The disease caused by Gymnosporangium spp. 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 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.