Clethodim resistance in Lolium rigidum (annual ryegrass) and its management in broadleaf crops

Abstract

Lolium rigidum is one of the most troublesome herbicide resistant weeds in Australia, which has so far evolved resistance to eleven major herbicide groups. Clethodim, an inhibitor of acetyl–coenzyme A carboxylase (ACCase), is a selective post-emergent herbicide used to control annual and perennial grasses in a wide variety of broadleaf crops and has been used by many farmers to manage L. rigidum in continuous cropping rotations. However, repeated use of this herbicide during the last two decades has resulted in the appearance of L. rigidum populations that are highly resistant to clethodim. Studies on the resistance mechanisms, genetics, and fitness of resistant alleles were undertaken to better understand the evolution of clethodim resistance in L. rigidum. Field studies were also undertaken to investigate the performance of alternative herbicides for the management of clethodim-resistant L. rigidum in faba bean and canola. Dose–response experiments were conducted on twelve populations of L. rigidum collected from different locations in Australia. All the populations were confirmed resistant to clethodim with resistance levels ranging from 3-34-fold as compared to the susceptible control. These resistant populations have also evolved cross-resistance to butroxydim. Sequencing of the target-site ACCase gene identified five known ACCase mutations (Leu-1781, Asn-2041, Gly-2078, Arg-2088, and Ala-2096) in these populations. In the highly clethodim-resistant populations, the level of clethodim resistance was influenced by the occurrence of frost close to herbicide application. A significant reduction in the level of clethodim efficacy was observed in resistant populations when plants were exposed to frost for three nights before or after clethodim application. However, there was no effect of frost on the response of the susceptible population suggesting that the mechanism present within the resistant populations interacts with frost to further reduce clethodim efficacy. The inheritance of clethodim resistance was investigated by cross pollinating the susceptible and five resistant populations. The results of the inheritance study showed different patterns of inheritance of clethodim resistance in L. rigidum; which included a single gene, partially dominant, nuclear encoded trait, two different patterns of two-gene inheritance and an example of maternal inheritance of the resistance trait. The fitness of three resistant alleles (Leu-1781, Asn-2041, and Gly-2078) was also studied by determining the change in the frequency of resistant alleles in two generations of L. rigidum in the absence of clethodim use. The results of this experiment showed that there was no significant change in the frequency of Leu- 1781 and Asn-2041 alleles in L. rigidum populations from one generation to other but the frequency of Gly-2078 allele increased significantly (7 to 16%; P ≤ 0.05). Studies were also undertaken to identify alternative herbicides for the control of clethodim resistant L. rigidum with a range of pre-emergent herbicides in broadleaf crops. In both faba bean and canola crops, pre-emergent herbicides alone were insufficient to effectively manage clethodim-resistant L. rigidum. The application of effective soil residual herbicides followed by the post-emergent tank-mixture of clethodim and butroxydim provided acceptable control of some clethodim resistant L. rigidum populations.