Catclaw Rust

There is a Fungus Among Us

CatClawYou may have noticed brown clusters growing on your Catclaw Acacia trees. Known as Catclaw rust, the fungus infects the terminal ends of branches and causes a distorted, bunchy growth. It produces spores on the leaves of the tree in the late spring and summer that eventually cluster together and appear dark brown in color. Unfortunately, the cooler temperatures throughout May and the beginning of June were ideal conditions for the fungus. The monsoon season brought higher levels of humidity, yet low levels of rainfall this year, allowing the fungal spores to spread readily and rapidly.


Is There a Treatment for the Fungus?

According to Dr. Mary Olsen, Plant Pathology Extension Specialist at the University of Arizona, there is no known effective and practical treatment for Catclaw rust. The fungus is a normal occurrence in nature and prompts a “survival of the fittest” environment among the trees. The fungus typically has a two year life cycle from the start of each new spore growth. Under favorable conditions, the outbreak could be similar next year.

Catclaw RustRemoval or branch pruning of the Catclaw Acacia is not advisable at this time. Eliminating the affected branches may further weaken the tree by creating an open wound during the growing season. Furthermore, the fungal spore can be easily spread through pruners and disseminated in air currents.

What Can You Do?

The good news is that the less-than desirable brown clusters can be removed when the tree is in dormancy (December through the end of February). Here are some guidelines for removing the brown clusters:

  • Infected branches should be cut off from the union of two branches only when the tree is dormant.
  • Do not cut in the middle of the branch or the fungus is likely to growth back.
  • Remember that it will take two years for the fungus to “run its course.” Fungus that appears next spring may take another 2 years to dissipate.
  • Trees that are completely infested should be removed from the landscape.
  • Continue to monitor your trees and have patience.

It is expected that most trees, unless heavily infested, will recover. For more information visit the University of Arizona Plant Pathology Extension website:

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