Abstract
AbstractItalian ryegrass [Lolium perenne L. spp. multiflorum (Lam.) Husnot] is one of the most troublesome weeds worldwide. Lolium multiflorum is also a grass seed crop cultivated on 50,000 ha in Oregon, where both diploid and tetraploid cultivars are grown. For this work, we will refer to the species as L. multiflorum, since the common names annual ryegrass and Italian ryegrass both refer to the same species. A survey was conducted to understand the distribution and frequency of L. multiflorum and its susceptibility to selected herbicides used in its control. The herbicides selected were clethodim, glufosinate, glyphosate, mesosulfuron-methyl (mesosulfuron), paraquat, pinoxaden, pyroxsulam, quizalofop-P-ethyl (quizalofop), 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 resistances were to acetyl-CoA carboxylase (ACCase), acetolactate synthase (ALS), 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitors, and combinations thereof. Multiple resistance 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. Populations resistant to ALS and ACCase inhibitors 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.
Highlights
Weed management is one of the most significant challenges of any agricultural system and, over time, has become even more difficult
These are the crops most affected by herbicide-resistant L. multiflorum, according to grower reports from the region; CropScape data did not differentiate between grass species grown for seed species; many of the randomized points were grass seed species other than tall fescue
Of the 150 fields surveyed in 2017 and 2018, L. multiflorum was present in 50% (75 fields), with a larger percentage in fields surveyed in 2017 (Supplemental Table 1)
Summary
Weed management is one of the most significant challenges of any agricultural system and, over time, has become even more difficult. With the continuous increase of the human population and the continually expanding demands for more efficient agriculture, minimizing yield losses through weed competition is ever more critical (Zahid et al 2016) These high-yield demands have culminated in intensive use of herbicides that have selected herbicide-resistance traits in weed populations, which makes management more challenging and complicated (Bararpour et al 2017). Multiflorum (Lam.) Husnot] is one of the most troublesome weeds worldwide, with resistance documented to eight different mechanisms of action (Heap 2020) Traits such as obligate outcrossing and large seed production make L. multiflorum an excellent candidate species for the evolution of herbicide resistance (Bararpour et al 2017; Brunharo and Hanson 2018; Busi et al 2012)
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