Glyphosate-resistant hairy fleabane and horseweed populations in the Pacific Northwest

Contributed by Fernando Oreja, Jennifer Gourlie, and Judit Barroso, Oregon State University

Hairy fleabane (Erigeron canadensis L.; syn. Conyza canadensis L. Cronq.) (Image 1) and horseweed or marestail (Erigeron bonariensis L.; syn. Conyza bonariensis L. Cronq.) (Image 2) are among the most problematic weed species across the world. In the Pacific Northwest (PNW), they are considered troublesome annual weeds during fallow periods. Several factors contribute to their population increase, including high fertility, small seed size, wind dispersion, fall and spring emergence, and the appearance of herbicide-resistant biotypes. The prevalence of these species in no-till systems might be attributed to favorable conditions for seed germination and survival. Both species exhibit sensitivity to soil burial; even a shallow burial depth significantly reduces their seed germination. Additionally, the reliance on herbicides in no-till systems can lead to repetitive application of specific modes of action, contributing to selection pressure and the development of resistant weed populations that are difficult to manage.

Wheat farmers from the Columbia River Basin of Oregon have reported insufficient control of Erigeron populations despite glyphosate application, prompting inquiries about resistance screening. Consequently, an experiment was conducted to investigate potential glyphosate resistance in these populations. Seeds from suspected populations of both horseweed (HW1 and HW2) and hairy fleabane (HFB1 and HFB2) were collected from fallow fields in Umatilla and Wasco Counties, respectively. Additionally, potential susceptible horseweed populations (HW3 and HW4) were collected at the Columbia Basin Ag. Research Center (CBARC) in Umatilla County. The seeds were sown in 4-inch diameter cups filled with commercial potting mix and placed in the greenhouse. Each pot was thinned to one seedling, and four pots (replicates) were allocated per treatment. When the hairy fleabane seedlings were about 4 to 5.5 inches tall and the horseweed rosettes were between 4 – 5 inches in diameter, herbicides were applied. The applications were conducted using a spray chamber equipped with a single nozzle (Even Flat Spray TP8002E tip, Teejet Technologies) set to deliver 15 gal/ac. Glyphosate (Gly Star® 5 Extra, Albaugh, Ankeny, IA, USA) was sprayed at 0X, 1X, 2X, 4X, and 8X, with the commercial rate of X = 18 fl oz/ac. Plant injury was evaluated visually 21 days after treatment (DAT) using a five-point scale. Plant biomass was also assessed at that time (Image 3). For simplicity, data are provided as a percentage relative to the control (untreated plants; Figure 1).

The log-logistic model fit to the experimental data indicates that the glyphosate rate to control 50% of the suspicious populations (HW1, HW2, HFB1 and HFB2) was 3.3, 3.9, 4.0 and 39 times higher than for the susceptible populations (HW3 and HW4). Notably, populations HFB1 and HFB2 from Wasco were not killed completely even at 8X (Figure 1). Conversely, both susceptible populations, which had never been directly exposed to glyphosate, were effectively controlled at 1X (Figure 1). This experiment confirmed the presence of glyphosate resistance in the suspicious populations.

These findings underscore the importance of integrated weed management approaches that incorporate diverse control methods and the use of diverse herbicide modes of action to maintain agricultural productivity in affected regions and to mitigate the spread of resistant populations.