Cedar Apple Rust

I found this in my juniper tree and had to learn more.

Bizarre, dried up fungal growth (Kate Russell)

It ends up, cedar apple rust has made its way to my yard.


Cedar apple rust is a fungal disease that attacks, you guessed it, apple and cedar trees. Predominantly found in the eastern U.S., cedar apple rust has made its way to California and many other states. Knowing what to look for and what to do about it can protect your apple trees.


Cedar apple rust is caused by a fungus (Gymnosporangium juniperi-virginianae) that needs both apples and cedar trees to complete its lifecycle. Crabapples, junipers, hawthorn, and quince can also be carriers.


​Symptoms of cedar apple rust

Apple trees may show symptoms on leaves, young twigs, and fruit. In spring, small, yellow spots may appear, growing and turning bright red or orange as the disease intensifies. Concentric rings may be visible within these spots. You may also see orange droplets. As the disease progresses, black dots may become visible. By late summer, strange, tubelike structures start to appear on the underside of leaves. Similar lesions may be visible near the blossom end of fruit.

Cedar apple rust gall with telial horns (Amy Manjon) CC BY-SA 4.0

Infected eastern red cedars produce reddish-brown galls that can be up to 1” in diameter. They look a little like cones, at first, but not really. If you look closely, you can see tiny pimples all over these growths. After spring rains, these pimples swell and produce spore-producing, gelatinous orange arms that look like something out of a horror movie.


Cedar apple rust life cycle

These fungi produce four different types of spores: aeciospores, basidiospores, spermatia, and teliospores. Those bizarre gooey orange horns seen on cedar and juniper are the teliospore stage. These horns germinate to produce basidia. Basidia are club-shaped, spore-producing structures. The basidia release basidiospores which then travel, for up to two miles, on any breezes that happen by.

Cedar apple rust on juniper (Littleinfo) Public Domain

These wind-blown basidiospores land on leaves, twigs, and flowers. Here, they germinate and enter nearby plant tissue, where they will produce another structure, called spermogonia, which produces spermatia. Spermatia spores are released in a sticky liquid that is attractive to insects. Insects then carry the spores around, resulting in fertilization of the spores. These spores grow through their leaf hosts and produce aecia on the underside of the leaves. The aecia release aeciospores, which are then blown back onto the cedar trees, where they will germinate and create galls that will produce next year’s telial horns.

Cedar apple rust on the underside of a crab apple leaf (Littleinfo) Public Domain

Cedar apple rust control

If you have cedar or juniper trees near your apple trees, regularly monitoring for symptoms is a good idea. In commercial apple groves, cedar trees within one mile are generally removed, as a preventative measure.


Fungicides are effective against cedar apple rust. Apple trees are sprayed four times, at 7- to 10-day intervals, just as buds start to open, as a prevention. Infected trees must be treated with systemic fungicides. Cedar trees are generally sprayed in summer.