Modern high-throughput molecular and analytical tools offer exciting opportunities to gain a mechanistic understanding of unique traits of weeds. During the past decade, tremendous progress has been made within the weed science discipline using genomic techniques to gain deeper insights into weedy traits such as invasiveness, hybridization, and herbicide resistance. Though the adoption of newer “omics” techniques such as proteomics, metabolomics, and physionomics has been slow, applications of these omics platforms to study plants, especially agriculturally important crops and weeds, have been increasing over the years. In weed science, these platforms are now used more frequently to understand mechanisms of herbicide resistance, weed resistance evolution, and crop–weed interactions. Use of these techniques could help weed scientists to further reduce the knowledge gaps in understanding weedy traits. Although these techniques can provide robust insights about the molecular functioning of plants, employing a single omics platform can rarely elucidate the gene-level regulation and the associated real-time expression of weedy traits due to the complex and overlapping nature of biological interactions. Therefore, it is desirable to integrate the different omics technologies to give a better understanding of molecular functioning of biological systems. This multidimensional integrated approach can therefore offer new avenues for better understanding of questions of interest to weed scientists. This review offers a retrospective and prospective examination of omics platforms employed to investigate weed physiology and novel approaches and new technologies that can provide holistic and knowledge-based weed management strategies for future.

Italian ryegrass [Lolium perenne L. spp. multiflorum (Lam.) Husnot] is a troublesome weedy species in many regions of California. Its control has been chiefly dependent on herbicides due to their effectiveness and practicality and, as result, herbicide-resistant populations have been selected. Poor control of a population of L. multiflorum with paraquat was recently reported in a prune orchard in Hamilton City, CA. A series of experiments were carried out to characterize the response of this population to several POST herbicides, study the mechanisms of resistance, and investigate alternative chemical management options in tree crops. A known susceptible (S) and the suspected resistant population (PRHC) were subjected to greenhouse dose–response experiments with clethodim, fluazifop-P-butyl, glufosinate, glyphosate, paraquat, pyroxsulam, rimsulfuron, and sethoxydim. A 310-bp fragment of the EPSPS gene containing position 106 was sequenced from PRHC and S. Field experiments were carried out in a prune orchard with PRE herbicides commonly used by perennial crop growers in California. Greenhouse dose–response experiments confirmed that PRHC is resistant to paraquat, as well as multiply resistant to clethodim and glyphosate. The EPSPS gene of PRHC is heterozygous for glyphosate resistance at position 106, where one allele exhibited proline substituted by serine and the other by alanine. Field experiments with PRE herbicides indicated that tank mixes containing indaziflam and flumioxazin can provide adequate L. multiflorum control up to 150 d after treatment. Poor weed management practices, such as overreliance on a single site of action, have frequently been associated with the selection of herbicide-resistant L. multiflorum populations around the world, and adequate herbicide-resistance management programs are necessary for growers to maintain economic sustainability even after evolution of herbicide-resistant weeds in their fields.

Palmer amaranth (Amaranthus palmeri S. Watson), a dioecious summer annual species, is one of the most troublesome weeds in U.S. cropping systems. The evolution of resistance to protoporphyrinogen oxidase inhibitors in A. palmeri biotypes is a major cause of concern to soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.) growers in the midsouthern United States. The objective of this study was to confirm and characterize the non–target site mechanism in a fomesafen-resistant accession from Randolph County, AR (RCA). A dose–response assay was conducted to assess the level of fomesafen resistance, and based on the GR50 values, the RCA accession was 18-fold more resistant to fomesafen than a susceptible (S) biotype. A TaqMan allelic discrimination assay and sequencing of the target-site genes PPX2 and PPX1 revealed no known or novel target-site mutations. An SYBR Green assay indicated no difference in PPX2 gene expression between the RCA and S biotypes. To test whether fomesafen resistance is metabolic in nature, the RCA and the S biotypes were treated with different cytochrome P450 (amitrole, piperonyl butoxide [PBO], malathion) and glutathione S-transferase (GST) (4-chloro-7-nitrobenzofurazan [NBD-Cl]) inhibitors, either alone or in combination with fomesafen. Malathion followed by (fb) fomesafen in RCA showed the greatest reduction in survival (67%) and biomass (86%) compared with fomesafen alone (45% and 66%, respectively) at 2 wk after treatment. Interestingly, NBD-Cl fb fomesafen also resulted in low survival (35%) compared with the fomesafen-only treatment (55%). Applications of malathion or NBD-Cl preceding fomesafen treatment resulted in reversal of fomesafen resistance, indicating the existence of cytochrome P450– and GST-based non–target site mechanisms in the RCA accession. This study confirms the first case of non–target site resistance to fomesafen in A. palmeri.

