Cedar-apple rust is caused by the fungus Gymnosporangium juniperi-virginianae. To complete its life cycle, the fungus must infect apple or crabapple and an alternate host. Alternate hosts most commonly include native red cedar (Juniperus virginiana) and Rocky Mountain juniper (J. scopulorum).

Figure 1 Cedar-apple rust horns on juniper Photo credit: U of MN Department of Plant Pathology

Figure 2 Cedar-apple rust lesions on apple leaf Photo credit: U of MN Plant Disease Clinic

In the spring, orange jelly-like horns arise from gray to brown colored fungal galls on the branches of infected cedars and junipers (Fig. 1). These horns produce wind blown spores, which infect apple and/or crabapple trees up to three miles away.

Symptoms on apple and crabapple initially appear on the upper leaf surface as small yellow spots, which later enlarge and turn orange (Fig. 2). The fungus then grows through the leaf, causing yellow-brown lesions to appear on the underside of the leaf. In late summer, these lesions form small, horn-like structures, containing rust-colored spores which are wind blown to susceptible cedars and junipers. There they start new infections.

The following spring, small greenish-brown swellings form on the branches of cedars and junipers. These swellings may enlarge up to two inches in diameter over the summer. The following spring, these galls produce orange gelatinous horns and spores, beginning a new infection cycle. Dead galls on cedar and juniper may remain attached for a year or more.

In most cases, apple and crabapple trees are not seriously damage by cedar apple rust. However, severe defoliation can cause reductions in fruit size and weakening of the tree. Avoid planting susceptible apple or crabapple trees near cedars or junipers, or remove existing cedars or junipers if feasible. Planting resistant varieties of apple and crabapple will also help reduce the severity of cedar- apple rust. Ornamental varieties rarely require fungicide treatment. In severe cases, chlorothalonil (Daconil 2787) or thiophanate-methyl (Cleary=s 3336) may be applied to ornamental apples and crabapples in the spring. To manage cedar apple rust on edible apples and crabapples, apply Thiophanate-methyl (Cleary=s 3336) in the spring.

Cedar-apple rust does not usually cause serious damage to cedar or juniper. To reduce the severity of rust, avoid planting susceptible cedar and juniper varieties near apple or crabapple trees. If susceptible varieties of cedar or juniper are already planted near apple trees, removal of the apple or crabapple tree may be an effective form of control. However, spores may be blown from neighboring trees allowing the disease to continue. If cedar and juniper galls have already formed, the galls may be pruned from infected trees to help reduce the number of spores available for infection next spring. Fungicides containing copper are available to protect susceptible cedar and juniper varieties.

Resistance to Cedar-Apple Rust
	Apples	Edible Crabapples	Ornamental Crabapples
Most resistant	Keepsake, Liberty, McIntosh, Northwestern Greening, Red Delicious, Regent	Chestnut	Adams, Candied Apple, Kelsey, Sargent, Snowdrift, Thunderchild
Intermediate	Connell Red, Cortland, Duchess, Fireside, Golden Delicious, Haralson, Honeycrisp, Honeygold, Mantet, Minjon, Oriole, Prairie Spy, Red Baron, Redwell, State Fair, Sweet Sixteen, Zesta	Dolgo	Dolgo, Radiant, Red Splendor, Spring Snow
Most susceptible	Beacon, Lakeland, Wealthy
Resistance information updated by Dave Bedford, Scientist at the University of MN Horticultural Research Center, and Doug Foulk, Extension Educator for Ramsey County. The Horticultural Research Center will continue to screen varieties for susceptibility, so readers may wish to check in the future for updated information.

Read fungicide labels carefully and apply only as directed.

Representative trade names may be included along with generic names. This information is supplied with the understanding that no discrimination is intended and no endorsement is implied.

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