I have had my eye on these strange brown golf ball shaped growths growing on the twigs of a cedar in my neighborhood for about a year now. I first took notice of them late last spring. They looked pretty nasty but I knew they had to be something interesting. Indeed, interesting doesn't even come close to the reality.
These odd little growths are actually a single stage in the complex life cycle of a group of fungi in the genus Gymnosporangium. Collectively they are referred to as cedar-apple galls. Its a group of fungi whose hosts include junipers and relatives of the apple. Wherever these two lineages coexist you are bound to find this fungus.
Gymnosporangium have a rather interesting life cycle that includes multiple hosts. The golf ball shaped galls will appear on the twigs of a juniper nearly a year after being infected with spores. They grow in size until they reach a point in which they will barely fit in the palm of your hand. The gall itself is covered in a series of depressions, making it look quite out of place in a natural setting. After a year on the tree, the galls enter into their next stage of development.
Usually triggered by the first warm rains of spring, strange gelatinous protrusions start to poke out of each depression on the gall's surface. These protrusions continue to swell until the entire gall is covered in bright orange finger-like masses. These are where the spores are produced. These spores, however, cannot infect another juniper. Instead, they need to land on the next host to complete their life cycle.
If the spores land on a member of the family Rosaceae (though usually apples - genus Malus - are preferred), then the second stage of the life cycle begins. Spores can germinate on both the leaves and the fruit but instead of turning into a large brown gall, they take on a different appearance. This is what makes this fungus readily apparent as a type of rust. A patch of orange will begin to grow. Upon closer inspection one can see that the orange patch is actually a series of small cup-like structures full of spores.
Come fall, the spores are ready to be dispersed by wind. With any luck, these spores will land back on a juniper tree and the cycle will start anew. Because of its propensity for apple crops, cedar-apple rust fungi are considered to be quite a pest. In a more natural setting, however, it is one of the most unique and interesting fungi you can find. It looks truly alien if you aren't already aware of its existence.