There are three rust diseases: cedar-apple rust, hawthorn rust, and quince rust. All three fungi spend part of their life cycle on the eastern red cedar and are problems only when red cedar is found close to the orchard. The most common is cedar-apple rust. These diseases can cause economic losses in several ways. Severe leaf infection and defoliation may make trees susceptible to winter injury. Severe defoliation reduces fruit size and quality, and infected fruit is deformed, sometimes very seriously. Cedar-apple rust occurs on leaves and fruit of apple and crabapple trees. Hawthorn rust occurs on leaves of pear, hawthorn, apple, and crabapple. Quince rust occurs on the leaves and fruit of quince and the fruit of pear, apple, and crabapple.

On leaves, cedar-apple rust, caused by the fungus Gymnosporangium juniperi-virginianae, first appears as small, pale-yellow spots on the upper surfaces. The spots enlarge and eventually, tiny, black, fruiting bodies (pycnia) become visible. Often a number of orange-yellow pustules, called aecia, are produced in each spot on the leaf.

Infected leaves may remain on the tree or may become yellow and drop. Quince rust, caused by the related fungus Gymnosporangium clavipes, does not infect apple leaves but does infect leaves and fruit of quince and hawthorn and can be observed in the spring on cedar branches. Fruit lesions are somewhat like leaf lesions but are much larger and often cause fruit to become disfigured or to develop unevenly.

Light-brown to reddish-brown galls form on the branches of red cedar. When they are dry and hard they may be 1/2 to 2 inches in diameter and are known as "cedar apples." The gall surface is covered with depressions much like those on a golf ball. In the spring, when the "cedar apples" become wet, a yellow-orange, gelatinous horn (telial horn) up to 2 inches long protrudes from each depression. Fruit infected by quince rust are usually puckered and distorted at the calyx end.

golf ball-like depressions, telial "horns, quince rust on apple

Spores discharged from these gelatinous telial horns are windborne, infecting apple leaves and fruit. Spore discharge begins about the pink stage of apple bloom and is usually completed in a few weeks. Following a few wet periods, the cedar galls die. Spots on apple leaves can be seen about 10 days after infection. Visible fruit infections require a somewhat longer time.

Aecia (yellow lesions) on the undersides of apple leaves or on fruit lesions themselves produce spores. Borne by winds, the spores may be carried back to the red cedar. After lodging in leaf axils or in crevices on cedar twigs, they germinate, infect the twig, and produce tiny galls the following spring. One year later, these galls produce gelatinous horns bearing spores that can infect apple trees.

Remove red cedar trees from the vicinity of apple trees. Plant scab-resistant apple cultivars, which may also be resistant to rusts. Fungicide applications should be made at the pink bud stage of apple and repeated at 7- to 10-day intervals through petal fall.