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)
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.
External links
Canadian Food Inspection Agency Factsheet: Gymnosporangium yamadae (http://www.inspection.gc.ca/english/sci/surv/data/gymyame.shtml)
Photo of Gymnosporangium clavariaeforme (http://www.iespana.es/caesaraugusta/Fotos/G/Gymnosporangium%20clavariaeforme.jpg) on Common JuniperJuniperus communis
Photo of Gymnosporangium sabinae (http://www.uni-essen.de/botanik/Exkursionen/Gymnosporangium_sabinae.jpg) on Juniperus sabina
Common broadleaf hosts of cedar-apple rust (Gymnosporangium juniperi-virginianae) include apples and crabapples, and the usual conifer hosts are eastern red cedar, Rocky Mountain juniper, as well as a few Chinese, common and prostrate junipers.
Gymnosporangium rust spores blow from galls or infected branches on the conifer host during rainy periods in April and May. If conditions are cool and wet when these spores land on the leaves, fruit or green shoots of susceptible broadleaf hosts, infection occurs.
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