Skip to main content

Look for Rust Fungi on Trees

Michelle Grabowski, UMN Extension Educator

A group of pathogenic fungi known as rusts can be easily seen and identified on evergreen trees this time of year. Mild temperatures and spring rains cause the rust fungi to produce brightly colored orange or yellow spore producing structures on infected branches.

Some diseases caused by rust fungi are minor problems and only affect the aesthetics of the tree. Others can be fatal to the tree if proper action is not taken in time. It is therefore critical for gardeners to understand how to identify the rust fungi that they are seeing on their plants.

The following rusts may be catching your eye.

Photo 1: Cedar Apple Rust Gall with gelatinous orange spore producing structures M. Grabowski, UMN Extension.

Cedar Apple Rust

Cedar Apple Rust (CAR) is caused by the rust fungus Gymnosporangium juniperi-virginianae. This fungus infects several species of juniper including eastern red cedar (Juniperus virginiana) Chinese juniper (J. chinensis), creeping juniper (J. horizontalis) and low juniper (J. communis var. depressa). At a different stage of its life cycle, it can also infect crabapple and apple trees and occasionally hawthorns.

What to look for:

· Round brown woody galls that are about ½ to 1 inch across on juniper.
· When wet, galls develop bright orange tentacle like gelatinous spore producing structures. These dangle off the gall like an octopuses tentacles and can be an inch or more long.
· Later in summer yellow to orange leaf spots with a red border develop on nearby apple or crabapple trees.

What to do:

· Although odd looking, CAR galls do not do major damage to the juniper tree. No action needs to be taken to protect the health of the juniper. Galls will die after they have released their spores this spring.
· Many varieties of apple and crabapple grown in Minnesota have resistance to CAR and need no further protection.
· Susceptible apple varieties should not be grown near eastern red cedar or juniper trees.

Photo 2: Japanese apple rust gall on juniper USDA-ARS.

· For more information on CAR on apple trees - visit IPM for home apple growers

Japanese Apple Rust

Japanese Apple Rust (JAR) is caused by the fungus Gymnosporangium yamadae. To date, this Asian fungus has not been found in Minnesota but has been identified in several north eastern states. The JAR fungus infects Juniperus chinensis and J. squamata. These two species of juniper are sold as ornamental trees and shrubs in Minnesota. Like Cedar Apple Rust, Japanese Apple Rust infects apple and crabapple trees at a later stage of its life cycle.

What to look for:

· Round brown woody galls that are about ½ to 1 inch across on juniper.
· When wet galls develop bright orange gelatinous spore producing structures. The spore producing structures of JAR are short stubby projections that only stick out less that ¼ of an inch from the surface of the gall.

Photo 3: Red flag on a white pine M.Grabowski, UMN Extension.

What to do:

· Report Japanese apple rust the MN Department of Agriculture Arrest the Pest Hotline
651-201-6684 Metro Area 1-888-545-6684 Greater Minnesota
· JAR infections are not likely to do serious damage to Junipers but it is unknown how Minnesota varieties of apple and crabapple will respond to the fungus. In Japan, JAR causes leaf spots and defoliation of apple trees.

White Pine Blister Rust

White pine blister rust (WPBR) is caused by the rust fungus Cronartium ribicola. In Minnesota, this fungus infects eastern white pine (Pinus strobus). Infection starts on the needles. The fungus then moves into the twig and continues on into larger branches and even the main trunk of the tree. Once the fungus has reached the main trunk of the tree, it will eventually girdle the trunk, killing everything above the infection. If the infection is identified in a twig or branch, however, these infections can be pruned out, halting the progression of the fungus and curing the tree.

What to look for:

· One or more random branches where all of the needles have been killed and turned reddish orange. These branches are often called red flags.
· Swollen, rough, cracked and discolored bark on the red flag branch.

Photo 4: Blister rust canker with spores on white pine branch M.Grabowski, UMN Extension.

· Raised cushion like pustules emerging from cracks on the infected branch. These break open to release powdery yellow orange spores.

What to do:

· Prune out infected branches before the bark discoloration is within 4 inches of the main trunk.
Print Friendly and PDF