. On ash, rust may cause some twisting of foliage and leaf drop, but it is not a serious disease and fungicide control is not needed. On hawthornes, apples and crabapples, it would require several years of severe infection and defoliation to harm trees.
Ash rust can be found on leaves, though infection can be found on all the current year’s growth. Initial symptoms are yellow to orange spots on the upper leaf surface and chlorotic spots on twigs and petioles. Then about two weeks later bright orange lesions appear on petioles, stems and lower leaf surfaces. Leaf distortion and galls on twigs and stems may eventually appear.
Typically ash rust does not kill trees but it will disfigure, defoliate and weaken them. Repeated infections can weaken trees. Weakened trees are more susceptible to winter damage and winter die-back. If the leader of a young tree dies due to infection, the tree is more likely to become a multi-stemmed tree. These trees are more likely to be weaker and shorter-lived. Train the young tree through pruning in late February to March.
Fungicides can be applied to susceptible cultivars. Application need to begin just as trees are leafing in spring to be effective. Systemic fungicides will have longer acting control thus reducing the number of applications. A few fungicides are available to homeowners for use on ash trees. Applications made now will do very little to control disease. Select and plant resistant cultivars to reduce this disease in the landscape or orchard.