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Pathogenicity of Agrobacterium Species from the Noxious Rangeland Weeds Euphorbia esula and Centaurea repens . A. J. CAESAR, USDA-ARS, Rangeland Weeds Laboratory, Biological Control of Weeds Research Unit and Department of Plant Pathology, Montana State University, Bozeman 59717-0056. Plant Dis. 78:796-800. Accepted for publication 3 May 1994. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society. 1994. DOI: 10.1094/PD-78-0796.

Disease surveys were made of Russian knapweed (Centaurea repens) and leafy spurge (Euphorbia esula), two noxious weeds that severely infest large areas of rangelands in the northern Great Plains. Strains of Agrobacterium tumefaciens were isolated from Russian knapweed exhibiting crown galls in New Mexico in 1991. Strains pathogenic to one or more of Helianthus annuus, Lycopersicon esculentum, Nicotiana tabacum, Datura stramonium, or Russian knapweed and known strains of biovars 1 and 2 of A. tumefaciens and A. vitis were inoculated on Russian knapweed and two other knapweed species, diffuse (C. diffusa) and spotted (C. maculosa). Some strains from New Mexico were strongly pathogenic to diffuse knapweed, causing rapidly developing galls that typically girdled, stunted, and caused death of the host. The biovar 1 strains were pathogenic to all three knapweed species, strains of biovar 2 were pathogenic to diffuse and spotted knapweed, and the single A. vitis strain was pathogenic only to diffuse knapweed, forming small galls. Stunted and chlorotic plants of leafy spurge with crown galls, collected in Glacier County, Montana, and plants with root galls collected in eastern North Dakota were infected with A. tumefaciens, which was identified as biovar 1. Leafy spurge plants exhibiting galls on roots collected in North Dakota were infected with strains identified as biovars 1 and 2. Host ranges among nine pathogenic strains from Russian knapweed, leafy spurge, and known strains representing biovars 1 and 2 of A. tumefaciens and A. vitis varied greatly, with six of the nine strains being pathogenic to no more than one additional species besides the original host. These findings indicate that A. tumefaciens may be effective as a biological control of these important rangeland weeds and especially of diffuse knapweed.