Herbicide-resistant Populations Research Articles

Adjustment of Weed Hoeing to Narrowly Spaced Cereals

Weed hoeing can be successfully performed in wide row crops, such as sugar beet, maize, soybean and wide spaced cereals. However, little experience is available for hoeing in narrow cereal row spaces below 200 mm. Yet, mechanical weed control can pose an alternative to herbicide applications by reducing the herbicide resistant populations present in the field. In this experiment, it was investigated whether hoeing is feasible in cereals with 150 and 125 mm row spacings. The trial was set up at two locations (Ihinger Hof and Kleinhohenheim) in southwest Germany. Three different conventional hoeing sweeps, a goosefoot sweep, a no-till sweep and a down-cut side knife were adjusted to the small row widths, and hoeing was performed once with a tractor and a standard hoeing frame which was guided by a second human operator. The average grain yield, crop and weed biomass, and weed control efficacy of each treatment were recorded. The goosefoot and no-till sweep were tested at driving speeds of 4 and 6 km·h−1. The down-cut side knife was applied at 4 km·h−1. The results indicate that hoeing caused no yield decrease in comparison to a conventional herbicide application or manual weeding. The highest yield with a mechanical treatment was recorded for the no-till sweeps at both trial locations. Hoeing was performed successfully in narrowly spaced cereals of 150 and 125 mm, and the weed control efficacy of the mechanical treatments ranged from 50.9% at Kleinhohenheim to 89.1% at Ihinger Hof. Future experiments are going to focus on more distinct driving speeds ranging from 2 to 10 km·h−1 and performing more than one pass with the hoe. Additionally, combining the mechanical weeding tools with a camera-steered hoeing frame could increase accuracy, allow for higher working speeds and substitute the second human operator guiding the hoe.

Efficacy of Rinskor™ (florpyrauxifen-benzyl ester) on Herbicide Resistant Barnyardgrass (Echinochloa crus-galli) in Rice Fields of Mekong Delta, Vietnam

Barnyardgrass (Echinochloa crus-galli) seed samples were collected in rice fields in different locations at Mekong delta in Vietnam for herbicide resistance tests. The ALS-resistant and synthetic auxin-resistant E. crus-galli were confirmed at several locations in the Mekong Delta. The average LD90 value of bispyribac, penoxsulam and quinclorac for assessed weed populations was 33.1, 15.1 and 550.2 g a.i.ha−1 respectively. There were cross resistant barnyardgrass populations to bispyribac and penoxsulam, the LD90 value of the two ALS inhibitors for E. crus-galli was positively correlated at R2=0.39, the cross resistant population was 33.3% of total sample. The correlation analysis was not useful to evaluate the multiple resistance between quinclorac and the two ALS inhibitors, the R2 value was lower than 0.05, however, the percentage of multiple resistance weed was 36.2% of population. There was no cross resistance or multiple resistance among the 3 tested herbicides and the new synthetic auxin herbicide Rinskor™. All tested weed samples, including quinclorac-resistant populations, were effectively controlled by Rinskor™. There was no difference between control from Rinskor™ in the different herbicide resistant populations. Average LD90 value of Rinskor™ in all tested barnyardgrass was 17.1 g a.i ha−1.

Fitness of ALS-Inhibitors Herbicide Resistant Population of Loose Silky Bentgrass (Apera spica-venti).

Herbicide resistance is an example of plant evolution caused by an increased reliance on herbicides with few sites of action to manage weed populations. This micro-evolutionary process depends on fitness, therefore the assessment of fitness differences between susceptible and resistant populations are pivotal to establish management strategies. Loose silky bentgrass (Apera spica-venti) is a serious weed in Eastern, Northern, and Central Europe with an increasing number of herbicide resistant populations. This study examined the fitness and growth characteristics of an ALS resistant biotype. Fitness and growth characteristics were estimated by comparing seed germination, biomass, seed yield and time to key growth stages at four crop densities of winter wheat (0, 48, 96, and 192 plants m-2) in a target-neighborhood design. The resistant population germinated 9–20 growing degree days (GDD) earlier than the susceptible population at 10, 16, and 22°C. No differences were observed between resistant and susceptible populations in tiller number, biomass, time to stem elongation, time to first visible inflorescence and seed production. The resistant population reached the inflorescence emergence and flowering stages in less time by 383 and 196 GDD, respectively, at a crop density of 96 winter wheat plants m-2 with no differences registered at other densities. This study did not observe a fitness cost to herbicide resistance, as often hypothesized. Inversely, a correlation between non-target site resistance (NTSR), earlier germination and earlier flowering time which could be interpreted as fitness benefits as these plant characteristics could be exploited by modifying the timing and site of action of herbicide application to better control ALS NTSR populations of A. spica-venti.

