Cedar Apple Rust

Did you have an orange octopus on your juniper this spring? Confused? These orange structures may indicate that you have an interesting landscape disease called cedar-apple rust.

Cedar-apple rust is a fungal disease with a complex life cycle. In order to complete its life cycle, the disease organism needs two different, unrelated hosts: a juniper and a rosaceous plant.

Juniper hosts include eastern red cedar and Southern red cedar. In the winter to early spring, tan to brownish golf ball sized galls can be observed on the stems of the juniper host. During moist weather in the spring, the galls produce bright orange, gelatinous “horns”, which produce spores. The gall has an appearance of an orange octopus at this point. Spores are distributed by wind and can travel over two miles to infect a rosaceous host. These spores will not reinfect the juniper. Although galls are unsightly on the juniper host, they rarely cause major damage to this host.

Rosaceous hosts are plants in the rose family including apple, hawthorn, and crabapple. In late spring, distinctive yellow or orange lesions are visible on the upper surface of the leaf. Later, cup-shaped structures will form on the lower surface of the leaf. These structures will produce spores that will infect junipers. Cedar-apple rust can cause leaves to drop prematurely from the rosaceous host and in a severe infection, could cause death of the tree. Fruit may also be infected, resulting in smaller, deformed, unmarketable apples.

The best way to control this disease is to use plants with genetic resistance to cedar-apple rust. Contact our Center for recommendations on resistant apples, junipers, and crabapples.

Although eradication of host plants (either juniper or apple) seems like a logical solution, it often fails to work since spores can travel miles in the wind. Also, do not plant susceptible hosts next to each other.

If the juniper host is small and galls are easy to reach, hand removal of the galls in late winter or early spring can be an effective control. Be sure to remove the galls before the orange horns are produced to avoid spore dispersal.

Fungicides can be used to prevent infection of the rosaceous host. Treating the juniper host is often not practical unless all trees in a two mile radius can be treated! Be careful using pesticides, since some fungicides may not be able to be used on both food crops (apples) and ornamentals (crabapples). Whenever using pesticides, always read and follow the label.

Cedar-apple rust is a complicated, but interesting disease. Keep in mind that once you see symptoms on the rosaceous host, it is too late to spray this year. For more information on cedar-apple rust, contact our Center at 775-5624.

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