This blog post would not have been possible without Victor Riberio, Oregon State University.
Downy brome (Bromus tectorum L.) is a difficult-to-control winter annual weed species in the dryland winter wheat fields of the Pacific Northwest. Lack of downy brome control can cause wheat yield losses up to 92%. Growers heavily rely on very limited options of selective POST herbicides to control downy brome in wheat. Most POST herbicides registered for downy brome control in wheat are from Group 2 (ALS inhibitors). The overreliance on herbicides, particularly with the same site of action, has resulted in the evolution of resistant downy brome populations.
In summer 2021, we collected downy brome seeds from wheat fields in Eastern Oregon to test for resistance. In total, 21 downy brome populations were collected from four counties (Figure 1).
Figure 1. Seed collection of downy brome populations in Eastern Oregon in 2021.
Downy brome seeds were tested at Oregon State University, Corvallis, OR for resistance to Group 1, Group 2, and Group 9 herbicides (Table 1). The herbicide rates included in the screening consisted of the herbicides’ label rate (1X) and two times the label rate (2X) plus an untreated control (Table 1). The materials used in this experiment were for research purposes only and not recommendations for use on winter wheat. Additionally, the 2X is an experimental use above the legal label rate and not a recommendation. Clethodim and glyphosate are not registered for use in winter wheat, but they are used in fallow. Quizalofop is registered for use in CoAXium Wheat only with Aggressor herbicide but not Assure II. Similarly, imazamox is only registered for use in Clearfield Wheat.
Table 1. Herbicide, trade name, site of action (Group #), and rates used in the herbicide screening.
a COC at 1% v/v total spray volume added to spray solution
b NIS at 0.25% v/v total spray volume + AMS at 15 lbs/100 gallons added to spray solution
c NIS at 0.25% v/v total spray volume + AMS at 3 lbs/acre added to spray solution
d NIS at 0.25% v/v total spray volume + AMS at 17 lbs/100 gallons added to spray solution
Herbicide screenings were conducted in the greenhouse using a completely randomized block design with three replications. Herbicides were applied when the downy brome plants reached the two- to three leaf stage using a research track sprayer, calibrated to deliver 15 GPA. Downy brome populations were visually assessed as dead or alive 21 days after treatment.
The study is ongoing. Sixteen populations have been tested thus far. Downy brome resistance to Group 2 herbicides is prevalent in wheat fields in Eastern Oregon (Table 2). Mesosulfuron resistance was the most predominant with 81% of the downy brome populations resistant to this herbicide (Table 2). A significant percentage of resistant populations were observed for pyroxsulam (64%), sulfosulfuron (54%), propoxycarbazone (50%), and imazamox (42%). Around 20-30% of the populations showed reduced sensitivity to Group 2 herbicides which means they are in the process of evolving resistance (R/S = 1X; at least 20% survival) (Table 2). All populations were susceptible to the Group 1 herbicides (ACCase inhibitors) clethodim and quizalofop, and to glyphosate (Table 2).
Table 2. Herbicide resistance screening of downy brome populations collected from wheat fields in Eastern Oregon*
*Abbreviations: NT, Not tested; R, Resistant (≥ 50% survival; red); S, Susceptible (green); R/S, At least 20% survival; yellow.
Figure 2. Photos of the herbicide screenings at 21 days after herbicide application. Rates are sorted from 0 to 2X for each herbicide. Label rate is in red. A-D: population GIL3; E-H: population UMA2. A: clethodim; B: quizalofop; C: imazamox; D: mesosulfuron; E: propoxycarbazone; F: pyroxsulam; G: sulfosulfuron; H: glyphosate.
The widespread occurrence of ALS- resistant downy brome populations limits effective post-emergence herbicide options in wheat. The use of Aggressor (quizalofop) with CoAXium Wheat offers an effective control option for ALS-resistant downy brome populations. However, stewardship guidelines and recommendations must be considered to preserve and optimize the efficacy of this technology while preventing the spread and selection of resistant weeds with the repetitive use of quizalofop. Glyphosate is still an effective option for downy brome control in fallow, but alternative sites of action or tank mixes should be considered to preserve its use.
A full dose-response study will be conducted to quantify the levels of resistance in the resistant populations. Additionally, investigations of the mechanisms of resistance to ALS inhibitors in these populations will be performed to increase understanding of cross-resistance patterns. Downy brome populations collected in summer 2022 will be investigated this fall.
We would like to thank the Oregon Wheat Commission for funding this research.
Some of the pesticides discussed in this presentation were tested under an experimental use permit granted by ODA. Application of a pesticide to a crop or site that is not on the label is a violation of pesticide law and may subject the applicator to civil penalties up to $7,500. In addition, such an application may also result in illegal residues that could subject the crop to seizure or embargo action by WSDA, ODA, and/or the U.S. Food and Drug Administration. It is your responsibility to check the label before using the product to ensure lawful use and obtain all necessary permits in advance.