The disease caused by Gymnosporangium spp., commonly called cedar-apple rust, is actually a complex of three diseases caused by three rust fungi: apple rust by Gymnosporangium juniperi-virginianae, hawthorn rust by G. globosum, and quince rust by G. clavipes. All three diseases can infect most varieties of eastern red cedar (Juniperus virginiana) and many cultivars of juniper, apple and crabapple. In addition to these plants, the hawthorn and quince rusts can infect mountain ash, pear and serviceberry. Quince rust can also infect chokecherry, cotoneaster, photinia and dwarf Japanese quince.
Symptoms and Cause: The symptoms on the deciduous hosts are familiar to most growers and landscapers. The leaves have yellow spots on the upper surface. In late summer, brownish clusters of threads or cylindrical tubes appear beneath the yellow leaf spots, or on fruits and twigs. The spores formed in the threads or tubes infect the leaves and twigs of junipers during wet, warm weather in late summer and early fall.
Galls and swellings on the junipers appear about seven months later and form gelatinous masses of spores after about 18 months. The rust diseases are very conspicuous on red cedar and junipers during spring, when the galls are covered with the orange-brown, gelatinous masses. Rust spores formed on the gelatinous masses can not infect other junipers. They infect the twigs, fruits or leaves of deciduous hosts during wet, rainy weather in early spring.
The symptoms of quince rust on red cedar and juniper include perennial, elongated swellings on the branches, which may crack and form cankers. In damp spring weather, cushion-shaped, orange gelatinous blisters burst through the bark of the branch swellings. Quince rust disease damages the ornamental value of susceptible cedars and junipers, killing young branches and weakening plants when cankers occur on the main trunk.
The symptoms of the apple rust and hawthorn rust on red cedar and juniper include brown, spherical galls on the branches. Apple rust galls are often over 2 inches in diameter; those of hawthorn rust are seldom over 1/2 inch in diameter. They form about 18 months after infection occurs and may remain on the juniper branches for years. Apple rust galls produce spores for only one year, though the hawthorn rust galls will produce spores for several years. The apple rust galls have circular markings on the surface that make them resemble the surface of a golf ball. From the centers of the circular markings protrude cylindrical, orange-brown, gelatinous horns, masses of spores, in wet spring weather. The hawthorn rust galls on junipers do not have the circular pattern on the surface, and the orange-brown gelatinous horns are tongue-shaped rather than cylindrical.
Control: If practical, destroy nearby junipers or prune off galls in late fall or early spring before crabapples bloom. Avoid planting susceptible deciduous hosts nearby red cedars.
Plant resistant hawthorn varieties and cultivars; Washington hawthorn (Crataegus phaenopyrum) is highly resistant, whereas, green hawthorn (C. viridis) and downy hawthorn (C. mollis) are susceptible in the north central states.