Stephen B Powles

• B.Sc., M.Sc., PhD
• Managing Director at The University of Western Australia

BACKGROUND Chlorsulfuron resistance and genetic dominance was evaluated in Raphanus raphanistrum genotypes homozygous (122‐RR, 376‐RR), heterozygous (122‐RS, 376‐RS) and compound heterozygous (122‐R/376‐R) for the target‐site resistance mutations Ala‐122‐Tyr and Asp‐376‐Glu in the AHAS (acetohydroxyacid synthase) gene. RESULTS At the AHAS level, 1...

A wild radish population (R) has been recently confirmed to be cross-resistant to 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides without previous exposure to these herbicides. This cross-resistance is endowed by enhanced metabolism. Our study identified one 2-oxoglutarate/Fe(II)-dependent dioxygenase gene (Rr2ODD1) and two P450 ge...

Background We have previously demonstrated that an aldo‐keto reductase (AKR) from Echinochloa colona (EcAKR4‐1) can metabolize glyphosate and confers glyphosate resistance. This study aims to investigate if the EcAKR4‐1 orthologs from Lolium rigidum also play a role in glyphosate resistance in non‐target‐site based, glyphosate‐resistant (R) L. rigi...

BACKGROUND Eleusine indica (L.) Gaertn. (goosegrass) is a major weed in global cropping systems. It has evolved resistance to glyphosate due to single Pro‐106‐Ser (P106S) or double Thr‐102‐Ile + Pro‐106‐Ser (TIPS) EPSPS target site mutations. Here, experiments were conducted to evaluate the single effect of soybean competition and its combined effe...

The introduction of harvest weed seed control (HWSC) techniques and associated machinery has enabled the routine use of an alternative weed control technology at a novel weed control timing in global grain cropping fields. Driven by the significant threat of widespread populations of annual ryegrass (Lolium rigidum) with multiple-herbicide resistan...

BACKGROUND Lolium rigidum is the most important weed in Australian agriculture and pre‐emergence dinitroaniline herbicides (e.g., trifluralin) are widely and persistently used for Lolium control. Consequently, evolution of resistance to dinitroaniline herbicides has been increasingly reported. Resistance‐endowing target‐site α‐tubulin gene mutation...

Glufosinate is an important and widely used non-selective herbicide active on a wide range of plant species. Resistance evolution to glufosinate in weedy plant species (including the global weed Eleusine indica) is underway. Here, we established the molecular basis of target-site glufosinate resistance in Eleusine indica. Full-length E. indica glut...

BACKGROUND Trifluralin is widely used in Australia as one of the important pre‐emergence herbicides to control annual ryegrass (Lolium rigidum Gaud.) populations. Trifluralin resistance evolution and mechanisms have been identified in some ryegrass populations. RESULTS In this study, 21 putative resistant field survey populations from Western Aust...

Significance Glyphosate is the world’s dominantly used herbicide to control weedy plant species in a wide range of situations, especially in global field crops of soybean, maize, canola, and cotton with genetically engineered glyphosate resistance. Persistent glyphosate selection has led to worldwide evolution of glyphosate-resistant weeds. Several...

Introduction Concurrent natural evolution of glyphosate resistance single and double point EPSPS mutations in weed species provides an opportunity for the estimation of resistance fitness benefits and prediction of equilibrium resistance frequencies in environments under glyphosate selection. Methods and Results Assessment of glyphosate resistance...

Dinitroanilines are microtubule inhibitors, targeting tubulin proteins in plants and protists. Dinitroaniline herbicides, such as trifluralin, pendimethalin and oryzalin, have been used as pre-emergence herbicides for weed control for decades. With widespread resistance to post-emergence herbicides in weeds, the use of pre-emergence herbicides such...

The combination of herbicides with different modes of action has been adopted not only to improve weed control but also to increase the environmental sustainability of plant-protection products. In this study, we showed a synergistic effect of the auxin herbicide 2,4-D amine with the PSII-inhibiting herbicide metribuzin to control the global grass...

Rapid and widespread evolution of multiple herbicide resistance in global weed species endowed by increased capacity to metabolize (degrade) herbicides (metabolic resistance) is a great threat to herbicide sustainability and global food production. Metabolic resistance in the economically damaging crop weed species Lolium rigidum is well known but...

