Fig. 11. Cedar-apple Rust gall on juniper before spore release.
Photo: M. Grabowski.
Fig. 12. Cedar-apple Rust gall on juniper during spore release.
Photo: L. Haugen, Bugwood.org.
Fig. 13. Cedar-apple Rust infection on apple leave.
Photo: M. Grabowski.
Cedar-apple rust is a fungal disease that spends half of its life cycle infecting apple or crab apple trees, and the other half infecting Eastern Red Cedar (Juniperus virginiana) or other species of juniper (Juniperus sp.). This disease can cause damage to leaves and fruit of very susceptible apple varieties, but is only a minor problem on resistant or partially resistant trees.
Cedar-apple rust causes leaf spots on apple and crab apple trees. Leaf spots are first yellow, then bright orange-red. Leaf spots often have a bright red border and may have small raised black dots in the center of the spot. These lesions grow through the leaf and develop small, brownish, spiky projections on the lower surface of the leaf. Very infrequently, fruit may exhibit a similar infection.
If there are red cedars or junipers in the yard or in nearby landscapes or wild areas, it may be useful to recognize that stage of the disease. Round brown galls up to two inches in diameter form on branches. These galls produce gelatinous, orange, horn-like projections during wet spring weather. While the galls may go unnoticed, the horns are bright orange and easily seen.
The cedar-apple rust fungus must alternately infect apple trees and red cedars or junipers and cannot survive without both host plants. The orange horns on red cedars or junipers produce spores in response to wet weather in the spring. These spores are carried on wind and can infect apple trees over a mile away. Galls on red cedars and juniper plants dry up and die in warm dry summer weather and no new infections on apple will occur until the following season. Apple leaf lesions produce spores that infect red cedars or junipers and start the cycle all over again.
Planting resistant varieties is the best strategy for protecting backyard apple trees from cedar-apple rust. Among varieties commonly grown in Minnesota, only Beacon and Wealthy are very susceptible to cedar-apple rust. Most other varieties have some resistance to the disease. There are no effective fungicides available to gardeners to control this disease on fruit producing trees.
The majority of cedar-apple rust infections come from nearby red cedars. If the gelatinous orange tentacled galls are common on nearby cedars or junipers, it is possible scout for the round woody galls with orange horns in spring, and remove the growths to prevent the spread of spores.