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Rust Disease of Apple
There are three rust diseases: cedar-apple rust, hawthorn rust, and quince rust. The most common is cedar-apple rust. All three must spend part of their life cycles on red cedar. 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. Of hawthorn rust, hosts are leaves of pear, hawthorn, apple, and crabapple; and of quince rust, hosts are the leaves and fruit of quince and the fruit of pear, apple, and crabapple.
Symptoms
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 to about 1/8 inch in diameter. Eventually, tiny, black, fruiting bodies (pycnia) become visible. Often a number of orange-yellow protuberances, called aecia, are produced in each spot on the underside of the leaf. Infected leaves may remain on the tree or may become yellowed and drop.
Fruit lesions appear on the blossom (calyx) end. They are somewhat like leaf lesions but 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 galls' surfaces are covered with depressions much like those on a golf ball. In the spring, when the "cedar apples" become wet, a yellow, gelatinous horn (telial horn) up to 2 inches long protrudes from each depression.
Disease cycle
Spores discharged from these gelatinous telial horns on red cedar are easily 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 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 become able to produce gelatinous horns bearing spores that can infect apple trees.
Disease management
Fungicide applications should be made at the pink bud stage of apples.