Spillage of Viable Seeds of Oilseed Rape along Transportation Routes: Ecological Risk Assessment and Perspectives on Management Efforts

Seed spillage during handling and transportation promotes establishment and invasion of feral crops into adjacent semi-natural habitats. This is also the case for oilseed rape (OSR, Brassica napus), where seed spillage may lead to establishment of herbicide resistant OSR populations in countries without cultivation of genetically modified OSR. Using data from Austria – where cultivation and import of genetically modified OSR are banned – as a prime example, we demonstrate that ports, oil mills, switchyards, and border railway stations to countries with different electric current systems – where trains have to stop – are the sites of primary concern with respect to seed spillage. Based on the results of the Austrian case study we discuss common measures to limit crop seed spillage which include intensified controls at border railway stations and the mode of seed packing during transportation. We further recommend sufficient cleaning both of goods wagons and of loading areas of trucks and ships as well as an appropriate weed management.

Biology and management of Avena fatua and Avena ludoviciana: two noxious weed species of agro-ecosystems.

Avena fatua and Avena ludoviciana are closely related grass weed species infesting a large number of crops around the world. These species are widely distributed in diverse agro-ecosystems from temperate to sub-tropical regions due to their unique seed traits, successful germination ecology, high competitive ability, and allelopathic potential. A. fatua is more widespread, adaptable, and problematic than A. ludoviciana. Both these species infest major winter and spring crops, including wheat, oat, barley, canola, maize, alfalfa, and sunflower, causing up to 70% yield losses depending on crop species and weed density. Chemical control has been challenged by large-scale herbicide resistance evolution in these weed species. A. fatua is the most widespread herbicide-resistant weed in the world, infesting about 5 million hectares in 13 countries. The use of alternative herbicides with different modes of action has proved effective. Several cultural practices, including diverse crop rotations, cover crops, improved crop competition (using competitive cultivars, high seed rates, narrow row spacing, altered crop geometry), and allelopathic suppression, have shown promise for controlling A. fatua and A. ludoviciana. The integrated use of these cultural methods can reduce the herbicide dose required, and lower dependency on herbicides to control these grasses. Moreover, integrated management may successfully control herbicide-resistant populations of these weed species. The use of integrated approaches based on the knowledge of biology and ecology of A. fatua and A. ludoviciana may help to manage them sustainably in the future.

Mechanism of resistance to mesotrione in an Amaranthus tuberculatus population from Nebraska, USA.

Amaranthus tuberculatus is a troublesome weed in corn and soybean production systems in Midwestern USA, due in part to its ability to evolve multiple resistance to key herbicides including 4-hydroxyphenylpyruvate dioxygenase (HPPD). Here we have investigated the mechanism of resistance to mesotrione, an important chemical for managing broadleaf weeds in corn, in a multiple herbicide resistant population (NEB) from Nebraska. NEB showed a 2.4-fold and 45-fold resistance increase to mesotrione compared to a standard sensitive population (SEN) in pre-emergence and post-emergence dose-response pot tests, respectively. Sequencing of the whole HPPD gene from 12 each of sensitive and resistant plants did not detect any target-site mutations that could be associated with post-emergence resistance to mesotrione in NEB. Resistance was not due to HPPD gene duplication or over-expression before or after herbicide treatment, as revealed by qPCR. Additionally, no difference in mesotrione uptake was detected between NEB and SEN. In contrast, higher levels of mesotrione metabolism via 4-hydroxylation of the dione ring were observed in NEB compared to the sensitive population. Overall, the NEB population was characterised by lower levels of parent mesotrione exported to other parts of the plant, either as a consequence of metabolism in the treated leaves and/or impaired translocation of the herbicide. This study demonstrates another case of non-target-site based resistance to an important class of herbicides in an A. tuberculatus population. The knowledge generated here will help design strategies for managing multiple herbicide resistance in this problematic weed species.