Pennsylvania smartweed [Persicaria pensylvanica (L.) M. Gómez] is a common weed of rice (Oryza sativa L.) in the midsouthern United States and has recently become a concern for farmers because of reduced tillage systems. Acetolactate synthase (ALS) inhibitors have been extensively used for controlling smartweeds in imidazolinone-resistant and conventional rice. In the present study, we confirmed resistance to commonly used ALS inhibitors in rice and characterized the underlying resistance mechanism in a P. pensylvanica biotype from southeast Missouri. A dose–response experiment was conducted in the greenhouse using bensulfuron-methyl, imazethapyr, and bispyribac-sodium to determine the resistance index (resistance/susceptibility [R/S]) based on GR50 estimates. The target-site ALS gene was amplified from R and S plants, and sequences were analyzed for mutations known to confer ALS-inhibitor resistance. The P. pensylvanica biotype in question was found to be resistant to bensulfuron-methyl (R/S=2,330), imazethapyr (R/S=12), and bispyribac-sodium (R/S=6). Sequencing of the ALS gene from R plants revealed two previously known mutations (Pro-197-Ser, Ala-122-Ser) conferring resistance to sulfonylureas and imidazolinones. This is the first report of ALS-inhibitor resistance in P. pensylvanica.

Seedbank sampling and quantification methods vary in their ability to describe weed diversity and density, so proper method selection is critical for studying weed communities. The germinable seedbank (GSB) method is commonly preferred over extractable seedbank method (ESB), because the latter is more time-consuming. However, these two methods have only been compared using a few weed species and a relatively small number of samples. A total of 204 weed seedbank samples were used to compare both methods for weed density, richness, evenness, and Shannon-Weiner diversity using a split-sample approach. The two methods yielded dramatically different results. The ESB had 418% higher density and 35% more species per sample but 11% less evenness than the GSB. Diversity was estimated to be only 9% higher using the extractable compared with the germinable method. While the extractable method had higher density and richness overall, this was not true for every species, with only 7 of 14 common species detected in higher amounts by the extractable method. The results indicate the two methods are not strongly correlated, limiting the possibility of generating a conversion factor between methods. Nevertheless, evenness and Shannon-Weiner diversity might allow comparisons between both methods when the emphasis is on characterizing predominant weed species. The GSB is a practical approach to compare treatments or conditions; however, the ESB is more useful to accurately quantify weed species richness, diversity, and density.

Dicamba-resistant (DR) kochia [Bassia scoparia (L.) A. J. Scott] has been reported in six U.S. states and one Canadian province. To develop effective B. scoparia control tactics, it is necessary to understand the seed germination pattern of DR B. scoparia. The objective of this study was to compare the germination characteristics of DR versus dicamba-susceptible (DS) B. scoparia populations from Montana and Kansas under constant (5 to 35 C) and/or alternating temperatures (5/10 to 30/35 C). DR B. scoparia lines from Montana were generated after three generations of recurrent selection of field-collected populations with dicamba. Seeds of DR or DS lines from Kansas were obtained after one generation of restricted self-pollination. DR B. scoparia lines from both Montana and Kansas had a lower maximum cumulative germination than the DS lines across all temperature treatments. A majority of DR B. scoparia lines from Montana showed a temperature-mediated seed germination response, with a higher thermal requirement (30 to 35 C or 25/30 to 30/35 C) to attain the maximum cumulative germination compared with DS lines. Germination rates at 5 to 30 C were lower for DR versus DS B. scoparia lines from Kansas. All DR lines from Montana took more time than DS lines to initiate germination at 5 and 10 C or 5/10 and 20/25 C. Similarly, there was a delayed onset of germination of the DR versus DS line from Kansas at 5, 10, 15, and 20 C. Furthermore, the DR B. scoparia from both Kansas and Montana had a slower germination pattern relative to the DS B. scoparia. Diversified crop rotations using winter wheat (Triticum aestivum L.), fall-sown cover crops, or early-spring planted crops (e.g., wheat or barley [Hordeum vulgare L.]) that are competitive against late-emerging B. scoparia in conjunction with strategic tillage and late-season weed control tactics should be used to facilitate depletion of DR B. scoparia seedbanks.

Waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] is a troublesome weed occurring in cropping systems throughout the U.S. Midwest with an ability to rapidly evolve herbicide resistance that could be associated with competitive disadvantages. Little research has investigated the competitiveness of different A. tuberculatus populations under similar environmental conditions. The objectives of this study were to evaluate: (1) the interspecific competitiveness of three herbicide-resistant A. tuberculatus populations (2,4-D and atrazine resistant [2A-R], glyphosate and protoporphyrinogen oxidase [PPO]-inhibitor resistant [GP-R], and 2,4-D, atrazine, glyphosate, and PPO-inhibitor susceptible [2AGP-S]) with soybean [Glycine max (L.) Merr.]; and (2) the density-dependent response of each A. tuberculatus population within a constant soybean population in a greenhouse environment. Amaranthus tuberculatus competitiveness with soybean was evaluated across five target weed densities of 0, 2, 4, 8, and 16 plants pot−1 (equivalent to 0, 20, 40, 80, and 160 plants m−2) with 3 soybean plants pot−1 (equivalent to 300,000 plants ha−1). At the R1 soybean harvest time, no difference in soybean biomass was observed across A. tuberculatus populations. At A. tuberculatus densities <8 plants pot−1, the 2AGP-S population had the greatest biomass and stem diameter per plant. At the R7 harvest time, the 2AGP-S population caused the greatest loss in soybean biomass and number of pods compared with the other populations at densities of <16 plants pot−1. The 2AGP-S population had greater early-season biomass accumulation and stem diameter compared with the other A. tuberculatus populations, which resulted in greater late-season reduction in soybean biomass and number of pods. This research indicates there may be evidence of interspecific competitive fitness cost associated with the evolution of 2,4-D, atrazine, glyphosate, and PPO-inhibitor resistance in A. tuberculatus. Focus should be placed on effectively using cultural weed management practices to enhance crop competitiveness, especially early in the season, to increase suppression of herbicide-resistant A. tuberculatus.

Palmer amaranth (Amaranthus palmeri S. Watson) is the most problematic weed in agronomic crop production fields in the United States. The objective of this study was to determine the effect of degree of water stress on the growth and fecundity of A. palmeri using soil moisture sensors under greenhouse conditions. Two A. palmeri biotypes collected from Nebraska were grown in loam soil maintained at 100%, 75%, 50%, 25%, and 12.5% soil field capacity (FC) corresponding to no, light, moderate, high, and severe water stress levels, respectively. Water was regularly added to pots based on soil moisture levels detected by Watermark or Decagon 5TM sensors to maintain the desired water stress level. Amaranthus palmeri plants maintained at ≤25% FC did not survive more than 35 d after transplanting. Amaranthus palmeri at 100%, 75%, and 50% FC produced similar numbers of leaves (588 to 670 plant−1) based on model estimates; however, plants at 100% FC achieved a maximum height of 178 cm compared with 124 and 88 cm at 75% and 50% FC, respectively. The growth index (1.1×105 to 1.4×105 cm3 plant−1) and total leaf area (571 to 693 cm2 plant−1) were also similar at 100%, 75%, and 50% FC. Amaranthus palmeri produced similar root biomass (2.3 to 3 g plant−1) at 100%, 75%, and 50% FC compared with 0.6 to 0.7 g plant−1 at 25% and 12.5% FC, respectively. Seed production was greatest (42,000 seeds plant−1) at 100% FC compared with 75% and 50% FC (14,000 to 19,000 seeds plant−1); however, the cumulative seed germination was similar (38% to 46%) when mother plants were exposed to ≥50% FC. The results of this study show that A. palmeri can survive ≥50% FC continuous water stress conditions and can produce a significant number of seeds with no effect of on seed germination.

