Italian 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.

Reduced control of some glyphosate-resistant hairy fleabane populations with paraquat has raised concerns about evolved multiple resistance to both glyphosate and paraquat. The objective of this study was to confirm the presence of multiple-resistant (glyphosate and paraquat) hairy fleabane populations in California. A series of dose-response experiments was conducted to evaluate the effect of glyphosate and paraquat in a known susceptible (S) and putative multiple-resistant (R) population of hairy fleabane. The greenhouse experiments were conducted during summer, fall, and winter under controlled temperature and natural light conditions. Multiple-resistant hairy fleabane was identified; however, the level of resistance to glyphosate varied substantially among seasons. During the summer, the glyphosate rate required to reduce growth by 50% (GR₅₀) for the R population was 0.94 kg ae ha⁻¹, 5.2-fold more than for the S population. In the fall and winter experiments, however, the R population response to glyphosate was similar to the S population with a GR₅₀of 0.22 kg ae ha⁻¹or less. Multiple-resistant plants were controlled in the fall and winter at rates that did not control the same population during summer. GR₅₀of paraquat varied among seasons (0.94, 0.24, and 0.07 kg ai ha⁻¹during summer, fall, and winter, respectively); however, plant mortality was more consistent. This is the first reported case of glyphosate–paraquat resistance in hairy fleabane and the multiple-resistant population could pose a significant challenge to annual no-till and perennial cropping systems in California. Further research on the mechanisms of resistance and the physiological factors underlying the seasonally variable response to glyphosate is needed.