Gymnosporangium

Gymnosporangium is a genus of heteroecious 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 Maloideae (apples, pears, quinces, shadbush, hawthorns, rowans and their relatives).

In junipers (the primary hosts) (see photo), the fungus forms a ball about 2-4 cm in diameter which produces a set of orange tentacle-like spore tubes called telial horns. These horns expand and have a jelly like consistency when wet. The spores are released and travel on the wind until they infect an apple, pear, or hawthorn tree.

On the secondary hosts, 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. 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.


 * Selected species

References and external links

 * Phillips, D. H., & Burdekin, D. A. (1992). Diseases of Forest and Ornamental Trees. Macmillan.
 * Scharpf, R. F., ed. (1993). Diseases of Pacific Coast Conifers. USDA Forest Service Agricultural Handbook 521.
 * Penn State Extension Gymnosporangium Fact Sheet
 * Forestry Images: Gymnosporangium
 * Canadian Food Inspection Agency Factsheet: Gymnosporangium yamadae
 * Photo of Gymnosporangium sabinae on Juniperus sabina
 * Photo of Gymnosporangium juniperi-virginianae on an apple leaf