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. The hosts of cedar-apple rust are leaves and fruit of apple and crabapple trees. The hosts of Hawthorn rust include the leaves of pear, hawthorn, apple, and crabapple. The hosts of quince rust include the leaves and fruit of quince and the fruit of pear, apple, and crabapple.
Cedar-apple rust is caused by the fungus Gymnosporangium juniperi-virginianae. Symptoms first appear as small, pale-yellow spots on the upper surfaces of leaves. 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 turn 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 caused by the cedar-apple rust fungus are somewhat like leaf lesions, they 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 surfaces of the galls are covered with depressions much like those on a golf ball. In the spring, when the "cedar apples" become wet, a yellow-orange, gelatinous horn up to 2 inches long protrudes from each depression. Fruit infected by quince rust is usually puckered and distorted at the calyx end.
Spores discharged from these gelatinous horns on red cedar are easily wind-borne and infect apple leaves and fruit. Spore discharge begins at 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 appear about 10 days after infection. Visible fruit infections require a somewhat longer time. Aecia on the undersides of apple leaves or on fruit lesions themselves produce spores. These spores can be carried back to the red cedar by wind and rain. 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 become able to produce gelatinous horns bearing spores that can reinfect apple trees.
Remove red cedar trees from the vicinity of apple trees. Plant scab-resistant apple varieties. Some scab-resistant varieties are immune to cedar-apple rust (see Table 4.1).
Apply fungicides at the pink bud stage of apple.