Cedar-apple cedar-quince, cedar-hawthorn, or Japanese apple rust
Bright-yellow or yellow-orange spots form on leaves. On the upper surface of the leaf spot, small, black fungal fruiting structures form. Later, clusters of cup-shaped structures with fringed edges can be observed on the underside of the infected leaves.
Gymnosporangium juniperi-virginianae (cedar-apple)
G. globosum (cedar-hawthorn)
G. yamadae (Janpanese-apple)
Remove unwanted junipers growing near crabapples. Do not plant junipers close to crabapples. Remove the galls from juniper branches during dormancy. Where the disease seldom occurs or few leaves are infected, no control is necessary. Where disease is frequent and severe, apply a fungicide first when crabapple flower bud tissue can be seen and at petal fall.