Two of the five species of European blackberry (Rubus fruticosus L. aggregate) along the West Coast of the United States are considered invasive. They are also similar in appearance. Biological control of invasive blackberry by Phragmidium violaceum, causal agent of a rust disease, had been under consideration when rust-diseased blackberry was discovered in Oregon in 2005. An investigation was initiated to determine whether this disease would be an important factor affecting population density of these blackberries. Surveys were made over a 5-yr period at more than 30 field sites in the Willamette Valley and along the Pacific coast of Oregon. Diseased and nondiseased blackberry specimens were collected for artificial greenhouse inoculations and for identification. The two blackberry species, Rubus armeniacus and R. praecox, were identified as the most invasive. They were readily distinguished morphologically on the basis of inflorescence and flower characteristics and to a certain extent by differences in primocane leaf and leaflet shape. Artificial greenhouse inoculation studies revealed that R. praecox was susceptible to the rust disease and that R. armeniacus was not. These results were confirmed during a field survey. Results of this investigation revealed that the rust disease will not be effective for biological control of R. armeniacus and other approaches to management of this particular species will be required.
Nomenclature: European blackberry, R. fruticosus L. aggregate, RUBFR; Armenian (=Himalaya) blackberry, Rubus armeniacus Focke, RUBDI; R. praecox Bertol. sensu lato; R. anglocandicans A. Newton; Phragmidium violaceum (Schultz) G. Winter.