The Carlisle Mosquito Online

Friday, May 25, 2007


Cedar-apple rust

The cedar-apple rust resembles a UFO that landed in the branches. (Photo by Liz Carpenter)

Name. The cedar-apple rust, Gymnosporangium juniperi-virginianae, is a fungus that requires both an apple (or crabapple) and a juniper to complete its life cycle. The "cedar" in the name actually refers to junipers (Juniperus species); this disease does not occur on true cedars (Cedrus species), which are not native to North America despite the common names of several juniper varieties. In our area, the native eastern red cedar (Juniperus virginiana) is a susceptible host.

Identification. I first saw this bizarre-looking organism on a red cedar branch in my back meadow in late April. It was a rubbery red sphere about two inches in diameter, covered with bright orange-yellow spikes about 3/4-inch long. The spikes become longer, thicker, and more gelatinous in wet weather. (We saw several in this stage at the far corner of the Towle Field on Ken Harte's bird walk on May 20.) The red sphere eventually turns into a dry brown gall (with little dimples where the spikes used to be) and drops from the tree in the summer.

On apples and flowering crabs, bright yellow-orange spots up to 1/4- inch long form on the upper surfaces of the leaves. The spots gradually enlarge and turn orange. Small black fruiting structures form in the center of the lesion. Eventually, an orange cup-like fungal structure forms on the bottom surface of the leaf directly under the lesion. Leaves with many spots drop during the summer.

(Photo by Kay Fairweather)
Life cycle. These rusts spend part of their lives on rosaceous hosts such as apple, flowering crab, and hawthorn, and another portion on species of juniper. The rust over-winters in the galls on the juniper. In spring, the galls swell during wet weather and begin to push out the gelatinous spikes mentioned above. Wind carries fungal spores from these structures to susceptible apple or crabapple plants. Infection occurs when the spores land on a susceptible plant during moist conditions. Spots appear on the leaf surface shortly after bloom. In late summer, spores are released from the fungal structures on the leaf underside and are blown by wind back to re-infect susceptible junipers, completing the life cycle. The disease must pass from junipers to rose family plants to junipers again; it cannot spread between rose family plants. The complete life cycle of the rust takes two years.

Control. Cedar-apple rust generally does little damage to juniper species; however, twig dieback can occur in very severe cases. The rust galls can be pruned out in late winter or early spring. Some varieties of juniper may be less susceptible.

Apples and crabs can be more seriously affected. Premature defoliation can weaken the tree, make it more susceptible to other diseases, and reduce fruit set and yield. The rust may also cause fruit lesions. Fungicides can be applied to apples and flowering crabs in the spring to prevent rust infection. Another way to avoid rust infection is to plant resistant cultivars of these species.

The fungal spores can travel as far as two miles, so avoiding planting these rose family and juniper species close together is not always an effective control mechanism.

References: Kansas State University, Diseases of Apple and Flowering Crab, K-State Research and Extension, publication C-678; Cedar Apple Rust Disease and Related Rust Diseases, K-State Research and Extension Electronic Publication; Horticulture and Home Pest News, "Cedar-Apple Rust", pp. 53-54, April 28, 1993; University of Saskatchewan, "Cedar Apple Rust",

2007 The Carlisle Mosquito