Gardening - Fruit: Pests & Diseases
Cedar-apple rust is a common disease of apple and crabapple. The fungus that causes the disease, Gymnosporangium juniperi-virginianae, is unusual because it must spend a phase of its life cycle as a parasite on Juniperus species, such as red cedar or ornamental junipers.
Cedar-apple rust can be severe on apple. Infections of apple fruit result in lower fruit quality and early fruit drop. Leaf spots may cause early defoliation, especially during dry summers. If trees are defoliated several years in a row, they become weakened and unthrifty. Fruit bud formation may be reduced after one year. The disease is not as harmful to juniper, causing galls but not severely affecting plant vigor.
"SP277-A-Cedar Apple Rust," The University of Tennessee Agricultural Extension Service, , http://trace.tennessee.edu/utk_agexgard/70