BACKGROUND Tubulin, the target site of dinitroaniline herbicides, is encoded by small gene families in plants. To better characterize the mechanisms of target‐site resistance to dinitroaniline herbicides in the globally important weedy species Lolium rigidum, attempts were made to amplify and sequence α‐tubulin transcripts. RESULTS Four α‐tubulin...

BACKGROUND Barnyardgrass (Echinochloa spp.) is a global weed in rice fields. Quinclorac is commonly used to control barnyardgrass. However, due to persistent use, quinclorac resistance has evolved. We obtained quinclorac‐susceptible (QS) and ‐resistant (QR1, QR2) lines from the progeny of a single resistant E. crus‐pavonis for a resistance mechanis...

BACKGROUND Resistance to the dinitroaniline herbicide trifluralin in Lolium rigidum (annual ryegrass) often is mediated by the enhanced capacity to metabolize the herbicide to less toxic polar conjugates and/or by functionally recessive target‐site mutations in α‐tubulin. RESULTS In two L. rigidum populations possessing enhanced trifluralin metabo...

BACKGROUND The photosystem II (PSII)‐inhibiting herbicides are important for Australian farmers to control Lolium rigidum Gaud. and other weed species in trazine tolerant (TT)‐canola fields. A L. rigidum population (R) collected from a TT‐canola field from Western Australia showed multiple resistance to PSII, acetyl‐coenzyme A carboxylase (ACCase)...

BACKGROUND Multiple‐herbicide resistance in Lolium rigidum and other weed species is increasingly exerting pressure on herbicide discovery research for solutions against resistance‐prone weeds. In this study we investigate: (i) the responses of L. rigidum populations and wheat to the new herbicide cinmethylin in comparison with other pre‐emergence...

BACKGROUND Diflufenican resistance has been reported in wild radish populations since 1998, but the resistance mechanisms have not been investigated. Recently, we identified a wild radish population (H2/10) from the Western Australian grain belt that is resistant (R) to the phytoene desaturase (PDS)‐inhibiting herbicide diflufenican. RESULTS Dose–...

BACKGROUND Relatively new herbicides that target 4‐hydroxyphenylpyruvate dioxygenase (HPPD) are now available for use on the world's great grain crops (rice, wheat, corn and soybean) and for other uses. With widespread and persistent use of HPPD‐inhibiting herbicides, the evolution of HPPD‐inhibiting herbicide resistant weeds is inevitable. Current...

Weeds are a major contributing factor to crop yield loss. Weed control is regularly practiced during the growing season, with many growers making a conscious effort to minimise weed-seed return to the soil seedbank during the cropping program. However, growers may be unintentionally introducing weed seeds through sowing of contaminated crop seed. U...

Background and aims: Resistance to the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4D) in wild radish (Raphanus raphanistrum) appears to be due to a complex, multifaceted mechanism possibly involving enhanced constitutive plant defence and alterations in auxin signalling. Based on a previous gene expression analysis highlighting the plasma m...

Glyphosate, the most commonly used herbicide in the world, controls a wide range of plant species, mainly because plants have little capacity to metabolize (detoxify) glyphosate. Massive glyphosate use has led to world-wide evolution of glyphosate-resistant weed species, including the economically damaging grass weed Echinochloa colona. An Australi...

Echinochloa crus-galli is a major weed in rice fields in China, and quinclorac has been long used for its control. Over-reliance of quinclorac has resulted in quinclorac resistance in E. crus-galli. Two resistant (R) E. crus-galli populations from Hunan, China were confirmed to be at least 78-fold more resistant to quinclorac than the susceptible (...

BACKGROUND A Lolium rigidum population collected from Western Australia was previously reported as highly resistant to dinitroaniline herbicides mainly due to a Val‐202‐Phe substitution in the target site α‐tubulin protein. To further determine the contribution of the 202 mutation to resistance, two sub‐populations, respectively comprising the 202...

Wild radish ( Raphanus raphanistrum L.) is a problematic and economically damaging dicotyledonous weed infesting crops in many regions of the world. Resistance to the auxinic herbicides 2,4-D and dicamba is widespread in Western Australian R. raphanistrum populations, with the resistance mechanism appearing to involve alterations in the physiologic...

