Alternate hosts are apple, crabapple, hawthorn and mountain ash.
A gall is an abnormal growth of plant tissues caused by the stimulus of a disease or insect. In the springtime, toothpaste-like spores erupt from these galls. Each mass is called a teliohorn. Spores from teliohorns are blown to an alternate host where they cause leaf spot. These spots are small, bright orange-yellow, and surrounded by a red halo.