Edamame [Glycine max (L.) Merr.] differs from grain-type soybean in several aspects, one being that edamame seeds are 65% to 100% larger than grain-type soybean seed. Crop seed size has implications for weed management in grain-type soybean; however, the extent to which this observation holds true for edamame is unknown. Because weed interference continues to be a barrier to domestic edamame production, the objective was to quantify the effect of edamame seed size on the crop’s ability to tolerate weed interference (CT) and the crop’s ability to suppress weeds (WSA). Five edamame cultivars plus one grain-type cultivar were each sorted to create “small” and “large” seed size classes. Seed lots were included in a split–split plot design, whereby an additional experimental factor was presence or absence of velvetleaf (Abutilon theophrasti Medik.). Crop and weed emergence and growth were monitored through 8 wk after emergence (WAE). Crop plants from large seed had higher tolerance to A. theophrasti than plants from small seed, as evidenced by crop height, area, and biomass. Edamame seed size had little effect on WSA; however, crop cultivars differentially reduced A. theophrasti leaf area and biomass at 4 and 8 WAE. While both seed size and edamame cultivar influence early-season crop competitive ability, the magnitude of these factors on CT and WSA underscores the importance of considering them not as stand-alone tactics but rather as useful additions to a more comprehensive integrated weed management system.

African turnipweed (Sisymbrium thellungii O. E.Schulz) is an emerging problematic broadleaf weed of the northern grain region of Australia. Laboratory experiments were conducted to evaluate the effects of temperature, light, salinity, pH, seed burial depth, and the amount of wheat crop residue on germination and emergence of two Australian S. thellungii weed populations (population C, cropped area; population F, fence line). Both populations behaved similarly across different environmental conditions, except in the residue study. Although the seeds of both populations of S. thellungii could germinate under complete darkness, germination was best (~95%) under light/dark conditions at the 20/10 C temperature regime. Both populations of S. thellungii germinated over a wide range of day/night temperatures (15/5, 20/10, 25/15, and 30/20 C). Osmotic stress had negative effects on germination, with 54% seeds (averaged over populations) able to germinate at −0.1MPa. Complete germination inhibition for both populations was observed at −0.8MPa osmotic potential. Both populations germinated at sodium chloride (NaCl) concentrations ranging from 50 to 100 mM, beyond which germination was completely inhibited. There were substantial reductions in seed germination, 32% (averaged over populations) under highly acidic conditions (pH 4.0) as compared with the control (water: pH 6.4). Seed germination of both populations on the soil surface was 77%, and no seedlings emerged from a burial depth of 1 cm. The addition of 6 Mg ha−1 of wheat (Triticum aestivum L.) residue reduced the emergence of the C and F populations of S. thellungii by 75% and 64%, respectively, as compared with the control (no residue). Information gathered from this study provides a better understanding of the factors favorable for germination and emergence of S. thellungii, which will aid in developing management strategies in winter crops, especially wheat, barley (Hordeum vulgare L.), and chick pea (Cicer arietinum L.).

Halauxifen-methyl is a new synthetic auxin herbicide for control of broadleaf weeds, including preplant applications for corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. The objective of this study was to investigate the efficacy of halauxifen-methyl in comparison to the current auxin standards, 2,4-D and dicamba, on glyphosate-resistant (GR) horseweed (Erigeron canadensis L.) at different plant heights. In field experiments, a foliar application of halauxifen-methyl at the recommended use rate of 5 g ae ha−1 resulted in 81% control. Dicamba applied at 280 g ae ha−1 provided a comparable level of efficacy of 80%, while 2,4-D at 560 g ae ha−1 resulted in 49% control. The addition of glyphosate improved GR E. canadensis control with 2,4-D more than with halauxifen-methyl or dicamba, possibly due to the higher level of control observed with halauxifen-methyl or dicamba alone. Even though applied at 50 to 100 times lower application rates, the efficacy of halauxifen-methyl on E. canadensis was similar to dicamba and greater than 2,4-D. Thus, halauxifen-methyl should be an effective tool for management of GR E. canadensis before planting both conventional and herbicide-resistant soybean varieties, and it precludes the extended preplant application interval required for dicamba in some soybean management systems.

