G. clavariaeforme (juniper-hawthorn rust)
G. cornutum (juniper-rowan rust)
G. globosum (juniper-hawthorn rust)
G. juniperi-virginianae (juniper-apple rust)
G. sabinae (juniper-pear rust)
G. clavipes (juniper-quince rust)
G. yamadae (Japanese juniper-apple rust)
Gymnosporangium is a fungus which infects alternately members of the genus Juniperus (junipers) and members of the family Rosaceae in the subfamily Maloideae (apples, pears, quinces, hawthorns, rowans and their relatives).
In junipers, the fungus forms a ball about 2-4 cm in diameter which produces a set of orange tentacle-like spore tubes. The spores are released and travel on the wind until they infect an apple, pear, or hawthorn tree.
On the Maloideae tree, the fungus produces yellowish depressions on the leaves. It also infects the fruit, which grows whitish tubes like a Medusa head. These are the spore tubes. The spores must then infect a juniper to complete the life cycle.
The fungus does not cause serious damage to junipers, but apple and pear trees can suffer serious loss of fruit production due to the effects of the fungus. Where orchards are of commercial importance, growers may have to remove any junipers in the area.
Note that the alternative name 'cedar-apple' rust is a misnomer based on the misapplication of the name 'cedar' to some junipers; cedars are immune.
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