BACKGROUND Weed resistance to foliar herbicides has dramatically increased worldwide in the last two decades. As a consequence, current practices of weed management have changed, with an increased adoption of soil‐applied herbicides to restore control of herbicide‐resistant weeds. We foresee metabolism‐based resistance and cross‐resistance to soil‐...

We reviewed the literature to understand the effects of glyphosate resistance on plant fitness at the molecular, biochemical and physiological levels. A number of correlations between enzyme characteristics and glyphosate resistance imply the existence of a plant fitness cost associated with resistance‐conferring mutations in the glyphosate target...

The increasing number of weedy species resistant to dinitroaniline herbicides warrants studies on the evolutionary factors contributing to resistance evolution, including genetic inheritance of resistance traits. In this study, we investigated the genetic control of trifluralin resistance in a well-characterised Lolium rigidum Gaud. population from...

There have been many studies on target-site resistance (TSR) to PSII-inhibiting herbicides, but only a few on the non-target-site resistance (NTSR). Here, we reported both TSR and NTSR to metribuzin in a wild radish population. Dose-response studies revealed a higher level of resistance to metribuzin in the resistant (R) compared to the susceptible...

Synthetic auxin herbicides such as 2,4-dichlorophenoxyacetic acid (2,4-D) are widely used for selective control of broadleaf weeds in cereals and transgenic crops. Although the troublesome weed wild radish (Raphanus raphanistrum) has developed resistance to 2,4-D, no populations have yet displayed an enhanced capacity for metabolic detoxification o...

BACKGROUND Wild radish (Raphanus raphanistrum) is a globally important weed of crops. Two atrazine‐resistant wild radish populations (R1 and R2), collected from the Western Australia grain belt, were investigated for resistance to photosystem II (PSII) herbicides. RESULTS Sequencing of the full‐length psbA gene revealed the well‐known Ser264–Gly s...

Harvest weed seed control (HWSC) is an Australian innovation, developed to target high proportions of weed seed retained at crop maturity by many major weed species. There is the potential, however, that a reduction in the average height of retained seed is an adaptation to the long-term use of HWSC practices. With the aim of examining the distribu...

This study confirmed the first case of glyphosate resistance in Tridax procumbens in Western Australia and investigated the glyphosate resistance mechanisms. Sequencing and cloning of the full EPSPS (5-enolpyruvylshikimate-3-phosphate synthase) coding sequences revealed a point mutation (ACC to TCC) at amino acid position 102, resulting in a novel...

Resistance to the pre-emergence herbicide trifluralin is increasing in Western Australian annual ryegrass (Lolium rigidum) populations. Three L. rigidum populations (SLR31, M2/6 and M4/8, hereafter named as R1, R2 and R3) collected from Australian grain fields were identified to be trifluralin resistant. Both target-site and non-target-site resista...

Background and aims: Resistance to synthetic auxin herbicides such as 2,4-dichlorophenoxyacetic acid (2,4-D) is increasing in weed populations worldwide, which is of concern given the recent introduction of synthetic auxin-resistant transgenic crops. Due to the complex mode of action of the auxinic herbicides, the mechanisms of evolved resistance...

Harvest weed seed control (HWSC) techniques have been implemented in Australian cropping systems to target and reduce the number of weed seeds entering the seedbank and thereby reduce the number of problematic weeds emerging in subsequent years to infest subsequent crops. However, the influence of HWSC on ameliorating herbicide-resistance (HR) evol...

The evolution of resistant weed populations in response to intensive herbicide selection pressure is a global issue. Resistance to post-emergence herbicides is widespread, whereas soil-applied pre-emergence herbicides can often remain effective. For example, in Australia pyroxasulfone is a new pre-emergence soil-applied herbicide which provides con...

The dinitroaniline herbicides (particularly trifluralin) have been globally used in many crops for selective grass weed control. Consequently, trifluralin resistance has been documented in several important crop weed species and has recently reached a level of concern in Australian Lolium rigidum populations. Here, we report novel mutations in the...

The late 1980s saw an explosion in the amount and diversity of herbicide resistance, posing a threat to crop production in many countries. The rapid escalation in herbicide resistance worldwide and in the understanding of resistance at the population, biochemical, and molecular level is the focus of this timely book. Leading researchers from North...

