Gymnosporangium globosum (fungus)

 

HOST Evergreen hosts: Eastern red cedar, Rocky Mountain juniper, occasion- ally on creeping and low junipers; Alternate hosts: hawthorn, apple, crabapple; occasionally serviceberry, quince, pear

DAMAGE/SYMPTOMS Juniper/cedar: Small, brown to reddish-brown galls form on evergreen twigs/branches that were infected the previous year. After rainy periods in spring, bright orange gelatinous tendrils form on the galls. Galls can cause dieback of small twigs. Hawthorn/apple/crabapple: Bright orange/ yellow spots appear on leaf surfaces in late spring to early summer. The spots enlarge and form whitish finger-like structures on the underside of leaves. Affected hawthorn leaves turn completely yellow and are dropped prematurely.

DISEASE CYCLE In spring, orange/yellow spores are produced on galls of infected evergreen plants, dispersed by wind and infect hawthorn, 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 can infect the evergreen hosts.

MANAGEMENT Plant resistant varieties whenever possible. Remove galls on evergreen plants in early spring to reduce the inoculum. Disinfect pruning tools between cuts with 70% ethyl alcohol or a standard household disinfectant spray. High value plants can be treated with a rust-labeled protectant fungi- cide containing the active ingredient myclobutanil, copper, or sulfur. 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 Bright yellow leaf spots on hawthorn. B Mature leaf spots with black fungal fruiting structures developing in centers. C Mature gall on cedar branch with emerging gelatinous spore structures. D Fully developed gelatinous gall on evergreen host.

 

Bright yellow leaf spots are visible on hawthorn leaves.    

Hawthorn leaves with visible mature orange-yellow leaf spots. Black fungal fruiting structures are visible in the centers of mature leaf spots.

This photo shows a mature cedar-hawthorne rust gall on a cedar branch with emerging gelatinous spore structures.    

A fully developed orange-colored gelatinous gall on the evergreen host.