Ash Rust

Encyclopedia Article

Need to know

  • Ash rust is a common fungal disease that impacts all species of ash trees. 
  • Infected leaves, petioles, and small twigs swell and may become twisted and distorted, with yellow to orange pustules developing and producing powdery spores.
  • Ash rust is caused by the fungus Puccinia sparganioides.
  • Although it is unsightly, ash rust is not a serious threat to the health of the tree, and because of this, control measures are not usually necessary.

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 the 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 your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each state's 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.

Want to submit a sample? Follow the instructions at Submitting Trees and Shrubs

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. 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.

 

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.  

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