Gymnosporangium is a genus of heteroecious plant-pathogenic fungi which alternately infect members of the family Cupressaceae, primarily species in the genus Juniperus (junipers), and members of the family Rosaceae in the subfamily Amygdaloideae (apples, pears, quinces, shadbush, hawthorns, rowans and their relatives).

[3] In junipers (the primary hosts, see photo), some species form a ball-like gall about 2–4 cm in diameter which produces a set of orange tentacle-like spore tubes called telial horns.

In other species, such as in G. clarvariforme, the telia are produced directly from the bark of the juniper with no obvious gall formation or swelling.

[4] The basidiospores are released and travel on the wind until they infect an apple, pear, hawthorn or suitable tree.

[5] Due to the economic impacts of the rusts in some areas where orchards are of commercial importance, some regions have attempted to ban the planting of and/or eradicate the coniferous hosts.