Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2019
Publication Date: 5/30/2019
Citation: Tehranchian, P., Nandula, V.K., Matzrafi, M., Jasieniuk, M. 2019. Multiple herbicide resistance in California Italian ryegrass (Lolium perenne ssp. multiforum) I: Characterization of ALS-inhibiting herbicide resistance. Weed Science. 67:273-280.

Interpretive Summary: Multiple resistance to glyphosate, sethoxydim, and paraquat was previously confirmed in two Italian ryegrass populations, MR1 and MR2, in northern California. Preliminary greenhouse studies revealed that both populations were also resistant to imazamox and mesosulfuron, both of which are acetolactate synthase (ALS)-inhibiting herbicides. Scientists from the Crop Production Systems Research Unit, USDA-ARS, Stoneville, Mississippi and University of California, Davis studied three subpopulations, MR1-A and MR1-P (from MR1), and MR2 (from MR2) to determine the resistance level to imazamox and mesosulfuron, evaluate other herbicide options for the control of these multiple resistant Italian ryegrass, and characterize the underlying ALS inhibitor resistance mechanism(s). Based on LD50 values, the MR1-A, MR1-P, and MR2 subpopulations were 38-, 29-, 8-fold, and 36-, 64-, and 3-fold less sensitive to imazamox and mesosulfuron, respectively, relative to the susceptible (Sus) population. Only MR1-P and MR2 plants were cross-resistant to rimsulfuron whereas both MR1 subpopulations were cross-resistant to imazethapyr. Pinoxaden [ACCase-inhibitor (Phenylpyrazoline 'DEN')] only controlled MR2 and Sus plants at the labeled field rate. However, all plants were effectively controlled (' 99%) with the labeled field rate of glufosinate. Based on I50 values, MR1-A, MR-P and MR2 plants was 712, 1104, and 3-fold and 10, 18, and 5-fold less responsive to mesosulfuron and imazamox, respectively, than the Sus plants. Sequence alignment of ALS gene of resistant plants revealed a missense SNP resulting in a Trp574-to-Leu substitution in MR1-A and MR1-P plants, heterozygous in both, but not in the MR2 plants. An additional homozygous substitution, Asp376-to-Glu, was identified in the MR1-A plants. The above results suggest an altered target- site is the mechanism of resistance to ALS inhibitors in MR1-A and MR1-P plants and a non-target site based resistance apparatus in the MR2 plants. An integrated weed management program incorporating a combination of chemical, mechanical, and cultural strategies is recommended for controlling multiple resistant Italian ryegrass from California and other states in the US .

Technical Abstract: Multiple resistance to glyphosate, sethoxydim, and paraquat was previously confirmed in two Italian ryegrass populations, MR1 and MR2 in northern California. Preliminary greenhouse studies reveled that both populations were also resistant to imazamox and mesosulfuron, both of which are acetolactate synthase (ALS)-inhibiting herbicides. In this study, three subpopulations, MR1-A (from seed of MR1 plants that survived a 16X rate of sethoxydim), MR1-P (from seed of MR1 plants that survived a 2x rate of paraquat), and MR2 (from seed of MR2 plants that survived a 16X rate of sethoxydim), were investigated to determine the resistance level to imazamox and mesosulfuron, evaluate other herbicide options for the control of these multiple resistant Italian ryegrass, and characterize the underlying ALS inhibitor resistance mechanism(s). Based on LD50 values, the MR1-A, MR1-P, and MR2 subpopulations were 38-, 29-, 8-fold, and 36-, 64-, and 3-fold less sensitive to imazamox and mesosulfuron, respectively, relative to the susceptible (Sus) population. Only MR1-P and MR2 plants were cross-resistant to rimsulfuron whereas both MR1 subpopulations were cross-resistant to imazethapyr. Pinoxaden [ACCase-inhibitor (Phenylpyrazoline 'DEN')] only controlled MR2 and Sus plants at the labeled field rate. However, all plants were effectively controlled (' 99%) with the labeled field rate of glufosinate. Based on I50 values, MR1-A, MR-P and MR2 plants was 712, 1104, and 3-fold and 10, 18, and 5-fold less responsive to mesosulfuron and imazamox, respectively, than the Sus plants. Sequence alignment of ALS gene of resistant plants revealed a missense SNP resulting in a Trp574-to-Leu substitution in MR1-A and MR1-P plants, heterozygous in both, but not in the MR2 plants. An additional homozygous substitution, Asp376-to-Glu, was identified in the MR1-A plants. Addition of malathion or piperonyl butoxide (PBO) did not alter the efficacy of mesosulfuron on MR2 plants. In addition, presence of 2,4-D had no effect on the response of mesosulfuron on the MR2 and Sus. The above results suggest an altered target- site is the mechanism of resistance to ALS inhibitors in MR1-A and MR1-P plants and a non-target site based resistance apparatus in the MR2 plants.