Russian thistle

Group 2 resistant Russian thistle increasing

Of 45 Russian thistle populations tested from central and southern Alberta in 2017, 31 (62%) were Group 2 (ALS inhibitor) resistant. No populations exhibited resistance to Group 9 (glyphosate).

A random survey of Russian thistle was conducted post-harvest in fall, 2017. A total of 45 populations were collected across 11 counties/MDs. Seedlings grown from the seeds of the collected plants were sprayed with glyphosate or Refine SG (16.6% tribenuron, 33.3% thifensulfuron).

No confirmed cases of glyphosate resistance

The survey did not find any cases of glyphosate-resistant Russian thistle. However, this result did not surprise the researchers as the initial discovery of resistance usually occurs through screening of samples submitted by farmers and not from random surveys. However, glyphosate resistant Russian thistle has been confirmed in Montana and Orgeon and will likely be confirmed on the southern Prairies at some point in the future.

Higher than expected Group 2 resistance

The first Group 2 resistant Russian thistle was discovered in Saskatchewan in 1989, and during the 2000s, relatively few (<10) were confirmed in southern and central Alberta, and Saskatchewan. Despite having similar biology as kochia, which has wide-spread Group 2 resistance, Russian thistle resistance to Group 2 was evolving more slowly.

However, that evolution appears to be changing. In the 2017 survey, Group 2 resistance was confirmed in 31 out of the 45 populations (62%) in 8 out of the 11 counties/MDs.

The researchers indicated that the incidence of resistance was greater than expected, given the relatively few cases previously. However, their experience with herbicide resistance in kochia is that once resistance to an herbicide site-of-action is selected in one or a few fields, the spread via seed and pollen across the region is very rapid. The average frequency of resistant Russian thistle plants in screened samples was 49%, indicating that most populations were segregating for resistance.

Resistant populations were found primarily in cereal stubble (two-thirds of sites), followed by field pea or lentil stubble (one-quarter of sites). The remaining resistant field populations were found in canola, potato, and chem-fallow (one field each). In addition, four resistant populations were located in ditch areas and two at oil well sites.

Management options

In cereal and oilseed crops, there are a number of options for controlling Group 2 resistant Russian thistle. However, options are very limited in pulse crops. In field pea, only bentazon (Group 6) can be used in-crop, and ethalfluralin or trifluralin (Group 3) can be applied pre-emergence. In lentil, trifluralin can be applied pre-emergence, while ethalfluralin only provides suppression of Russian thistle. The researchers note that careful crop rotation planning is required when growing pulse crops.

The researchers concluded that herbicide resistant Group 2 Russian thistle will spread quickly throughout all populations. Since seed and pollen gene flow in Russian thistle is similar to that of kochia, resistance will inevitably result in frequent and widespread single- or multiple-resistance trait stacking in individuals and populations. A collective community or regional response is required to mitigate the spread of herbicide resistance Russian thistle.

The Alberta Crop Industry Development Fund provided financial support of this project. The map was produced by D. Giffen, Agriculture and Agri-Food Canada, Saskatoon, SK (2018)

Beckie, H.J., Hall, L.M., Shirriff, S.W, Martin, E., Leeson, J.Y. 2019. Glyphosate and acetolactate synthase inhibitor resistance in Russian thistle (Salsola tragus L.) in Alberta. Can. J. Plant Sci. 99: 384–387 (2019)