Ash samples showing signs of rust have been arriving in the Iowa State University Plant Disease Clinic. The spores produced by the ash rust fungus, Puccinia sparganioides, are bright orange, making the disease easy to spot.
The fungus produces gall-like pustules that may occur on leaves, petioles, and green twigs. Diseased tissue swells, causing distorted leaves and sharp bends in petioles. Infection of the twigs and petioles may lead to browning of leaves in the early summer.
The spores produced on ash do not reinfect ash leaves and stems, but infect marsh and cord grasses, the alternate 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, such as preventative fungicide sprays. Spraying rust pustules on leaves and stems will not "cure" them or make them disappear. Infected trees may be more susceptible to environmental stresses. Good cultural practices such as watering during dry periods and mulching with an organic mulch such as wood chips can help maintain tree vigor.