Cedar apple rust on Zestar apple, M Grabowski UMN Extension
Several rust fungi are now showing up in brilliant colors on hawthorn, apple, crabapple and other trees and shrubs in the Rosaceae family. Infected leaves have bright orange, yellow or red spots. Infected fruit are covered with small tube like fungal structures, that pour out large quantities of powdery orange fungal spores. Cedar apple rust, hawthorn rust and quince rust are all caused by fungi in the genus Gymnosporangium. The Gymnosporangium rust fungi infect juniper for half of their life cycle. On cool wet spring days gelatinous bright orange spores can be found on galls or cankers on juniper trees. Those spores cause the infections on trees and shrubs in the Rosaceae family. In turn spores produced by the Rosaceae trees and shrubs will infect junipers.
Quince Rust on Hawthorn Fruit, M. Grabowski UMN Extension
As a result of this unique life cycle, leaf spots may grow slightly bigger, but no new leaf spots or fruit infections will form this year on the Rosaceae trees and shrubs. The Gymnosporangium rust fungi rarely cause damage to the health of trees and shrubs in Minnesota. So enjoy the colors and unique biology of these interesting fungi. No management is needed.