Ash rust is caused by the fungus Puccinia sparganioides. Like other rust diseases, typical symptoms include the presence of bright orange pustules. Pustules may occur on leaves, petioles, and green twigs. Diseased tissue swells, causing distorted leaves and sharp bends in petioles. 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.
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 spring.
The disease is rarely destructive enough to warrant special control measures.