In Australia, widespread evolution of multi-resistant weed populations has driven the development and adoption of harvest weed seed control (HWSC). However, due to incompatibility of commonly used HWSC systems with highly productive conservation cropping systems, better HWSC systems are in demand. This study aimed to evaluate the efficacy of the in...

Background: The pre-emergence dinitroaniline herbicides (such as trifluralin and pendimethalin) are vital to Australian no-till farming systems. A Lolium rigidum population collected from the Western Australian grain belt with a 12-year trifluralin use history was characterised for resistance to dinitroaniline, acetyl CoA carboxylase (ACCase)- and...

Weed population simulations can be useful to predict the effects of alternative management practices on herbicide resistance (HR) evolution. Almost all previous simulations have ignored the possibility of within-field spatial structure in a weed population, instead making the implicit assumption of perfect dispersal and spatial homogeneity in popul...

Palmer amaranth is the most troublesome weed problem in mid-southern US crop production. Herbicides continue to be the most commonly employed method for managing Palmer amaranth, despite the weed’s widespread resistance to them. Therefore, farmers need research and extension efforts that promote the adoption of integrated weed management (IWM) tech...

A novel glyphosate resistance double point mutation (T102I/P106S, TIPS) in the 5-enolpyruvylshikimate-3 phosphate synthase (EPSPS) gene has been recently identified for the first time only in the weed species Eleusine indica. Quantification of plant resistance cost associated with the TIPS and the often reported glyphosate resistance single P106S m...

HWSC systems that target weed seed production during harvest have been in use in Australian crop production systems for over 30 years. Until recently, though, grower adoption of these systems has been relatively low. It is now apparent with the introduction of a range of new weed seed targeting systems that there is renewed grower interest in the u...

Author Contributions RB and AP designed and performed the experiments. RB, AP, TG, SP wrote the manuscript. One sentence summary This study provides novel insight into herbicide resistance conferred by GST-based detoxification to allow proactive intervention to minimize weed resistance evolution. Abstract The evolution of herbicide-resistant weed...

Harvest weed seed control (HWSC) systems have been developed to exploit the high proportions of seed retained at maturity by the annual weeds rigid ryegrass, wild radish, bromegrass, and wild oats. To evaluate the efficacy of HWSC systems on rigid ryegrass populations, three systems, the Harrington Seed Destructor (HSD), chaff carts, and narrow-win...

Background: An Eleusine indica population has evolved resistance to glufosinate, a major post-emergence herbicide of global agriculture. This population was analysed for target-site (glutamine synthetase) and non-target-site (glufosinate uptake, translocation and metabolism) resistance mechanisms. Results: Glutamine synthetase (GS) activity extr...

A relatively low number of weed species have evolved resistance to auxinic herbicides despite their use for almost 70 years. This inheritance study with two Raphanus raphanistrum populations multiple-resistant 2,4-D and the ALS-inhibiting herbicide chlorsulfuron determined the number of genes and genetic dominance of 2,4-D resistance and investigat...

Background: Across several agricultural systems the evolution of herbicide resistance has occurred more rapidly to post-emergence than pre-emergence herbicides, however, the reasons for this are not clear. We used a new simulation model to investigate whether interactions between differences in order of application and weed cohorts affected could...

The management of glyphosate-resistant Palmer amaranth has been a challenge in southern United States cropping systems. Registration of dicamba-resistant crops will provide an alternative management option to control herbicide-resistant Palmer amaranth populations, particularly those having resistance to herbicide groups 2, 3, 5, 9, 14, and 27. How...

Background: Organo-phosphate insecticides can inhibit specific cytochrome P450 enzymes involved in metabolic herbicide resistance mechanisms leading to synergistic interactions between the insecticide and the herbicide. In this study we report synergistic versus antagonistic interactions between the organo-phosphate insecticide phorate and five di...

Background: Rice is a major field crop of paramount importance for global food security. However, the increased adoption of more profitable and resource-efficient direct-seeded rice systems (DSR) has contributed to greater weed infestations including weedy rice that has become a severe problem in several Asian regions. In this study we have develo...

