Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2007
Publication Date: 3/1/2008
Citation: Eskandari, F., Bruckart, W.L., Schaad, N.W., Sechler, A.J., Postnikova, E.N., Caesar, A.J., Coombs, E. 2008. First Report of Crown Gall Caused by Agrobacterium sp. on Diffuse Knapweed (Centaurea diffusa). Plant Disease. 92:487.
Interpretive Summary: An unusual bacterial disease that caused large galls on diffuse knapweed was discovered in Oregon. This is the first time such a disease has been seen on this plant. The bacterium that causes the disease was isolated and identified as an Agrobacterium species. Identification was based on the disease it causes, reactions in standard bacterial tests on growth media, and morphological characteristics. We have not been able to place it in any of the known species of Agrobacterium at this time. This is the first report of crown gall on diffuse knapweed in the United States (U.S.). It is important to know what diseases occur on introduced invasive U.S. plants, such as diffuse knapweed, particularly as it relates to developing and releasing biological control agents. The isolate and data on molecular characteristics have been deposited in appropriate collections and databases.
Technical Abstract: A specimen of diffuse knapweed (Centaurea diffusa, DK) with crown gall-like symptoms was collected July 27, 2004, in Mosier, Wasco Co., OR (N 45.6842, W 121.4021), and sent to the USDA at Ft. Detrick, MD, for identification. A bacterium was isolated on Potato Dextrose Agar that caused hyperplasia and hypertrophy on carrot disks and tomato stems, either after direct inoculation with a contaminated needle or after inoculation with pieces of galled tissue from symptomatic DK. In biochemical identification tests typical for Agrobacterium sp., the bacterium did not grow on 2% NaCl, grew on D1M agar, produced acid from erythritol but not from melezitose, converted malonic acid to base, and turned litmus milk alkaline. These results are characteristic of biovar 2 (= A. rhizogenes) except for the litmus milk reaction. Pathogenicity tests resulted in gall formation on Centaurea diffusa, C. solstitialis, C. maculosa, C. cyanus, Acroptilon repens, Lycopersicon esculentum, Carthamus tinctorius, Helianthus annuus, and Crupina vulgaris. 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 isolate most closely clustered with A. rubi (strains D12787 and AM181759), differing by 11 bp (99.2% similarity). Also on this basis, it was much different from A. tumefaciens (strains UBA PF2 and AJ389896), A. larrymoorei (strain EMBL Z30542), A. vitis (strains AB3 and S4), and A. sp. (strains O and D14506). Another tumorigenic species, A. tumefaciens (Biovar 1) was reported from New Mexico on A. repens, a species in the same tribe and sub-tribe of the Asteraceae as DK. Host range of the A. repens strain was similar to that of the DK strain studied herein. However, 16S sequencing, which confirmed identification of the A. repens strain as A. tumefaciens, showed it to differ from the DK strain. The DK strain also differed from A. rubi both on the basis of reported biochemical profiles and by 16S sequence comparisons. 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 and 16S sequence comparisons. This is the first report of crown gall on diffuse knapweed in North America. This strain has been entered into the International Collection of Phytopathogenic Bacteria at Fort Detrick as accession no. 60099. The 16S ITS sequence has been put into the GenBank database as accession EF687663.