Pacific Coast Pear Rust on mountain ash, Sorbus sp.

Mountain ash, Sorbus, is susceptible to Gymnosporangium libocedri and other Gymnosporangium spp. rust fungi. The alternate host on which the rust overwinters, is the incense cedar (Calocedrus decurrens). Cool rainy spring conditions and close proximity of rosaceous hosts such as hawthorn, mountain ash and serviceberry near the alternate host, incense cedar, allow infections to occur commonly in the Willamette valley.

For more information regarding the biology and management of this disease see:

Online Guide to Disease Management - Pear-Pacific Coast Pear Rust Note: chemical recommendations are for commercial pear production. For information about about similar Gymnosporangium rusts on ornamentals, see Online Guide to Disease Management - Serviceberry - Amerlanchier sp. - Rust. For nursery fungicide efficacy, see Comparison of fungicides for control of rust on serviceberry, 2006. J. W. Pscheidt and John P. Bassinette.

Online Guide to Disease Management - Mountain ash - Rust Image of rust on Sorbus fruit.

Gymnosporangium libocedri - Widely Prevalent Fungi of the United States. Distribution map and images.

Incense cedar rust - Forest Disease Management Notes. USDA/Forest Service, Pacific Coast Region. Fact sheet.

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Robin Rosetta