Does the Cost of Adaptation to Extremely Stressful Environments Diminish Over Time? A Literature Synthesis on How Plants Adapt to Heavy Metals and Pesticides

Populations adapted to locally stressful environmental conditions are predicted to carry costs in performance and fitness, particularly when compared to non-stress adapted populations in the absence of stress. However, empirical observations found fitness costs incurred by stress-resistant genotypes are often ambiguous or absent. Compensatory evolution may purge genotypes with relatively high costs over time, resulting in the recovery of fitness in a stress-resistant population. We assessed the magnitude of adaptation costs over time to test for a reduction in negative genetic effects by compiling published data on measures of fitness from plant populations inhabiting mine tailings and populations adapted to herbicides. Heavy metal contaminated sites represent a stress that is immediate and unchanging; herbicides represent a stress that changes over time with dosage or the type of herbicide as treated populations become more resistant. To quantify costs, for each comparison we recorded the performance of plants from stress and non-stress environments grown under benign conditions. Time since the initiation of the stress was determined to test whether costs change over time. Costs were overall constant through time. The magnitude of cost were consistent with trade-offs for heavy metal resistance and certain herbicide mechanisms (triazine and resistance via P450 enzyme), but not for other herbicides where costs were inconsistent and appear to be low if not absent. Superior stress-resistant populations with higher performance than non-stress populations were found from both herbicide and metal stress, with some extreme cases early from time since initiation. There was an increasing benefit to cost ratio over time for herbicide resistant populations. We found that adaptation to stressful environments is generally costly except in herbicide resistance, and that costs are not diminished over time. Stress-resistant populations without costs also arise infrequently, though these populations may often be restricted from spreading.

Selection of relatively exact reference genes for gene expression studies in goosegrass (Eleusine indica) under herbicide stress

Goosegrass (Eleusine indica) is one of the most serious annual grassy weeds worldwide, and its evolved herbicide-resistant populations are more difficult to control. Quantitative real-time PCR (qPCR) is a common technique for investigating the resistance mechanism; however, there is as yet no report on the systematic selection of stable reference genes for goosegrass. This study proposed to test the expression stability of 9 candidate reference genes in goosegrass in different tissues and developmental stages and under stress from three types of herbicide. The results show that for different developmental stages and organs (control), eukaryotic initiation factor 4 A (eIF-4) is the most stable reference gene. Chloroplast acetolactate synthase (ALS) is the most stable reference gene under glyphosate stress. Under glufosinate stress, eIF-4 is the best reference gene. Ubiquitin-conjugating enzyme (UCE) is the most stable reference gene under quizalofop-p-ethyl stress. The gene eIF-4 is the recommended reference gene for goosegrass under the stress of all three herbicides. Moreover, pairwise analysis showed that seven reference genes were sufficient to normalize the gene expression data under three herbicides treatment. This study provides a list of reliable reference genes for transcript normalization in goosegrass, which will facilitate resistance mechanism studies in this weed species.

Transcriptome sequencing identifies novel persistent viruses in herbicide resistant wild-grasses

Herbicide resistance in wild grasses is widespread in the UK, with non-target site resistance (NTSR) to multiple chemistries being particularly problematic in weed control. As a complex trait, NTSR is driven by complex evolutionary pressures and the growing awareness of the role of the phytobiome in plant abiotic stress tolerance, led us to sequence the transcriptomes of herbicide resistant and susceptible populations of black-grass and annual rye-grass for the presence of endophytes. Black-grass (Alopecurus myosuroides; Am) populations, displaying no overt disease symptoms, contained three previously undescribed viruses belonging to the Partititiviridae (AMPV1 and AMPV2) and Rhabdoviridae (AMVV1) families. These infections were widespread in UK black-grass populations and evidence was obtained for similar viruses being present in annual rye grass (Lolium rigidum), perennial rye-grass (Lolium perenne) and meadow fescue (Festuca pratensis). In black-grass, while no direct causative link was established linking viral infection to herbicide resistance, transcriptome sequencing showed a high incidence of infection in the NTSR Peldon population. The widespread infection of these weeds by little characterised and persistent viruses and their potential evolutionary role in enhancing plant stress tolerance mechanisms including NTSR warrants further investigation.

