Cedar-apple rust fungus begins its life cycle on cedars and other trees in the juniper family, but it completes its cycle on apple trees, where it can damage fruit production. You can control the rust by intervening at either stage.
Identification: Cedar Stage
Known scientifically as Gymnosporangium juniperi-virginianae, cedar-apple rust first appears as roundish or kidney-shaped galls on the small branches of cedar trees. These produce bright-yellow, gelatinous tendrils in spring. The overwinter galls can reach up to 2 inches in diameter. With the protrusions -- "telial horns" -- they can reach the size of a small orange.
Identification: Apple Stage
Rust becomes evident on apple trees as light spots on leaves. It is most obvious when it develops into bright yellow or orange lesions on the undersides of leaves. The lesions in turn form "aecia," hairlike structures that thicken into small cuplike shapes. Lesions and aecia also can form on the fruit itself.
Because both types of trees must be present, within a quarter-mile, for cedar-apple rust to mature, one simple way to prevent the fungus is to separate the trees. You can prevent infestation of apples by removing the woody galls from cedars by hand. They must be removed before the yellow, gelatinous "telial horn" protrusions appear, as these produce the spores that result in apple lesions. Fungicides can protect apple trees from rust, but be careful to use products that are labelled as safe for food crops, not those intended for use on ornamental plants.