Weird Tree Disease: Cedar Apple Rust
July 12, 2018
While weird plant diseases can certainly be harmful to your trees...some of them are just so interesting! One of the most odd and intriguing tree diseases is Cedar Apple Rust. Caused by a fungal pathogen, Gymnosporangium juniperi-virginianae, it spends part of its life cycle on cedar trees (mostly junipers here in Texas), and another part of its life on apple trees and related species. You may have seen these weird structures on trees before and wondered if it was infected with some sort of alien species! Now you know…
The life cycle of Cedar Apple Rust is long, with much of it- almost two years - spent on cedar trees. Cedars (junipers) are infected between June and September when spores are blown in from apple or crabapple leaves. Then, small, green/brown galls form the following summer. The galls will mature the following spring when they swell and produce curious, orange tendrils called telial horns. These tendrils look like something out of a horror movie and cover the golf ball to baseball-sized gall. While these galls look gross and scary, they do not usually detrimentally affect the health of the cedar. The spores are then carried by wind to nearby apple trees.
In order for infection to occur on apples, or related species, water needs to be present on the leaves. The spores germinate and multiple best at temps around 60 F, which means infection takes place in the damp, cool conditions during spring. After the spores begin to grow, you will notice orange/yellow rust-like lesions on the upper surface of the leaves. Later in the spring or summer, pinkish brown tubular structure - called aecia - grow on the bottom of leaves or on small fruits of apples, pears and quince. These spores then lead to reinfection of cedars.
Even though the disease spends most of its life cycle on cedar/juniper species, the real damage is done to apples, pears and crabapples. The spots can cause defoliation of the trees along with deforming any fruit produced. If you have ornamental pear or crabapple trees in your landscape, or you have a fruit tree orchard, then this disease will impact you the most. Otherwise, it shouldn’t be a big concern.
Since juniper species grow wild in Texas, it would be difficult to completely eradicate Cedar Apple Rust. You can help control the spread to your landscape if you have susceptible tree specimens or fruit trees.
1) The best way to manage Cedar Apple Rust is to plant resistant varieties of apples. Resistant varieties include Red Delicious, Liberty, McIntosh and Empire. Texas A&M offers additional information on apple varieties suited to our area. However, as apples aren’t terribly acclimated to our Texas climate, you’ll most often see this disease on pears.
2) If possible, remove any wild host plants growing nearby such as cedars, junipers, apples, pears, crabapples and quince.
3) Plant any host plants far apart—most research suggests at least ¼ mile. This won’t be possible though if your city neighbors have susceptible trees in their landscape.
3) Hand remove and destroy any galls on cedar trees early in the spring before the telial horns are formed.
4) The last resort is to use fungicides to control the fungus on apples and pears. As a precaution, please always follow label directions, or call in a professional.
Cedar Apple Rust can be a destructive disease on certain plants, if left untreated. If you are unsure if you have an infection, or need any clarification or direction on how to proceed, give us a call for an arborist evaluation.
Posted: July 12, 2018