Ash rust is a common fungal disease of all species of ash trees. The disease is rarely destructive enough to warrant special control measures.
Infected leaves, petioles, and small twigs swell and may become twisted and distorted. Yellow to orange pustules develop and produce powdery spores. The spores of the fungus, yellow-orange in color, appear over the swollen areas. The canker-like areas on twigs and petioles may lead to browning of leaves in the early summer.
Ash rust is caused by the fungus Puccinia sparganioides. The spores produced on ash are incapable of reinfecting ash, but infect the marsh and cord grasses, the alternative hosts of this rust fungus. The fungus overwinters on these grasses and infects ash during warm wet weather in the spring.
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.