|Yanke, L. Jay|
Submitted to: International Symposium on Biological Control of Weeds
Publication Type: Proceedings
Publication Acceptance Date: 10/16/2003
Publication Date: 11/1/2004
Citation: Zhang, W., Sulz, M., Mykitiek, T., Li, X., Yanke, L., Kong, H.N., Buyer, J.S., Lydon, J. 2004. A Canadian strain of Pseudomonas syringae causes white-colour disease of Cirsium arvense (Canada thistle). In: Cullen,J.M., Briese, D.T., Kriticos, D.J., Lonsdale, W.M., Morin, L. and J.K. Scott, editors.Proceedings of the XI International Symposium on Biological Control of Weeds, April 27 - May 2, 2003, Canberra, Australia. p. 215-220. Interpretive Summary: A disease of Canada thistle that stunts its growth and causes the tops o f the plants to turn white was found in several locations in Canada. A bacterial species, designated CT99B016C, was demonstrated to be the causal agent. When applied with the surfactant Silwet L-77, the strain caused disease in Canada thistle, and in annual sowthistle, spiny sowthistle, and dandelion plants. Biochemical, nutritional, and genetic analysis indicated that the bacteria is a Pseudomonas syringae species. In addition, these tests indicated that CT99B016C is not the same as Pseudomonas syringae pv. tagetis, a bacterium previously shown to cause the same symptoms in Canada thistle and other plant species. This study describes a new bacterial species with potential as a biological control agent of Canada thistle. The information could be useful to researchers who are developing bacterially-based biological control agents to stop the spread of this noxious, invasive weed. The ultimate benefit is an improved knowledge base from which decisions can be made on the proper use of biological control agents, which benefits farmers and the general public alike.
Technical Abstract: Patches of white-colored Canada thistle (Cirsium arvense) plants were recently found on roadsides, pastures, and market gardens in Devon, Mulhurst, Stony Plain, and Edmonton, Alberta, Canada. The diseased plants showed apical chlorosis, sometimes with dark and necrotic leaf spots. These symptoms were also associated with stunted growth, fewer shoots, inhibition of flowering, and/or sterility. A total of 101 bacterial strains were isolated from the leaves, stems, and flowers of white-colored Canada thistle plants. A bacterial species (one strain designated CT99B016C) was consistently isolated from diseased plants and was found to produce similar symptoms on Canada thistle under both greenhouse and field conditions. The organism was reisolated from inoculated, diseased plants, thereby fulfilling Koch's postulates. The optimal bacterial cell concentration to achieve maximum disease was within the range of 108 ¿ 109 cfu/ml, while the optimal surfactant concentration was 0.15-0.3% Silwet L-77â. The CT99B016C strain also caused severe disease of annual and spiny sowthistle (Sonchus oleraceus and S. asper) and dandelion (Taraxacum officinale). The disease severity on these two weed species was even greater than that on Canada thistle. Results of phenotypic tests and fatty acid analysis clearly placed the CT99B016C strain within the Pseudomonas syringae group. Fatty acid analysis also indicate that isolate CT99B016C is more closely related to P. syringae pv. tabaci and P. syringae pv. syringae than P. syringae pv. tagetis. Results from PCRs with primer sets TAGTOX-9 and TAGTOX-10 also indicate that CT99B016C is different from P. syringae pv. tagetis. The exact pathovar identification of CT99B016C remains to be determined.