Background: When used at effective doses, weed resistance to auxinic herbicides has been slow to evolve when compared to other modes of action. Here we report the evolutionary response of a herbicide-susceptible population of wild radish (Raphanus raphanistrum L.) and confirm that sub-lethal doses of 2,4-D amine can lead to the rapid evolution of...

An overreliance on herbicides in several important grain-and cotton-producing regions of the world has led to the widespread evolution of herbicide-resistant weed populations. Of particular concern are weed populations that exhibit simultaneous resistance to multiple herbicides (MHR). Too often, herbicides are the only tool used for weed control. W...

A large-plot field experiment was conducted at Keiser, AR from fall of 2010 through fall of 2013 to understand to what extent soybean in-crop herbicide programs and postharvest fall management practices impact Palmer amaranth population density and seed production. The effect of postemergence-only (glyphosate-only) (POST) or preemergence (PRE) foll...

Resistance to auxinic herbicides is increasing in a range of dicotyledonous weed species, but in most cases the biochemical mechanism of resistance is unknown. Using 14C-labelled herbicide, the mechanism of resistance to 2,4-dichlorophenoxyacetic acid (2,4-D) in two wild radish (Raphanus raphanistrum L.) populations was identified as an inability t...

In the last two decades the cultivation of transgenic crops has steadily increased worldwide. In Western Australia transgenic glyphosate-resistant canola (GR) has been cultivated since 2009. This study was conducted to examine the potential for transgene persistence outside agricultural fields after commercialization of GR crops. Propagule pressure...

Table S1. Parameter estimates (days to flowering) following early‐flowering time selection using the four‐parameter logistic model [1] used to estimate FD50 parameters. Table S2. Parameter estimates for the days to flowering following late‐flowering selection using the four‐parameter logistic model [1] used to estimate FD50 parameters.

Background: Weeds can be a greater constraint to crop production than animal pests and pathogens. Pre-emergence herbicides are crucial in many cropping system to control weeds that have evolved resistance to selective post-emergence herbicides. In this study we assessed the potential to evolve resistance to the pre-emergence herbicides prosulfocar...

RIM, or ‘Ryegrass Integrated Management’, is a model-based decision support system (DSS) for weed management in broadacre cropping systems that was updated to continue aid the delivery of key recommendations to manage herbicide resistance. This article complements earlier publications by documenting the rationales that underpinned the re-developmen...

Avena is a problematic weed of cropping regions of southern Australia and many areas of the world. In 2010, a random survey was conducted across 14 million hectares of the Western Australian grain belt to monitor the change in herbicide resistance levels by comparing resistance frequency results with a survey conducted in 2005. Screening Avena popu...

Herbicides have been the primary tool for controlling large populations of yield depleting weeds from agro-ecosystems, resulting in the evolution of widespread herbicide resistance. In response, nonherbicidal techniques have been developed which intercept weed seeds at harvest before they enter the soil seed bank. However, the efficiency of these t...

In Australia, triazine herbicides have routinely controlled the Vulpia species (Vulpia bromoides, Vulpia myuros, and Vulpia fasciculata; collectively referred to as silvergrass). However, a simazine-resistant silvergrass biotype, collected from Pingelly in the Western Australian grain belt in 2014, has been confirmed. Compared to the pooled mortali...

Light is an important resource that crops and weeds compete for and so increased light interception by the crop can be used as a method of weed suppression in cereal crops. This research investigated the impact of altered availability of photosynthetically active radiation (PAR) (from crop row orientation or seeding rate) on the growth and fecundit...

The rate of herbicide resistance evolution in plants depends on fitness traits endowed by alleles in both the presence and absence (resistance cost) of herbicide selection. The effect of two Lolium rigidum spontaneous homozygous target-site resistance-endowing mutations (Ile-1781-Leu, Asp-2078-Gly) on both ACCase activity and various plant growth t...

BACKGROUND This study confirms and characterises glyphosate resistance in two polyploid Echinochloa colona populations from north-eastern Australia.RESULTSGlyphosate dose response revealed that the two resistant populations were marginally (up to 2-fold) resistant to glyphosate. Resistant plants did not differ in non-target-site foliar uptake and t...

BACKGROUND Herbicides when applied at the correct plant stage and dose are highly toxic to plants. At reduced, low herbicide doses (below the recommended dose) plants can survive and display continuous and quantitative variation in dose-survival responses. Recurrent (directional) selection studies can reveal whether such a phenotypic variation in p...

