Ash Rust

Encyclopedia Article

Image of ash rust
Ash rust

Overview of ash rust 

Ash rust is a common fungal disease of all species of ash trees.  The disease is rarely destructive enough to warrant special control measures.

Signs and symptoms of ash rust

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.

Disease cycle of ash rust

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.

Type of Sample Needed for Diagnosis and Confirmation

The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents.  If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us

Image of ash rust
Ash rust 

Management of ash rust  

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.