Gymnosporangium clavariiforme is a species of rust fungus which alternately infects Juniperus and hawthorns. In junipers,the primary hosts, G. clavariiforme 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 a hawthorn tree. The fungus does not cause serious damage to junipers, but hawthorns can suffer serious loss of haw production due to the effects of the fungus.