Cedar Apple Rust is a fungus that causes leaf damage on trees of the Malus species such as the domestic apples (M. domestica) and the crabapples (M. sylvestris). During its life cycle it needs an alternate host and the alternate hosts are species of the Juniperus genus, especially in Minnesota, the Eastern Red Cedar, Juniperus virginiana. The fungus is Gymnosporangium juniperi-virginianae. It is more damaging to the apples than to the cedars.
The rust begins on the red cedar in the shape of a brown kidney shaped gall and then enlarges (above 1st photo) sometimes resembling a large caterpillar with spines. Those spines are called 'telial' horns'. When cool moist rainy days come in late spring, these enlarge to bright orange horns resembling gelatin (above 2nd photo). From these horns the spores, called 'teliospores', develop and these produce another smaller spore called a 'basidiospore' that affect apple trees when released. In late spring after release of the basidiospores (above 3rd photo) the galls dry up.
Meanwhile, the basidiospores have found leaves of the Malus genus and there they develop rust colored lesions which in turn produce and release a different form of spore, called 'aeciospores', which are carried by wind and rain back to the Juniperus species where they begin another life cycle, which all together, takes almost two years to complete. For further details check out this pdf fact sheet.
Below: Two examples of the rust gall in the enlarged telial horns stage.