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First Report of Crown Gall Caused by an Agrobacterium sp. on Diffuse Knapweed (Centaurea diffusa)

Diffuse knapweed (DK) plants were discovered in Mosier, Wasco County, OR (45.6842°N, 121.4021°W) with crown gall-like symptoms near the soil line. Specimens were collected on 27 July 2004 and sent to the USDA-ARS at Ft. Detrick, MD for identification of disease and pathogen. Pure culture of a bacterium was obtained on potato dextrose agar, and hyperplasia and hypertrophy developed on carrot disks and tomato stems after wound inoculation with a needle contaminated by the agar culture. The same bacterium was reisolated from the galls on DK, thus fulfilling Koch's postulates. Pathogenicity tests involving needle inoculations of stems and petioles resulted in gall formation on Acroptilon repens, Carthamus tinctorius, Centaurea solstitialis, C. maculosa, C. cyanus, Crupina vulgaris, Helianthus annuus, and Rubus armeniacus. In biochemical tests typically used for identification of Agrobacterium species (3), the DK strain grew on D1M agar but not on 2% NaCl medium, produced acid from erythritol but not from melezitose, converted malonic acid to base, and turned litmus milk alkaline. These results are characteristic of Agrobacterium rhizogenes (= Biovar 2), except for the litmus milk reaction. Using 16S rRNA cluster analysis by unweighted pair group method with arithmetic mean (UPGMA, 500 replicates) and basic local alignment search tool (BLAST), the DK strain clustered most closely with A. rubi (GenBank Accession Nos. D12787 and AM181759). The DK strain differed from A. larrymoorei (GenBank Accession No. Z30542), A. tumefaciens (GenBank Accession No. AJ389896), A. rhizogenes (GenBank Accession No. AB247607), and A. vitis (GenBank Accession No. AB247599) on the basis of 16S rRNA sequence cluster analysis. The DK strain differed from A. rubi on the basis of differential reactions with erythritol, litmus milk, and 2% NaCl medium (2,4); and the 16S rRNA sequence of the DK strain differed from that of A. rubi by 11 bp (99.2% similarity). Comparisons also were made between the DK strain and two strains (83A and 135A) of A. tumefaciens (= Biovar 1), described from New Mexico on A. repens (1), a plant species in the same tribe and subtribe of the Asteraceae as DK. Host range reported for the two A. repens strains after artificial greenhouse inoculations was similar to that of the DK strain and it included diffuse knapweed (1). However, 16S sequencing, which confirmed identification of both A. repens strains as A. tumefaciens, showed they differed from the DK strain. The DK strain belongs in the genus Agrobacterium, but it could not be assigned to any known species on the basis of data from phenotypic or 16S sequence comparisons. To our knowledge, this is the first report of crown gall on diffuse knapweed in the field. This strain has been deposited into the International Collection of Phytopathogenic Bacteria at Fort Detrick (Accession No. 60099), and the 16S rRNA sequence has been deposited into the GenBank database (Accession No. EF687663).

References: (1) A. J. Caesar, Plant Dis. 78:796, 1994. (2) B. Holmes and P. Roberts, J. Appl. Bacteriol. 50:443, 1981. (3) L. W. Moore et al. Page 17 in: Laboratory Guide for Identification of Plant Pathogenic Bacteria. 3rd ed. N. W. Schaad et al., eds. The American Phytopathological Society, St. Paul, MN, 2001. (4) K. Ophel and A. Kerr, Int. J. Syst. Bacteriol. 40:236, 1990.