Confirmation and distribution of paraquat-resistant Italian ryegrass (Lolium perenne L. ssp. multiflorum) in North Carolina

Italian ryegrass is a troublesome weed species commonly found across the United States. In North Carolina, biotypes resistant to herbicides from Groups 1, 2, and 9 have been confirmed. In fall 2020, multiple growers reported unsatisfactory control of Italian ryegrass after sequential burndown applications of paraquat in the Southern Piedmont region of the state. The objectives of this study were to confirm the presence of a paraquat-resistant Italian ryegrass biotype in the state through a whole-plant dose–response bioassay and to characterize the response of Italian ryegrass accessions from the same region to commonly used burndown herbicides. Greenhouse studies were conducted at the North Carolina State University weed science laboratories to evaluate the response of three putative paraquat-resistant Italian ryegrass biotypes (B, H, SB) and four putative susceptible biotypes (S1, S2, S3, and S4) to paraquat rates ranging from 52.5 to 26,880 g ai ha −1 and the response of 38 accessions to clethodim (271 g ai ha −1 ), glyphosate (1,260 g ae ha −1 ), glufosinate (880 g ai ha −1 ), nicosulfuron (34 g ai ha −1 ), and paraquat (840 g ai ha −1 ). The effective paraquat dose required to reduce biomass by 50% (GR 50 ) for the putative paraquat-resistant biotypes ranged from 570 to 1,729 g ai ha −1 , equivalent to 19- to 58-fold more resistant to paraquat compared to the average GR 50 of susceptible biotypes. This study confirms the presence of paraquat-resistant Italian ryegrass in North Carolina. Results from the accessions study reveal that 29% of biotypes tested were resistant to paraquat, all of which also exhibited resistance to glyphosate and nicosulfuron. Additionally, a wide distribution of multiple herbicide–resistant biotypes was observed in the Southern Piedmont region, with 97% and 74% of accessions tested resistant to ≥1 and ≥2 sites of action, respectively.