The Plant Disease Clinic has received several ash samples recently with bright orange spots on the leaves and petioles. Ash rust is a common fungal disease of all species of ash trees. Infected leaves, petioles, and small twigs swell and may become twisted and distorted. Yellow to orange pustules develop and produce powdery spores.
The spores produced on the ash tree do not reinfect ash, but rather infect cord and marsh grasses and the fungus overwinters on these plants. In the spring, the fungus produces spores that can infect ash, completing the life cycle.
Although it is unsightly, ash rust is not a serious threat to the health of the tree. Because of this, control measures are not usually necessary. A heavy infection may stress a young tree and make it more susceptible to winter injury. Cultural practices that reduce stress, such as watering during dry periods or mulching, can help to improve tree vigor.