No herbicide-resistant Russian thistle confirmed in Manitoba
A 2018 survey found that Russian thistle populations collected in Manitoba did not exhibit resistance to acetolactate synthase inhibitors (tribenuron/thifensulfuron; Group 2), synthetic auxins (2,4-D ester or fluroxypyr; Group 4), or glyphosate (Group 9).
The status of herbicide-resistant Russian thistle in Manitoba was unknown, so a baseline survey in Manitoba was conducted in 2018 to document the incidence of herbicide resistance in Russian thistle, and the impact of these biotypes on Manitoba crop production.
While this weed is more abundant in the dry southern regions of Alberta and Saskatchewan, it can also be problematic in western Manitoba where a 2016 survey of annual crops by Leeson et al. found Russian thistle as the 19th most abundant weed in the rural municipalities of North Cypress and Langford and 23rd most abundant weed species in Oakland and South Cypress.
A 2017 survey of Alberta highlights the risk of the development of herbicide resistant Russian thistle. It documented ALS inhibitor resistance (Group 2) in 62% of the Russian thistle populations sampled, which was a marked increase from previous estimates. Spread of herbicide-resistant Russian thistle can occur through wind-driven tumbleweeds and pollen-mediated gene flow, and may introduce these biotypes to Manitoba from western provinces or U.S. states.
A randomized survey of 315 sites covering 46 rural municipalities in Manitoba in October 2018 was conducted to collect Russian thistle samples. The sites were visited post-harvest during a three-week period in early October to ensure that mature seeds were collected for resistance testing. The samples were separately screened for herbicide resistance to tribenuron/thifensulfuron (Group 2), glyphosate (Group 9), 2,4-D ester (Group 4), and fluroxypyr (Group 4).
Russian thistle was found at only 14 of the 315 surveyed sites in Manitoba, including the rural municipalities of Arthur, Cameron, Daly, Edward, Grey, North Cypress, North Norfolk, Sifton, and South Norfolk. All of the populations were found in agricultural fields; eight following harvest of small grain cereals, two following canola, two following soybean, one following corn, and one following potato.
No resistant populations found
None of the Russian thistle populations were found to be resistant to any of the screened herbicides. This despite that ALS inhibitor-resistant biotypes have been confirmed in Saskatchewan since 1989.
Weed surveys have found that Russian thistle frequency in Manitoba has declined since the 1970s, coinciding with the introduction of Group 2 herbicides. However the rapid spread of Group 2 herbicide resistant Russian thistle in Alberta indicate that there is potential for herbicide resistance to develop in Manitoba.
The current survey suggests that herbicide-resistant Russian thistle is likely not present in Manitoba, or is present at an abundance below the survey detection threshold given the relatively small sample size. This is good news for growers in Manitoba because chemical tools to manage Russian thistle remain effective in these regions. Even though herbicide-resistant Russian thistle was not detected in the current survey, growers and agronomists should remain diligent and pay close attention to how this weed responds to herbicide applications in the field.
This research was funded through Agriculture and Agri-Food Canada and the Alberta Wheat Commission, Manitoba Pulse and Soybean Growers, Manitoba Wheat and Barley Growers Association, Saskatchewan Canola Development Commission, Saskatchewan Pulse Crop Development Board, and Western Grains Research Foundation.
Dr. Charles M. Geddes, Dr. Robert H. Gulden, Mrs. Tammy Jones, Ms. Julia Y. Leeson, Ms. Mattea M. Pittman, Dr. Shaun M. Sharpe, Mr. Scott W. Shirriff, and Prof. Hugh J. Beckie. Baseline survey of herbicide resistance in Russian thistle (Salsola tragus L.) finds no resistance in Manitoba. Canadian Journal of Plant Science.
Open Access https://doi.org/10.1139/CJPS-2020-0260
Photo courtesy Charles Geddes.