Frequency, distribution, and ploidy diversity of herbicide-resistant Italian ryegrass (Lolium perenne spp. multiflorum) populations of western Oregon

Abstract

Italian ryegrass [Lolium perenne L. spp. multiflorum (Lam.) Husnot] is one of the most troublesome weeds worldwide. L. multiflorum is also a grass seed crop cultivated on 50,000 ha in Oregon, where both diploid and tetraploid cultivars are grown. A survey was conducted to understand the distribution, frequency, and susceptibility of L. multiflorum to selected herbicides used to control L. multiflorum. The herbicides selected were clethodim, glufosinate, glyphosate, mesosulfuron-methyl (mesosulfuron), paraquat, pinoxaden, pyroxsulam, quizalofop-P-ethyl (quizolafop), pronamide, flufenacet + metribuzin, and pyroxasulfone. The ploidy levels of the populations were also tested. A total of 150 fields were surveyed between 2017 and 2018, of which 75 (50%) had L. multiflorum present. Herbicide-resistant populations were documented in 88% of the 75 populations collected. The most frequent mechanisms of action were resistance to Acetyl-CoA carboxylase (ACCase), Acetolactate Synthase (ALS), 5-enolpyruvylshikimate-3-phosphate (EPSPs) inhibitors, and combinations thereof. Multiple and cross-resistance, found in 75% of the populations, were the most frequent patterns of resistance. Paraquat-resistant biotypes were confirmed in six orchard crop populations for the first time in Oregon. Herbicide resistance was spatially clustered, with most cases of resistance in the northern part of the surveyed area. ALS and ACCase resistant populations were prevalent in wheat (Triticum aestivum L.) fields. Multiple-resistance was positively correlated with plant density. Tetraploid feral populations were identified, but no cases of herbicide resistance were documented. This is the first survey of herbicide resistance and ploidy diversity in L. multiflorum in western Oregon. Resistant populations were present across the surveyed area, indicating that the problem is widespread.

Evidence of multiple origins of glyphosate resistance evolution in Lolium multiflorum

“…A set of 16 L. multiflorum populations from agricultural fields in the Willamette Valley in Oregon were identified for this study (Figure 1, Supporting Information Table S1). The populations were collected in 2017-2018 as part of a broader survey of herbicide resistance (Bobadilla et al, 2021). A cultivated, public variety of annual ryegrass known as “Gulf’ and a previously characterized multiple-herbicide resistant L. multiflorum population called PRHC from California (Brunharo & Hanson, 2018) were included in the study, as was a cultivated variety of perennial ryegrass ( L. perenne ) used as an outgroup.…”

“…A cultivated, public variety of annual ryegrass known as “Gulf’ and a previously characterized multiple-herbicide resistant L. multiflorum population called PRHC from California (Brunharo & Hanson, 2018) were included in the study, as was a cultivated variety of perennial ryegrass ( L. perenne ) used as an outgroup. Gulf has been widely used as a reference susceptible population for L. multiflorum herbicide resistance characterization (Bobadilla et al, 2021), whereas PRHC is a population that exhibits resistance to four different herbicide mechanisms of action, including a known target-site mutation in EPSPS . The populations selected exhibited various herbicide resistance patterns, where resistance to ALS (mesosulfuron and pyroxsulam), acetyl-CoA-carboxylase (ACCase; clethodim, pinoxaden, and quizalofop), and EPSPS (glyphosate) inhibitors were the most common.…”

“…In Oregon, L. multiflorum is a weed in many crops, including perennial ryegrass, tall fescue, wheat, orchardgrass, and annual ryegrass grown for seed. Recently, 60 resistant L. multiflorum populations were identified in Oregon, with some populations resistant to up to four different herbicide mechanisms of action (Bobadilla et al, 2021). The widespread herbicide resistance in grass seed fields in Oregon poses a serious threat to local agricultural communities, because of potential contamination of grass seed lots with herbicide resistant weed seeds.…”

POST control of Italian ryegrass in hazelnut orchards

“…The genetic differentiation was not associated with geographic isolation but possibly with sitespecific crop management actions (Karn and Jasieniuk 2017). Evidence for potential Italian ryegrass population structuring was noted on the clustered distribution of multiple herbicide resistances (Bobadilla et al 2021). Because herbicide-resistant Italian ryegrass in the Willamette Valley is ubiquitous yet locally unique, site-specific recommendations are needed to optimize weed control.…”

“…Based on a recent survey in the area, 88% of the 75 tested Italian ryegrass populations were herbicide-resistant; among the resistant populations, three-quarters displayed resistance to more than one herbicide (Bobadilla et al 2021). Documented cases of herbicide-resistant Italian ryegrass include resistance to the inhibitors of acetyl-CoA carboxylase (ACCase), acetolactate synthase (ALS), 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), glutamine synthetase, long-chain fatty acids, and photosystem I diverter (Bobadilla et al 2021;Heap 2021).…”