Resistance to Propanil in Ricefield Bulrush (Schoenoplectus mucronatus) Is Conferred by apsbAMutation, Val219to Ile

Determining the mechanisms of herbicide resistance in weeds allows for the development and implementation of applied management practices aimed to control and to prevent further spread of herbicide-resistant populations in crop fields. This research was conducted to determine propanil resistance and cross-resistance to other photosystem II (PSII) inhibitors in ricefield bulrush biotypes and to elucidate the mechanism of propanil resistance. To this end, propanil-resistant (R) and propanil-susceptible (S) biotypes were selected from field-collected populations after propanil spraying at the field rate, and whole-plant, dose–response experiments were conducted to evaluate cross-resistance to PSII inhibitors and interactions between propanil and the insecticides malathion and carbaryl. In addition, thepsbAgene from R and S biotypes was sequenced for amino acid alterations following polymerase chain reaction (PCR) amplification. Plant survival data indicated the R biotype displayed a 14-fold increase in propanil resistance relative to the susceptible (S) biotype. In addition, the propanil-R biotype also had increased resistance to the PSII-inhibitors bromoxynil, diuron, and metribuzin but was more susceptible to bentazon than were propanil-S plants. Synergism between propanil and the insecticides carbaryl and malathion was greater in the S biotype than it was in the R biotype, indicating that, unlike propanil resistance in weedy grasses, enhanced degradation of the herbicide molecule is not a mechanism of resistance for propanil in ricefield bulrush. A Val219to Ile substitution in the propanil-R chloroplast D1 protein was identified following sequencing of thepsbAgene. This research suggests a single-point mutation at the target site causes resistance to propanil, diuron, metribuzin, and bromoxynil but increasing susceptibility to bentazon in propanil-R ricefield bulrush, a novel Val219–Ile feature. To our knowledge, this is the first instance of propanil resistance in weeds because of a mechanism other than enhanced herbicide metabolism. Tank-mixing bentazon and propanil, where permitted, can control both propanil-R and propanil-S biotypes.

Shifts in outcrossing rates and changes to floral traits are associated with the evolution of herbicide resistance in the common morning glory

Human-mediated selection can strongly influence the evolutionary response of natural organisms within ecological timescales. But what traits allow for, or even facilitate, adaptation to the strong selection humans impose on natural systems? Using a combination of laboratory and greenhouse studies of 32 natural populations of the common agricultural weed, Ipomoea purpurea, we show that herbicide-resistant populations self-fertilise more than susceptible populations. We likewise show that anther-stigma distance, a floral trait associated with self-fertilisation in this species, exhibits a nonlinear relationship with resistance such that the most and least resistant populations exhibit lower anther-stigma separation compared to populations with moderate levels of resistance. Overall, our results extend the general finding that plant mating can be impacted by human-mediated agents of selection to that of the extreme selection of the agricultural system. This work highlights the influence of human-mediated selection on rapid responses of natural populations that can lead to unexpected long-term evolutionary consequences.

Identifying Critical Control Points in the Wild Oat (Avena fatua) Life Cycle and the Potential Effects of Harvest Weed-Seed Control

