On leaves, cedar-apple rust first appears as small, pale yellow spots on the upper surfaces. The spots enlarge to about 1∕8 inch in diameter. Photo by K. Peter.
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.
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. Unlike other fungal diseases, rust leaf and fruit infections will not produce additional infections in the tree.
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 ½ 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.
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. Basidiospores are produced within 4 hours at 52-77°F (Table 2-2). Lesions on leaves begin to appear 10 to 14 days after infection. Visible fruit infections require a somewhat longer time. Quince rust is economically important primarily when an extended wetting period (more than 48 hours) with a mean temperature above 50°F occurs between tight cluster and late pink bud stages.
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.
To minimize cedar apple rust infections, remove cedar trees located near orchards. Fungicide applications using EBDCs, such as ferbam, ziram, manzate, should be made beginning at the pink bud stage of apples and continue through first cover. FRAC group 3 products are effective; several FRAC group 11 products are labeled for "suppression" only. Once rust lesions are observed on leaves and fruit, it is too late for control. Unfortunately, effective organic options are not available.