Gymnosporangium juniperi-virginianae (fungus)


HOST Eastern red cedar, Rocky Mountain juniper, common and prostrate junipers; Alternate hosts: apple, crabapple, occasionally hawthorn

DAMAGE/SYMPTOMS Juniper/cedar: Bulbous, brown to reddish-brown galls of various sizes form on evergreen twigs/branches that were infected the previous year. After rainy periods in spring, bright orange to yellow gelatinous tendrils form on the galls. Galls can cause dieback of small twigs. Apple/ crabapple: Bright orange/yellow spots appear on leaf surfaces in late spring to early summer. The spots enlarge and form finger-like structures on the underside of leaves. Affected leaves may drop during the summer. On fruits, orange/yellow spots can also form. Infection reduces fruit quality and may lead to premature fruit drop.

DISEASE CYCLE In spring, orange-yellow spores are produced on galls of infected juniper plants and dispersed by wind, infecting apple and/or crabapple trees in late spring to early summer. Leaf spots on alternate hosts produce a new set of spores on finger-like structures formed on the undersides of leaves in midsummer. This new set of spores is dispersed by wind and carried to juniper hosts.

MANAGEMENT Ideally the two hosts should be physically separated. Plant resistant varieties whenever possible. Remove galls on junipers in early spring to reduce the inoculum. Disinfect pruning tools between cuts with 70% ethyl alcohol or a standard household disinfectant spray. Consider applying a protectant fungicide (active ingredient myclobutanil, copper, or sulfur) that is labeled for rust. Apply to emerging leaves of ornamental alternate hosts during the time when the galls on the junipers are orange and gelatinous. Multiple applications might be necessary depending on the selected product.


A Cedar-apple rust gall on juniper and bright colored leaf spots on apple leaf. B Orange-yellow leaf spots on alternate host. C Orange spores developing on galls in spring. D Fully developed gelatinous gall on juniper host.


A brown, round cedar-apple rust gall is visible on a juniper branch and bright orange-yellow colored leaf spots are visible on an apple leaf.    

Close-up orange-yellow leaf spots on an alternate host leaf.

This photo shows gelatinous masses containing orange spores starting to emerge from a cedar-apple rust gall in spring.    

This photo shows fully developed orange-yellow gelatinous cedar-apple rust galls on a juniper host.