Cedar-apple rust usually causes only cosmetic damage to eastern red cedar or rocky mountain juniper, both commonly used in home landscaping. Showing up first as woody tumors on small twigs, the cedar rust galls indicate a systemic infection deep within the tree and are unaffected by sprays. Since this fungal infection depends on two hosts--apple and cedar--successful treatment and control depends on breaking the cycle of infection between the two.
Avoid planting both apples and cedars in the same yard. Although infection spreads through the air and may pass between these types of trees over a distance of two to four miles, wind speed and direction may increase or decrease that range. With cedars and apples growing close together, the infection could intensify and eventually kill both types of trees.
Spraying cedars with fungicide may prevent infection but already infected trees don't benefit from application of this poison. Treat nearby infected apple trees instead, since the fungal infection affects the surface tissue of apple trees and fungicide breaks that part of the infection cycle. Regular spraying in spring when cedar galls emit spores protects apples from the fungus. Fungicide applied to apple trees also prevents re-infection of cedars. Ferbam, myclobutanil and triadimefon treat rust in apple trees effectively.
Pruning out the woody galls which appear on cedar branches late in the summer will effectively limit the spread of the illness. Prune any time from late summer to early spring before warm spring weather arrives. Warm, wet weather in early April triggers the spore cycle of the gall. Clip off the twig hosting the gall, not just the gall itself. Cedar branches renew from the tip and a partially pruned twig isn't likely to re-grow. Unless a cedar hosts dozens of galls the damage from careful pruning won't show. If not reinfected by spores from sick apple trees, the disease cycle ends.
Wild junipers or cedars often show little resistance to cedar-apple rust. Replacing eastern red cedars and rocky mountain junipers with other evergreens of similar growing habit eliminates one of the host species without drastically changing the appearance of the landscaping. Spruce substitutes well for eastern red cedar, growing to a similar size and form, while euonymus can take the place of low-growing junipers. Planting disease-resistant apple cultivars offers the same solution from the other side of the problem. Red-free and Freedom apples as well as Liberty and William's Pride show complete immunity to cedar-apple rust.