Wild oat is a problematic weed species that requires new management techniques in the face of herbicide resistance; harvest weed-seed control (HWSC) may be an option. Wild oat demographic information was collected in long-term, rotational field studies in Lacombe, AB, Canada, in 2006 and 2007, and a periodic matrix model was parameterized using management extremes (no IPM, no herbicide to high IPM, and full herbicide). Population growth rates were calculated for each treatment and year. Prospective (elasticity) and retrospective (LTRE) analyses were conducted alongside a rearrangement of the model equation in which population growth rates were designated and the required proportion of newly shed seed survival that gives that growth rate was solved for. All populations had λ > 1 or increasing populations. Elasticity analyses indicated that λ was most-highly elastic to the overwinter seedbank (Esw = 1), followed by seedling survival, fecundity, and survival of newly shed seed (0.63 to 0.86 across treatments). The latter may be the most-accessible vital rate for management of herbicide resistant populations. LTRE exposed the stochasticity of wild oat population growth rates between years and their ability to take advantage of lapses in control. Decreasing the proportion of newly shed seeds (snew) that survives was the most-effective and available control strategy until reduced to 0.1 to 0.3 when the summer seedbank becomes more critical. When averaged across treatments, > 80% of newly shed seed must be eliminated to stop the population from growing, resulting in a stable population, but not a decline. Because of preharvest shattering, HWSC will likely not be effective enough alone to cause wild oat populations to decline. New management techniques for wild oat control that can be used in combination with HWSC and integrated weed management strategies are needed.

Rapid in-season detection of herbicide resistant Alopecurus myosuroides using a mobile fluorescence imaging sensor

WeedPAM has been introduced as a new chlorophyll fluorescence imaging sensor to detect herbicide stress in weeds a few days after treatment (DAT). In this study, it was investigated if the sensor could differentiate between 50 sensitive and herbicide resistant populations of Alopecurus myosuroides 5 DAT. Resistance profile of all populations had been analyzed in standard greenhouse bioassays. Populations were sown in winter wheat at several locations in Germany over two years. At 3–7 leaves growth stage, they were treated with four ALS- and three ACCase-inhibitors at recommended dosages. Five DAT, maximum quantum efficiency of PS II was measured with the WeedPAM sensor on 40 A. myosuroides plants per treatment. Based on the sensor data, populations were classified into sensitive and resistant populations. Classification was verified by a visual assessment of all treatments and populations 21 DAT. In total, 95% of the WeedPAM classifications 5 DAT were correct. We could demonstrate that WeedPAM is capable to detect herbicide resistant A. myosuroides populations shortly after treatment. This allows selecting alternative weed control methods against resistant weed populations in the same growing season.

Mycobiota of the weed Conyza canadensis (Asteraceae) in Brazil

Conyza canadensis is a noxious and notably problematic weed in Brazil whose control is greatly challenging due to the appearance, in the last years, of herbicide resistant populations. An investigation regarding the associated pathogenic fungi of this plant was conducted in Brazil aimed at starting a biological control program to help mitigate its economic impact. Ten fungal species were yielded and described: two ascomycetes (Leptosphaerulina conyzicola sp. nov. and Wentiomyces melioloides), five hyphomycete asexual morphs (Alternaria tenuissima, Cercospora conyzicola sp. nov., Cercosporella virgaureae, and Fusarium fujikuroi), two coelomycete asexual morphs (Phoma conyzaphthora sp. nov., Septoria erigerontis), one rust fungus (Aecidium conyzicola sp. nov.), and one oomycete (Basidiophora entospora). Four among the fungi that were collected represent new taxa, and the others represented either new host-records of known fungal species or new geographic records or both, except for C. virgaureae that had already been recorded on C. canadensis in Brazil. None of the discovered species have any potential to be used as a mycoherbicide or to be introduced somewhere else, nevertheless, the knowledge about this mycobiota will help guide the selection of potential biocontrol agents to be introduced in Brazil.

Manejo de Sagittaria montevidensis resistente aos mecanismos de ação da ALS e do FSII com saflufenacil associado a diferentes adjuvantes

