Cedar apple rust / juniper apple rust
Written by: Laine Hackenberg
Posted: July 11, 2023
Cedar apple rust is caused by the fungus Gymnosporangium juniperi-virginianae. It is present across North America and Europe. In eastern North America, cedar apple rust is the most important rust disease of apples. This disease requires two hosts to complete its lifecycle, including a juniper species, primarily eastern red cedar (Juniperus virginiana L.), and a rosaceous species, usually apple or crabapple.
In juniper species, cedar apple rust appears as brown spherical galls about 5-50 mm (0.2–2 inches) in diameter that eventually sprout bright orange gelatinous horns-shaped telia 10-20 mm (0.4–0.8 inches) in length.
In rosaceous plants, primarily apple and crabapple, cedar apple rust causes yellow, orange, or brown spots on the upper and lower surface of leaves, as well as on the surface of fruits. Fungal structures called aecia, which look like thick orange hairs, will form on the underside of the leaves.
- Cedar apple rust requires two hosts to complete its lifecycle: eastern red cedar (juniper) and apple.
- The fungus forms large brown spherical galls on juniper. These galls produce bright orange fungal structures after rain in spring and early summer.
- Fungal symptoms on apple include yellow, orange, or brown spots on the upper and lower surface of leaves and on the fruit.
- Resistant cultivars of both juniper and apple are available to help manage this disease.
Cedar apple rust galls on a juniper. Note the prominent orange telia. This fungal pathogen requires both a juniper and a rosaceous (apple) host to complete its lifecycle. Image credit: Department of Plant Pathology, North Carolina State University, Bugwood.org
Cedar apple rust gall on a juniper species with dried telia. Image credit: Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University,
Cedar apple rust leaf spots on a roseacous species. On the left, pycnia are pictured on the upper surface of the leaf. On the right, aecia are shown on the bottom side of the leaf. Image credit: Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Pathogen life cycle
In spring, galls on juniper trees absorb rain water and produce showy orange gelatinous structures known as telia. The telia produce teliospores that then germinate into basidospores. The basidiospores are released into the air and can travel up to 5 kilometers and infect the young leaves and fruits of apple and crabapple. Leaves are most susceptible to infection when they are between 4 and 8 days old, while fruits are susceptible from tight cluster (when buds are visible yet still tightly grouped in a cluster) to petal fall. Basidiospores germinate into pycnia which are yellow, orange, or brown in color and are located on the upper surface of the leaves or on fruit. One to two months after the pycnia appear, hair-like structures known as aecia form on the lower surface of the leaves or on fruit. The aecia produce aceiospores, which are released during dry conditions in late summer. The aceiospores are carried by the wind and can infect juniper and initiate new galls. New galls on juniper require two years to mature and the galls can then produce telia, completing the life cycle.
The most effective way to manage cedar apple rust is to not plant cedar and apple or crab apple trees near each other. Planting resistant cultivars of apple and juniper will also aid in disease management. Juniperus chinensis cultivars tend to be highly resistant, while Juniperus virginiana tend to be susceptible. There are some J. virginiana cultivars reported to have resistance, including ‘Admiral’, ‘Blue Mountain’, ‘Hillspire’ and ‘Oxford.’ Some apple cultivars reported to be resistant include Golden Supreme, Pioneer Mac, Sansa, Enterprise, ang Gala Supreme.
Mechanical control practices can also be employed, including pruning out the galls from the Juniperus hosts and destroying the galls before they germinate. Chemical fungicides are not recommended for this disease.
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Bergdahl, Aaron D.; Hill, Alison, tech. coords. 2016. Diseases of trees in the Great Plains. Gen. Tech. Rep. RMRS-GTR-335. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 229 p. 186-187
Biggs, A. R., Rosenberger, D. A., Yoder, K. S., Kiyomoto, R. K., Cooley, D. R., Sutton, T. B. 2009. Relative susceptibility of selected apple cultivars to cedar apple rust and quince rust. Plant Health Progress. Online. Plant Health Progress doi:10.1094/PHP-2009-1014-01-RS.
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