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 that 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. Originally found only in the eastern U.S., cedar apple rust has spread. Knowing what to look for and what to do about it can protect your apple trees.

Cedar apple rust fungi (Gymnosporangium juniperi-virginianae) need apples and cedar trees to complete their lifecycle. Crabapples, junipers, hawthorn, and quince can also be carriers.

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

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

Symptoms of cedar apple rust

Apple trees may show symptoms on leaves, young twigs, and fruit. Small, yellow spots may appear in spring, 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 appear on the underside of leaves. Similar lesions may be visible near the blossom end of the fruit.

Infected eastern red cedars produce reddish-brown galls up to 1” in diameter. They look somewhat like cones at first, but not really. Look closely, and 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 several types of structures and spores at various stages of their development. Bizarre gooey orange horns seen on cedar and juniper are one stage. Those horns germinate to produce club-shaped, spore-producing structures. They release those spores, which then travel for up to two miles on any breezes that might happen.

These wind-blown spores land on leaves, twigs, and flowers. Here, they germinate and enter nearby plant tissue, where they enter another stage. These spores are spewed forth in a sticky liquid attractive to insects. Insects then carry the spores around, resulting in fertilization of the spores. These spores grow through their leaf hosts and produce yet another stage on the underside of the leaves. This stage releases yet another batch of release spores, which are then blown back onto cedar trees. Here they germinate and create galls that produce the next round of infection.

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

Cedar apple rust control

Monitoring for symptoms of cedar apple rust is a good idea. Commercial apple growers remove cedar trees within one mile 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 open, as prevention. Infected trees must be treated with systemic fungicides by a professional arborist. Spray cedar trees in summer.