Thiocarbamate Resistance in Annual Ryegrass (Lolium Rigidum) from Southern Australia
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(Thesis)
Date
2020
Authors
Brunton, David John
Editors
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Preston, Christopher
Gill, Gurjeet
Boutsalis, Peter
Gill, Gurjeet
Boutsalis, Peter
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Abstract
Annual ryegrass (Lolium rigidum Gaudin) is a genetically diverse, obligate outcrossing species that poses a significant threat to the long-term sustainability of cropping regions across southern Australia. This thesis investigated resistance to thiocarbamate herbicides, which are important for the control of L. rigidum in southern Australia. Resistance to the thiocarbamates was confirmed in L. rigidum populations collected from cropping fields in southern Australia between 2014 to 2018. Resistant (R) populations (EP162, 375-14, 198-15, RAC1, 16.2 and A18) and two known susceptible (S) populations (SLR4 and VLR1) were used to quantify and characterise resistance, its possible mechanism, inheritance of resistance and resistance to alternative pre-emergent herbicides. L. rigidum populations from southern Australia identified as resistant to thiocarbamates were further characterised. The thiocarbamate R populations had varying amounts of cross-resistance to chloroacetamide and sulfonylisoxazoline herbicides, inhibitors of very-long chain fatty-acid (VLCFA) synthesis. The populations 198-15 and 375-14 exhibited the highest LD50 to triallate (44.7- and 26.3-fold), prosulfocarb (45.7- and 22.1-fold), S-metolochlor (31.5- and 19.8-fold), metazachlor (27.2- and 14.6-fold) and pyroxasulfone (13.5- and 14.9-fold). This study demonstrates field-evolved resistance to thiocarbamate, chloroacetamide, and sulfonylisoxazoline herbicides in L. rigidum. Phorate, a known inhibitor of cytochrome P450 enzymes was used to identify potential resistance mechanisms. L. rigidum populations were tested for their response to herbicides from three different site-of-action (SOA) in the presence or absence of phorate. Pre-treatment of L. rigidum seeds with phorate resulted in an antagonistic response to thiocarbamate herbicides in multiple resistant L. rigidum populations. In contrast, phorate synergized trifluralin and propyzamide activity in some populations, reducing the LD50 by 50%. Conversely, treatment with phorate had no significant effect on the LD50 for S-metolachlor or pyroxasulfone. This study highlights an antagonistic response in L. rigidum treated with thiocarbamate herbicides and no response to VLCFA-inhibiting herbicides which suggests the possible involvement of a complex of P450-mediated and other mechanisms in L. rigidum populations, similar to those conferring tolerance to these herbicides in wheat. There were diverse modes of inheritance to the thiocarbamate herbicide prosulfocarb between the populations of the segregating R and S phenotypes. Resistance in population 375-14 fitted a 15:1 ratio for the R and S individuals which is likely associated with the presence of two independent dominant alleles. Segregation in the population 198-15 fitted a 13:3 ratio which is expected for the presence of two independent alleles, one dominant and one recessive. The R: S ratio observed in the F2 of EP162 fitted a 9:7 ratio, which is expected for two additive dominant alleles. In contrast, the segregation of population 16.2 fitted a 7:9 ratio consistent with two independent recessive alleles contributing to resistance. These studies showed that inheritance of thiocarbamate herbicides is controlled by the presence of different alleles with varying levels of dominance. Populations EP162, 375-14 and 198-15 were also found to be resistant to the isoxazolidinone herbicides bixlozone and clomazone. After two generations of recurrent selection (RS2) at the recommended field rate, the LD50 to bixlozone in population 198-15 increased 26.9-fold as compared to the S population. The recurrent selection process also increased the level of resistance to bixlozone in populations EP162 and 375-14 compared to the S population. Phorate antagonized bixlozone and clomazone in the susceptible SLR4 (34.6- and 28.1-fold increase in LD50) and populations EP162 (46.6- and 36.5-fold), 375-14 (73.9- and 71.4-fold) and 198-15 (86.4- and 91.5-fold) compared to the absence of phorate. These results suggest possible involvement of P450-mediated metabolism, rather than a lack of herbicide activation. These findings highlight the elevated risk of rapid evolution of cross-resistance to bixlozone and clomazone in thiocarbamate-resistant populations. Field experiments conducted at Paskeville and Arthurton, South Australia, identified effective herbicide options for the control of thiocarbamate-resistant L. rigidum in wheat. Despite presence of resistance to multiple SOA herbicides, the mixture of two different SOA herbicides pyroxasulfone + triallate significantly improved L. rigidum control and wheat yield compared to each herbicide applied alone. The application of glyphosate as a weed seed set control treatment following pyroxasulfone + triallate (pre-plant incorporated) further reduced L. rigidum seed production at both field sites. This study showed effective strategies available to growers to manage thiocarbamate-resistant L. rigidum through the use of different SOA herbicides in combination with effective weed seed set control.
School/Discipline
School of Agriculture, Food and Wine
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2021
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