CORVALLIS, Ore. — Bright orange, jelly-like growths on incense cedar and deforming lesions on pear fruit often signal a fungal disease known as Pacific coast pear rust.
Pacific coast pear rust, caused by Gymnosporangium libocedri, has a distinctive two-host life cycle that moves between incense cedar and plants in the rose family, including pears. On incense cedar, the fungus produces bright orange masses that resemble thick marmalade.
“It typically cycles from incense cedars to hosts in the rose family in early springtime, around the time that pears go through bud break.”
“On cedar trees, it looks like someone threw orange marmalade all over the tree,” said Jay Pscheidt, plant pathologist with the Oregon State University Extension Service. “It typically cycles from incense cedars to hosts in the rose family in early springtime, around the time that pears go through bud break.”
The fungus begins on incense cedar, which carries but does not initially show disease for about three years. During that time, the jelly-like orange structures develop on cedar twigs and branches and release airborne spores. These spores drift to nearby pear trees and infect young fruit, green shoots and leaves.
In pears and other rosaceous plants, the infection often appears as bright orange or rust-colored spots on leaves and lesions on the fruit surface. Pears may become misshapen or cracked as fruit tissue swells around the infection. Ornamental flowering pears are also susceptible and may develop distorted shoots and premature leaf drop.
After the fungus completes its stage on pears or other rosaceous hosts, spores drift back to incense cedar and infect it again — this time producing a phase known as cedar broom rust.
Host plants and distribution in Oregon
The rust can infect many rose-family species, including:
- Tree hosts: crabapple, hawthorn, mountain ash, apple
- Shrubs: quince, serviceberry, wild roses
In Western Oregon and Southern Oregon — where incense cedar is more common and spring weather is wetter — Pacific coast pear rust is a recurrent problem. It is less common in Central and Eastern Oregon, where incense cedar is limited.
“Wet weather is more favorable for rust diseases, and we saw a ton of it last year because it was so rainy in the spring,” Pscheidt said. “So far this year it’s a medium-level problem.”
Damage and timing
The rust is not harmful to people, but it deforms fruit and reduces yield on pears and other rosaceous plants. Heavy infections on either host can cause branch dieback or overall decline. Symptoms tend to be most visible in May.
Pscheidt emphasized that the best defense is identifying and removing alternate hosts near susceptible pear trees.
Spores can survive on incense cedar for three years but live only a single year on rosaceous plants. Reinfection of pear and cedar happens annually where both hosts are present.
Management options
There are no home-use pesticide products for controlling the disease on incense cedar. Gardeners may apply the fungicide Immunox to protect flowering pears during the vulnerable early-spring period.
More detail about identifying and managing the disease is available in the Pacific Northwest Plant Disease Management Handbook, which includes guides to Pacific coast pear rust and incense cedar rust.
Previously titled Watch out for rust that can jump from incense cedars to fruit trees
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