A population of Echinochloa colona infesting agricultural fields in the northern region of Western Australia evolved glyphosate resistance after 10 years of glyphosate selection. This study identified two phenotypic (susceptibility 'S' vs resistance 'R') lines from within a segregating glyphosate-resistant population. Estimation of survival, growth...

RIM, or “Ryegrass Integrated Management,” is a model-based software allowing users to conveniently test and compare the long-term performance and profitability of numerous ryegrass control options used in Australian cropping systems. As a user-friendly decision support system that can be used by farmers, advisers, and industry professionals, RIM ca...

Glyphosate is the most important and widely used herbicide in world agriculture. Intensive glyphosate selection has resulted in the widespread evolution of glyphosate resistant weed populations, threatening the sustainability of this valuable once-in-a-century agrochemical. Field-evolved glyphosate resistance due to known resistance mechanisms is g...

Lolium rigidum populations in Australia and globally have demonstrated rapid and widespread evolution of resistance to acetyl coenzyme A carboxylase (ACCase)- and acetolactate synthase (ALS)-inhibiting herbicides. Thirty-three resistant L. rigidum populations, randomly collected from crop fields in a most recent resistance survey, were analysed for...

Random surveys conducted in the Western Australian (WA) grain belt have shown that herbicide-resistant Lolium rigidum and Raphanus raphanistrum are a widespread problem across the cropping region. In 2010, a random survey was conducted to establish the levels of herbicide resistance for common weed species in crop fields, including the minor but em...

Raphanus raphanistrum is a problematic weed, which has become increasingly difficult to control in Australian cropping regions. In 2010, a random survey was conducted across 14 million ha of the Western Australian grain belt to establish the frequency of herbicide resistance in R. raphanistrum and to monitor the change in resistance levels by compa...

Transgenic glyphosate-resistant canola was first commercially grown in Western Australia (WA) in 2010, providing an opportunity to obtain important baseline data regarding the level of glyphosate resistance in weeds following the exclusive use of glyphosate for in-crop weed control. In this study, two surveys (2010 and 2011) were conducted across t...

Dormancy in Lolium rigidum (annual ryegrass) seeds can be alleviated by warm stratification in the dark or by application of fluridone, an inhibitor of plant abscisic acid (ABA) biosynthesis via phytoene desaturase. However, germination and absolute ABA concentration are not particularly strongly correlated. The aim of this study was to determine i...

An Eleusine indica population was previously reported as the first global case of field-evolved glufosinate resistance. This study re-examines glufosinate resistance and investigates multiple resistance to other herbicides in the population. Dose–response experiments with glufosinate showed that the resistant population is 5-fold and 14-fold resist...

RIM, or "Ryegrass Integrated Management,'' is a user-friendly weed management software that integrates long-term economics. As a model-based decision support system, RIM enables users to easily build 10-year cropping scenarios and evaluate the impacts of management choices on annual rigid ryegrass populations and long-term profitability. Best used...

Herbicide resistance continues to escalate in weed populations infesting global wheat (Triticum aestivum L.) crops, threatening grain production and therefore, food supply. Conservation wheat production systems are reliant on the use of efficient herbicides providing low-cost, selective, weed control in intensive cropping systems. The resistance dr...

Acetohydroxyacid synthase (AHAS) inhibitor herbicides currently comprise the largest site-of-action group (with 54 active ingredients across five chemistries) and have been widely used in world agriculture since they were first introduced in 1982. Resistance evolution in weeds to AHAS inhibitors has been rapid and identified in populations of many...

Problem weeds in agriculture, such as Lolium rigidum Gaud., owe some of their success to their large and dormant seed banks, which permit germination throughout a crop-growing season. Dormant weed seed banks could be greatly depleted by application of a chemical that stimulates early-season germination and then kills the young seedlings. Fluridone,...

Background In Australia, glyphosate has been used routinely to control wild radish (Raphanus raphanistrum L.) for the past 40 years. This study focuses on two field evolved glyphosate resistant populations of wild radish collected from the grainbelt of Western Australia.ResultsTwo wild radish biotypes were confirmed to be glyphosate resistant by co...