Gymnosporangium juniperi-virginianae Schwein. 1822
Gymnosporangium macropus Link
Gymnosporangium virginianum Sprengel
Cedar apple rust, American apple rust
Cedar apple rust lives out part of its life on two different species of trees (making it a heteroecious parasite). Each of the trees, eastern red cedar and apple trees, must be fairly close together. (Sometimes the fungus inhabits quince or hawthorn instead of apple trees.)
Identification: This fungus transmutes itself through several stages of development. In the phase where it takes up residence in eastern red cedar, it produces uneven light brown sacs about an inch around. The sacs become rounder and resemble an oak gall. That is, until they begin pushing out orange columns, creating a structure that looks a little like a rusty World War II mine. The columns continue to exude, becoming a Medusa-like cluster of orange, slimy tentacles. The tentacles are actually “spore horns,” and they release spores that develop for a time and become airborne. Some of these land on apple trees, where they grow, coating the leaves with an orange-yellow blister-like “rust.” This in turn eventually releases more spores that infect new eastern red cedars, continuing the cycle. For a more detailed explanation, see Tom Volk’s description.
Cedar apple rust does substantial damage to apple trees, and is considered an invasive species in some regions.
Edibility: Please tell me you’re kidding!
Gymnosporangium juniperi-virginianae on Tom Volk's Fungi site, at the Department of Biology at the University of Wisconsin
Gymnosporangium juniperi-virginianae on Squamules
Gymnosporangium juniperi-virginianae on Michael Kuo's MushroomExpert.com
Gymnosporangium juniperi-virginianae on extension.illinois.edu
Gymnosporangium juniperi-virginianae on Wikipedia
Gymnosporangium juniperi-virginianae description by Thomas H. Kent, last updated 25 May 2020.