The fungus of concern -- Gymnosporangium juniperi-virginianae -- successfully lives in many other common U.S. landscape plants in addition to between eastern red cedar (Juniperus virginiana) and apple trees. The spores of the fungus also prosper and infest other juniper shrubs and ground cover evergreens as well as crabapples. While no eastern redcedar trees may be within several hundred feet of an apple tree, other ubiquitous junipers can lead to the apple or crabapple tree becoming infected. Apple and crabapple trees do not infect each other -- the fungus must live back and forth between them and a cedar or juniper plant.
Fungal Life Cycle
The spores of the fungus spread in the wind during the summer. The fungus tends to proliferate when the humidity is high and temperatures hover in the 50 to 75 F range. On cedar trees and junipers, the spores develop into galls or ball-like tumors that survive the winter. In spring, the galls grow and mature to about quarter coin-sized and look light greenish brown and warty. Anytime it rains, brown telia, hornlike reproductive fungal structures, elongate and turn orange on the galls to release spores into the wind. These spores land on apple trees and cause tiny lesions. After two to three weeks, the center of the lesions on leaf undersides become jagged with bright orange aecia bodies. An aecia is a cuplike fungal reproductive structure. The aecia releases spores, called aeciaspores, into the wind that land on cedar trees and junipers. The life cycle perpetuates between the plants.
Planting Apples Near Cedars
The general guideline is to not plant apple trees in proximity of cedar trees or junipers in an effort to diminish any likelihood of contracting cedar-apple rust. Ideally, do not plant apples within several hundred feet of cedar trees, according to the Plant Disease Diagnostic Clinic at Cornell University. But be aware, wind still might deposit spores among plants located thousands of yards away. Apple trees infected with cedar-apple rust might not produce satisfactory fruits and could be considered useless to the gardener.
Contact your local cooperative extension office and ask about recommendations for planting apples in proximity to cedar trees or landscape junipers. Depending on your climate and soils, certain apple tree varieties demonstrate excellent resistance to the fungus. If you cannot rid your property of cedar trees or junipers, planting only resistant apple cultivars might prove the best option. Pruning off galls or spraying fungicides becomes costly and tedious. The University of Missouri mentions a few apples that don't succumb to cedar-apple rust problems. Cultivars 'Liberty,' 'Nova Easygro,' 'Novamac,' 'Priscilla,' 'Redfree' and 'William's Pride' possess the highest resistance. 'Enterprise,' 'Freedom,' 'Pristine' and 'Trent' apple varieties also offer considerable resistance to the effects of the fungus.