Blackberry Rust Fungus: Possible New Biological Control

Background Blackberry (Rubus species) is one of the most widespread and devastating invasive weeds on the west coast. Himalayan blackberry (R. armeniacus), and wild evergreen blackberry (also referred to as cut-leaf blackberry or wild R. laciniatus), are major weed pests of the Pacific Northwest (PNW) and northern California. Recently, a rust fungus, Phragmidium violaceum, was discovered on Himalayan blackberry plants in Oregon. Plants of both species are now known to occur from south of San Francisco Bay in California to the Canadian border in Washington and west of the Sierra Nevada and Cascade mountains. Economic losses are estimated in the millions of dollars. Blackberry encroachment results in lost acreage on cattle ranches, reduced range and pasture productivity, displacement of native species, and reduced overall watershed health. Nearly every section of waterway in western Oregon has been impacted by blackberry thickets smothering out other important plant species. Available cultural, chemical, and mechanical control treatments are effective on a small scale and will continue to be part of an integrated pest management program for blackberry. Biological control is an important addition to currently available control methods to significantly reduce landscape-level impacts by this aggressive weed.