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The disease 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, many cultivars of juniper, and 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.
The symptoms 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.
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.