Objetivou-se com esse trabalho avaliar o efeito de doses de saflufenacil e de adjuvantes no controle de sagitária resistente aos herbicidas inibidores de ALS e do Fotossistema II (FS II), e a fitotoxicidade a cultivar de arroz Epagri 108. Foram utilizados os biótipos de sagitária (Sagittaria montevidensis), SAGMO 10 - com resistência cruzada a herbicidas inibidores da ALS, e SAGMO 32 - com resistência múltipla a herbicidas inibidores da ALS e do FS II. Os tratamentos constituíram-se da aplicação de doses de saflufenacil: 75; 112,5 e 150 g i.a. ha-1; da adição dos adjuvantes Dash HC® (0,5% v/v); Assist® (1% v/v); Iharaguen-S® (0,5% v/v) e Veget’Oil® (1 L ha-1) para cada dose do herbicida aplicado nos biótipos de sagitária. Avaliou-se o controle da sagitária, a fitotoxicidade e a massa seca da parte aérea (MSPA) do arroz. O saflufenacil, nas doses de 75 a 150 g i.a. ha-1 não proporcionou controle eficiente dos biótipos de sagitária. A adição dos adjuvantes Veget’Oil®, Assist® e Iharaguen-S® ao saflufenacil não alterou a eficiência de controle de sagitária resistente a herbicidas. A cultivar Epagri 108 apresenta tolerância ao saflufenacil até 150 g i.a. ha-1, não ocorrendo redução da MSPA. A fitotoxicidade ocasionada às plantas de arroz pelo saflufenacil é menor ao se adicionar o Veget’Oil®. A massa seca das plantas do arrroz não foi afetada independente do adjuvante utilizado. O saflufenacil pode ser usado, até a dose de 150 g i.a. ha-1, associado ao adjuvante Veget’Oil® para o manejo de sagitária infestante do arroz cultivar Epagri 108, porém não proporciona controle adequado dos biótipos SAGMO 10 e SAGMO 32.

Germination Ecology of Herbicide-Resistant Population of Littleseed Canarygrass from North–Western India

ABSTRACTLittleseed canarygrass (Phalaris minor Retz.) is a most dominant weed of wheat (Triticum aestivum L.) in the north-western states of India. Little information is available on germination of littleseed canarygrass and wheat under similar ecological conditions. The present study was conducted under controlled conditions to study germination ecology of littleseed canarygrass population possessing multiple-herbicide resistance, along with wheat. Germination of littleseed canarygrass and wheat was independent of light. Optimum temperature for germination of littleseed canarygrass and wheat was 20°C. Germination of both species was completely inhibited at osmotic potentials lower than −0.4 MPa. Seeds of littleseed canarygrass and wheat were able to germinate even at NaCl concentration of 200 mM, with complete inhibition at 300 mM. Germination of both species was favored in a pH range of 5–8. Moisture, salinity, and pH stresses significantly reduced the speed of germination, with a concomitant increase in mean germination time, thus exposing seedlings to longer periods of less favorable conditions. This information should help in predicting potential distribution of littleseed canarygrass to new areas and also assist in formulating effective weed-management strategies for the future.

Multiple-Herbicide Resistance Is Widespread in Roadside Palmer Amaranth Populations.

Herbicide-resistant Palmer amaranth is a widespread issue in row-crop production in the Midsouthern US. Palmer amaranth is commonly found on roadside habitats in this region, but little is known on the degree of herbicide resistance in these populations. Herbicide resistance in roadside Palmer amaranth populations can represent the spread of an adaptive trait across a selective landscape. A large-scale survey was carried out in the Mississippi Delta region of eastern Arkansas to document the level of resistance in roadside Palmer amaranth populations to pyrithiobac and glyphosate, two important herbicides with broad history of use in the region. A total of 215 Palmer amaranth populations collected across 500 random survey sites were used in the evaluations. About 89 and 73% of the surveyed populations showed >90% survival to pyrithiobac and glyphosate, respectively. Further, only 3% of the populations were completely susceptible to glyphosate, while none of the populations was completely controlled by pyrithiobac. Among the 215 populations evaluated, 209 populations showed multiple resistance to both pyrithiobac and glyphosate at varying degrees. Dose-response assays confirmed the presence of high levels of herbicide resistance in the five selected populations (≥ 25-fold compared to a susceptible standard). Results demonstrate the prevalence of multiple-herbicide resistance in roadside Palmer amaranth populations in this region. Growers should be vigilant of Palmer amaranth infestation in roadsides adjacent to their fields and implement appropriate control measures to prevent likely spread of herbicide resistance into their fields.

