CEDAR-APPLE RUST is a fungal disease that requires two different host species to complete its life cycle; a primary host and a secondary host. Both must be growing in close proximity for the disease to develop. Primary hosts are cedar trees, and secondary hosts include apple, crabapple, saskatoon or service berry, pear, and mountain ash. A closely related, but more common species of rust in Montana is the JUNIPER-HAWTHORN RUST. Many of our ornamental evergreens are often mistakenly called "cedar", but are actually junipers, which are one of the two plant species necessary for the rust to complete its life cycle. Symptoms and management for both rust species are similar.
The disease overwinters as reddish-brown galls up to two inches in diameter on the branches of the juniper or cedar host. In spring, the galls swell, and are covered with spikes. Later they form gelatinous, yellow-orange "sporehorns" that discharge disease spores. The spores are carried on the wind to infect the secondary hosts such as hawthorn, mountain ash, or apple. In Montana, we seldom see this pest on apple trees. On the broadleaf trees, pale yellow-orange spots form on leaves and fruit, and may develop into numerous thread-like projections about 1/4 inch long.
To manage the disease, prune the galls from the infected evergreen trees at least eight inches below the infected area. Burn the debris, and disinfect pruning tools in a solution of one part household bleach to nine parts water, allowing them to soak about ten seconds. Preventative applications of fungicides may be needed to protect susceptible broadleaf trees that have infected cedar or juniper trees growing in close proximity. Although some twig damage can result on broadleaf trees, the disease will not spread into the rest of the tree from the leaves, twigs, and fruit. When planting trees, select resistant cultivars when possible.
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Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997