It’s Not a Tumor: The Woes of Cedar-Apple Rust

Conifer aficionados–be on the lookout! Your cedars and their cultivars are being exploited by a fungus called cedar-apple rust (Gymnosporangium juniperi-virginianae). These spiky, red galls are formed due to the pesky fungus settling in bases of needles and twig cracks. The stage of the fungus we commonly see on cedars is the overwintering, stagnant stage that does not hurt the cedar. 

But wait… there’s more. The life cycle of the fungus requires two years: spring #1 is for the gall to mature on infected cedars, while spring #2 is for the gall to spread its spores and wreak havoc on nearby apple trees.

 Left to right: View of various Cedar-Apple Rust galls. Try to cut them off, before they spread their spores!

Be on the lookout and be proactive…

As the second spring gets wetter and warmer, the galls project squishy, orange horns that contain spores. Galls may have variation in the condition of the horns (see photo), which is due to the presence or absence of precipitation. Because spores are like dust, they have a large dispersal radius and can travel via wind to an apple tree over a mile away. After the gall has spread its spores, it drops off the tree, where the cycle continues on apple trees or any plant from the Rosaceae family such as hawthorn, serviceberry, and crabapple.

A way to break this cycle and protect your nearby apple trees from the damaging effects of this fungus is to physically remove the rust galls from cedars using shears, before they have a chance to spread their spores in early spring. Ensuring that fallen leaves and twigs are raked up and disposed of from around the tree can also be beneficial. If the tree has severe infection, it may need to be removed entirely.

…to prevent the spread of Cedar-Apple Rust!

Proactive actions can be taken such as choosing rust-resistant cultivars as well as applying copper fungicides to cedar plants between late August and late October. Fungicides can also be applied once to apple trees near bud break to prevent infection; however, once they exhibit signs of the fungus, there is not much that can be done to prevent them from infecting nearby cedars and continuing the cycle.

Be on the lookout and be proactive to prevent the spread of Cedar-Apple Rust!

Darien Becker
Environmental Educator, WEC