Corn poppy (Papaver rhoeas L.) is the most widespread broadleaf weed species infesting winter cereals in Europe. Biotypes that are resistant to both 2,4-D and tribenuron-methyl, an acetolactate synthase (ALS) inhibitor, have evolved in recent decades, thus narrowing the options for effective chemical control. Though the effectiveness of several integrated weed management (IWM) strategies have been confirmed, none of these strategies have been tested to manage multiple herbicide–resistant P. rhoeas under no-till planting. With the expansion of no-till systems, it is important to prove the effectiveness of such strategies. In this study, a field experiment over three consecutive seasons was conducted to evaluate and compare the effects of different weed management strategies, under either direct drilling (i.e., no-till) or intensive tillage, on a multiple herbicide–resistant P. rhoeas population. Moreover, evaluations were carried out as to whether the proportions of ALS inhibitor–resistant individuals were affected by the tillage systems for each IWM strategy at the end of the 3-yr period. The IWM strategies tested in this research included crop rotation, delayed sowing, and different herbicide programs such as PRE plus POST or POST. All IWM strategies greatly reduced the initial density of P. rhoeas each season (≥ 95%) under either direct drilling or intensive tillage. After 3 yr, the IWM strategies were very effective in both tillage systems, though the effects were stronger under direct drilling (~95%) compared with intensive tillage (~86%). At the end of the study, the proportion of ALS inhibitor–resistant plants was not different between the IWM strategies in both tillage systems (94% on average). Therefore, crop rotation (with sunflower [Helianthus annuus L.]), delayed sowing, or a variation in the herbicide application timing are effective under direct drilling to manage herbicide-resistant P. rhoeas. Adoption of IWM strategies is necessary to mitigate the evolution of resistance in both conventional and no-till systems.

Fomesafen is a protoporphyrinogen oxidase–inhibitor herbicide with an alternative mode of action that provides PRE weed control in strawberry [Fragaria×ananassa (Weston) Duchesne ex Rozier (pro sp.) [chiloensis×virginiana]] produced in a plasticulture setting in Florida. Plasticulture mulch could decrease fomesafen dissipation and increase crop injury in rotational crops. Field experiments were conducted in Balm, FL, to investigate fomesafen persistence and movement in soil in Florida strawberry systems for the 2014/2015 and 2015/2016 production cycles. Treatments included fomesafen preplant at 0, 0.42, and 0.84 kg ai ha−1. Soil samples were taken under the plastic from plots treated with fomesafen at 0.42 kg ha−1 throughout the production cycle. Fomesafen did not injure strawberry or decrease yield. Fomesafen concentration data for the 0.0- to 0.1-m soil depth were described using a three-parameter logistic function. The fomesafen 50% dissipation times were 37 and 47 d for the 2014/2015 and 2015/2016 production cycles, respectively. At the end of the study, fomesafen was last detected in the 0.0- to 0.1-m depth soil at 167 and 194 d after treatment in the 2014/2015 and 2015/2016 production cycles, respectively. Fomesafen concentration was less than 25 ppb on any sampling date for 0.1- to 0.2-m and 0.2- to 0.3-m depths. Fomesafen concentration decreased significantly after strawberry was transplanted and likely leached during overhead and drip irrigation used during the crop establishment.

Egyptian broomrape (Phelipanche aegyptiaca Pers.) is a root-parasitic weed that severely damages many crops worldwide, including tomato (Solanum lycopersicum L.). In Israel, the management protocol used for P. aegyptiaca in open-field tomato includes PPI sulfosulfuron at 37.5 g ai ha−1 to the top 10-cm soil layer. The objective of this study was to investigate the co-effect of sulfosulfuron application timing and variable degradation rate in soil on the control efficacy of P. aegyptiaca in tomato. Degradation of sulfosulfuron (80ng g−1 soil) at a temperature of 15C, measured in soil samples from three farms using liquid chromatography–tandem mass spectrometry, followed a first-order kinetics with variable degradation rate constant among sites (0.008 to 0.012 d−1). Incubation at 25 C increased sulfosulfuron degradation rate constant by a factor of 2 to 2.7 in soils from the different sites, with a similar degradation rate order among soils. A higher degradation rate in the soil resulted in a shorter period of residual activity, measured using a sorghum [Sorghum bicolor (L.) Moench.] bioassay. Phelipanche aegyptiaca management in open-field tomatoes was investigated in five independent field experiments. Sulfosulfuron soil concentration throughout the growing season (following preplant incorporation of 37.5 g ha−1) was calculated from laboratory-measured degradation rates, which were corrected to represent the effect of recorded temperatures at each field. At the end of the tomato growing season, control efficacy of P. aegyptiaca varied among experiments (70.4% to 100%) and positively correlated with predicted sulfosulfuron concentration at the critical period for seedling control (R2=0.67). The current study confirms that sulfosulfuron is degraded in soil to nonphytotoxic metabolites and that rapid degradation rates would result in reduced injury to P. aegyptiaca seedling and, consequently, lower control efficacy.