Characterization and inheritance of dicamba resistance in a multiple-resistant waterhemp (Amaranthus tuberculatus) population from Illinois

Waterhemp (Amaranthus tuberculatus (Moq.) Sauer) is one of the most troublesome agronomic weeds in the midwestern US. The rapid evolution and selection of herbicide-resistance traits in A. tuberculatus is a major challenge in managing this species. An A. tuberculatus population, designated CHR, was identified in 2012 in Champaign County, IL, and previously characterized as resistant to herbicides from six site-of-action groups: 2,4-D (Group 4), acetolactate synthase inhibitors (Group 2), protoporphyrinogen oxidase inhibitors (Group 14), 4-hydroxyphenylpyruvate dioxygenase inhibitors (Group 27), photosystem II inhibitors (Group 5), and very long chain fatty acid synthesis inhibitors (Group 15). Recently, ineffective control of CHR was observed in the field after dicamba application. Therefore, this research was initiated to confirm dicamba resistance, quantify the resistance level and investigate its inheritance in CHR. Multiple field trials were conducted at the CHR location to confirm poor control with dicamba and compare dicamba treatments with other herbicides. Greenhouse trials were conducted to quantify the resistance level in CHR and confirm genetic inheritance of the resistance. In field trials, dicamba did not provide more than 65% control, while glyphosate and glufosinate provided at least 90% control. Multiple accessions were generated from controlled crosses and evaluated in greenhouse trials. Greenhouse dicamba dose-response experiments indicated a resistance level of 5 to 10-fold relative to sensitive parental line. Dose-response experiments using F1 lines indicated that dicamba resistance was an incompletely dominant trait. Segregation analysis with F2 and backcross populations indicated that dicamba resistance had moderate heritability and was potentially a multigenic trait. Although dicamba resistance was predominantly inherited as a nuclear treat, minor maternal inheritance was not completely ruled out. To our knowledge, CHR is one of the first cases of dicamba resistance in A. tuberculatus. Further studies will focus on elucidating the genes involved in dicamba resistance.

Rapid increase in glyphosate resistance and confirmation of dicamba-resistant kochia (Bassia scoparia) in Manitoba

Increased adoption of crops with stacked traits conferring glyphosate and dicamba resistance, and recent confirmation of kochia [<i>Bassia scoparia</i> (L.) A.J. Scott] biotypes resistant to these herbicides in Alberta and Saskatchewan, warrant surveillance of herbicide-resistant kochia in Manitoba. A randomized-stratified survey of 315 sites in Manitoba was conducted in the fall of 2018. Overall, 58% of the kochia populations tested were glyphosate-resistant, while 1% were dicamba-resistant. This survey documents rapid increase in glyphosate-resistant kochia over a five-year time frame, and also confirms the first cases of kochia in Manitoba with dicamba resistance alone and in combination with glyphosate resistance.

Fluroxypyr-resistant kochia [Bassia scoparia (L.) A.J. Scott] confirmed in Alberta

Recent confirmation of dicamba-resistant kochia [<i>Bassia scoparia</i> (L.) A.J. Scott] in Alberta warrants investigation of resistance to other commonly used synthetic auxin herbicides like fluroxypyr. A randomized-stratified survey of 305 sites in Alberta was conducted in 2017 to determine the status of fluroxypyr-resistant kochia. Overall, 13% of the kochia populations were fluroxypyr-resistant. Only 4% of the populations were both fluroxypyr- and dicamba-resistant, indicating that different mechanisms may confer resistance to these herbicides. When combined with estimates of dicamba resistance, about 28% of kochia populations sampled in Alberta in 2017 were resistant to at least one synthetic auxin herbicide.

Survey reveals frequency of multiple resistance to glyphosate and dicamba in kochia (Bassia scoparia)

Glyphosate-resistant (GR) kochia has been reported across the western and midwestern United States. From 2011 to 2014, kochia seed was collected from agronomic regions across Colorado to evaluate the frequency and distribution of glyphosate-, dicamba-, and fluroxypyr-resistant kochia, and to assess the frequency of multiple resistance. Here we report resistance frequency as percent resistance within a population, and resistance distribution as the percentage and locations of accessions classified as resistant to a discriminating herbicide dose. In 2011, kochia accessions were screened with glyphosate only, whereas from 2012 to 2014 kochia accessions were screened with glyphosate, dicamba, and fluroxypyr. From 2011 to 2014, the percentages of GR kochia accessions were 60%, 45%, 39%, and 52%, respectively. The percentages of dicamba-resistant kochia accessions from 2012 to 2014 were 33%, 45%, and 28%, respectively. No fluroxypyr-resistant accessions were identified. Multiple-resistant accessions (low resistance or resistant to both glyphosate and dicamba) from 2012 to 2014 were identified in 14%, 15%, and 20% of total sampled accessions, respectively. This confirmation of multiple glyphosate and dicamba resistance in kochia accessions emphasizes the importance of diversity in herbicide site of action as critical to extend the usefulness of remaining effective herbicides such as fluroxypyr for management of this weed.

Triple-resistant kochia [Kochia scoparia (L.) Schrad.] in Alberta

A randomized stratified survey was conducted in Alberta in 2017 to determine the distribution and abundance of multiple-resistant [acetolactate synthase (ALS) inhibitor, glycine, and synthetic auxin] kochia. All populations were ALS inhibitor resistant, with glyphosate and dicamba resistance confirmed in 50% and 18% of populations, respectively. Ten percent of populations exhibited resistance to all three mode-of-action herbicides.