Características biológicas, resistência a herbicidas e manejo de Amaranthus palmeri em agroecossistemas

Populações da espécie A. palmeri com resistência a herbicidas foram detectadas em lavouras brasileiras em 2014. Plantas dessa espécie nunca foram identificadas anteriormente no Brasil. Em outros locais do mundo, A. palmeri causa muitos prejuízos, em função das características biológicas que a torna muito competitiva em sistemas agrícolas. Além disso, populações resistentes aos herbicidas são problemas importantes no manejo das plantas, especialmente quando ocorre resistência múltipla. Na presente revisão são descritas as características biológicas dessa espécie que auxiliam no entendimento da sua adaptação, os prejuízos potenciais para as culturas e, também, na expectativa de encontrar vulnerabilidades que possam ser utilizadas no manejo das plantas. São descritos os casos de resistência de A. palmeri registrados no mundo, bem como os mecanismos causais da mesma. Também são discutidas ações que permitam o manejo e o controle dessa espécie nas culturas do algodão, soja e milho.

Incidence of Herbicide Resistance, Seedling Emergence, and Seed Persistence of Smooth Barley (Hordeum glaucum) in South Australia

Smooth barley has emerged as a problematic weed in cereal crops of South Australia. After the recent reports of herbicide resistance and increase in seed dormancy in smooth barley, it was considered important to determine the herbicide resistance status and seedbank behavior of field populations of this weed species. A field survey was undertaken in the Upper North and Eyre Peninsula regions of South Australia in October 2012. Of the 90 smooth barley populations screened for resistance to quizalofop, 15% exhibited some level of resistance and 85% were susceptible. Resistance to acetolactate synthase (ALS)-inhibiting herbicides was low, with only 3 and 12% of populations classified as developing resistance to imazamox + imazapyr and sulfosulfuron, respectively. No multiple resistance patterns were observed; however, two ALS-inhibiting herbicide-resistant populations had sulfonylurea-to-imidazolinone cross-resistance. At the start of the growing season, the majority of smooth barley populations emerged rapidly (median 50% time to emergence [T50] = 8 d). In contrast, some populations of smooth barley displayed an extremely slow emergence pattern, withT50of > 20 d. No direct linkage between seed dormancy and herbicide resistance was observed. However, two acetyl coenzyme A carboxylase-inhibiting herbicide-resistant populations were highly dormant and exhibited delayed emergence. The majority of smooth barley populations showed low-level or no seedbank persistence, but a few populations persisted for 1 yr. However, some weed populations had up to 20% seedbank persistence from 1 yr to the next. Overall there was a strong negative relationship between smooth barley seedling emergence and the level of seed persistence (R2= 0.84, P < 0.05). This association indicated that greater seed dormancy could be responsible for extended persistence of the seedbank of this weed species. The study provides valuable insights into the general pattern of herbicide resistance and the behavior of the seedbank of smooth barley populations on South Australian farms.

Evaluating Annual Bluegrass Herbicide Resistance Evolution in Golf Course Fairways

Annual bluegrass is one of the most diverse plant species in the world and is the most problematic winter annual weed in commercial turfgrass. Continuous application of the same herbicide mechanism of action for annual bluegrass control on golf courses has increased herbicide-resistant populations. The purpose of this research was to simulate six herbicide-use strategies to evaluate the risk of annual bluegrass resistance evolution to glyphosate. In a worst-case scenario of yearly glyphosate applications at dormancy, resistance evolved within 10 yr and was predicted to evolve in > 90% of populations by yr 20. When glyphosate was rotated on alternate years with a unique mechanism of action, resistance was delayed for 12 to 15 yr. Season-long control of annual bluegrass often requires multiple herbicide applications. Therefore, additional strategies were simulated in which glyphosate was applied at dormancy with combinations of PRE and/or POST herbicides at various timings. Resistance was most effectively delayed with a PRE application in late summer, a POST application in fall, and alternating glyphosate with a different POST option at dormancy. This delayed resistance by 25 yr and a 35% risk was predicted after 50 yr. Strategies utilizing three annual herbicide applications with unique mechanisms of action were more effective for controlling population growth compared to other strategies. Resistance was predicted to evolve within 35 yr for each of the strategies simulated. However, these results indicate annual bluegrass herbicide resistance can be managed by using an integrated herbicide program, rotating unique mechanisms of action as frequently as possible.