Red Growth On Aspen Trees are Diplodia Galls (Diplodia tumefaciens)

Observation by Brandie Radigan:

These growths are all over many of the aspens at Liberty Falls.

Comments by LEO Editors:

During the August 2020 LEO Webinar on Forest Health in Alaska, presenter Lori Winton, pathologist with the US Forest Service, was able to identify the red growth. Lori shared that:

"This is a Diplodia gall, it's a disease caused by a microorganism - a fungus - that is very difficult to see on there but what it does it makes the plant over produce hormones so it kind of makes a tumor kind of like cancer. The tree is growing out of control in this localized spot because it has an infection from this fungus Diplodia. the damage that this fungus does is a gall...this is more cosmetic usually, it doesn't kill trees directly but what it can do is promote breakage, especially when there are snow-loads on or above the gall. It can make the tree break easier at that point. But aspen are pretty resilient to sprouting so it's probably going to make the form not-so-nice but its not going to kill the trees.

In her presentation, Lori shared the Common Forest Insects & Diseases in Alaska webpage from the Region 10 US Forest Service website. By searching that site, we found more information on Diplodia galls. In 2019, the Diplodia gall was recorded at 11 sites in Southcentral and Interior Alaska. The disease has been recorded across North America on trembling aspen, balsam poplar, and other Populus tree species, but in Alaska, Diplodia galls have only been found on aspens in areas generally 2 acres in size. Erica Lujan