Many plants, from grasses to pine trees, get rust diseases. A common experience is to walk through tall grass infected with rust and emerge with shoes coated in orange-red spores. The rust diseases produce several different spores, each spore capable of infecting a different plant. The cedar-apple rusts are a good example. Spores formed in galls on cedar infect apple leaves, causing leaf spots.
Spores formed on apple leaves infect only the cedar needles, causing galls.
Rusts generally infect healthy, vigorous plants in preference to weak plants.