Rigid ryegrass, an important annual weed species in cropping regions of southern Australia, has evolved resistance to 11 major groups of herbicides. Dose–response studies were conducted to determine response of three clethodim-resistant populations and one clethodim-susceptible population of rigid ryegrass to three different frost treatments (−2 C). Clethodim-resistant and -susceptible plants were exposed to frost in a frost chamber from 4:00 P.M. to 8:00 A.M. for three nights before or after clethodim application and were compared with plants not exposed to frost. A reduction in the level of clethodim efficacy was observed in resistant populations when plants were exposed to frost for three nights before or after clethodim application. In the highly resistant populations, the survival percentage and LD50 were higher when plants were exposed to frost before clethodim application compared with frost after clethodim application. However, frost treatment did not influence clethodim efficacy of the susceptible population. Sequencing of the acetyl coenzyme A carboxylase (ACCase) gene of the three resistant populations identified three known mutations at positions 1781, 2041, and 2078. However, most individuals in the highly resistant populations did not contain any known mutation in ACCase, suggesting the resistance mechanism was a nontarget site. The effect of frost on clethodim efficacy in resistant plants may be an outcome of the interaction between frost and the clethodim resistance mechanism(s) present.

Metsulfuron is used for POST control of spotted spurge in many warm-season turfgrasses. A suspected resistant (R) biotype of spotted spurge was collected from turfgrass in Georgia with a history of exclusive metsulfuron use. Research was conducted to evaluate the resistance level of this biotype to metsulfuron, efficacy of other mechanisms of action for control, and the molecular basis for resistance. Compared with a susceptible (S) biotype, the R biotype required >90 and >135 times greater metsulfuron rates to reach 50% injury and reduce biomass 50% from the nontreated, respectively. The R biotype was also resistant to trifloxysulfuron but was injured equivalent to the S biotype from dicamba, glyphosate, and triclopyr. Gene sequencing of the R biotype revealed a Trp574 to Leu substitution that has conferred resistance to acetolactate synthase (ALS) inhibitors in previous research. This is the first report of ALS resistance in spotted spurge. More importantly, this is the first report of a herbicide-resistant broadleaf weed from a turfgrass system in the United States.

Secondary plant metabolites may influence plant–plant interactions and plant invasions. Distinguishing such chemicals requires integrating varying chemical ecology approaches, information on the amounts and persistence of specific chemicals in nature, and measures of effects (e.g., phytotoxicity assays) to judge the importance of the chemicals (e.g., allelochemicals). The invasive plant croftonweed has caused substantial ecological and economic losses in China. We examined contents and degradation of croftonweed chemicals in the soil and their potential phytotoxic effects on conspecific and five allospecific plant species. Soils in which croftonweed was grown had four phytotoxins: DEHP, DBP, DTD, and HHO. All chemicals were detected in croftonweed-invaded soil, with contents ranging from 0.013 (for DEHP) to 0.353 (for DTD) µg g−1 of soil. All four compounds were degraded rapidly in 1 wk. Combinations of the chemicals inhibited seed germination or seedling growth of four of the six plants, including croftonweed itself, at mean contents found in the soil. The putative allelochemicals degraded rapidly in the soil, and the low levels of allelochemicals observed in the soil may be sufficient to affect seed germination and plant growth.

Henbit is a facultative broadleaf winter annual in the Lamiaceae family. Acetolactate synthase (ALS) inhibitors are primarily used to control a broad spectrum of weeds, including henbit. During 2012 to 2013, field applications of ALS-inhibiting herbicides were ineffective in controlling a henbit population from Marion County, KS (MCK). To confirm field-evolved resistance to ALS inhibitors, response of MCK henbit and a known susceptible henbit population from Kansas (DPS) to varying doses of three different ALS inhibitors were examined: chlorsulfuron, imazamox, and propoxycarbazone. Results of the dose–response experiments suggest that the MCK population is highly resistant to chlorsulfuron (resistance index [R/S] > 1,000) and propoxycarbazone (R/S = 331) but is susceptible to imazamox. A full-length ALS gene sequence obtained using the 5′- and 3′- rapid amplification of complementary DNA ends approach revealed a Pro197 to Arg point mutation (a common mutation that confers resistance to sulfonylurea herbicides, e.g., chlorsulfuron) in the MCK henbit. No other known resistance-conferring mutations were found in the study. Evolved resistance to major classes of ALS inhibitors in the MCK henbit will reduce herbicide options for its control. To our knowledge, this is the first case of evolution of herbicide resistance in henbit.

Transfer of herbicide resistance among closely related weed species is a topic of growing concern. A spiny amaranth × Palmer amaranth hybrid was confirmed resistant to several acetolactate synthase (ALS) inhibitors including imazethapyr, nicosulfuron, pyrithiobac, and trifloxysulfuron. Enzyme assays indicated that the ALS enzyme was insensitive to pyrithiobac and sequencing revealed the presence of a known resistance conferring point mutation, Trp574Leu. Alignment of the ALS gene for Palmer amaranth, spiny amaranth, and putative hybrids revealed the presence of Palmer amaranth ALS sequence in the hybrids rather than spiny amaranth ALS sequences. In addition, sequence upstream of the ALS in the hybrids matched Palmer amaranth and not spiny amaranth. The potential for transfer of ALS inhibitor resistance by hybridization has been demonstrated in the greenhouse and in field experiments. This is the first report of gene transfer for ALS inhibitor resistance documented to occur in the field without artificial/human intervention. These results highlight the need to control related species in both field and surrounding noncrop areas to avoid interspecific transfer of resistance genes.

This research investigated the temperature and light requirements for seed germination and emergence patterns of pinnate poppy, violet horned-poppy, and nodding hypecoum, three annual Papaveraceae species found in arable lands in the Mediterranean region. Two experiments performed in growth chambers (1) analyzed light (complete darkness or 12 h light) and temperature (10/5, 15/5, and 20/10 C day/night temperatures) requirements for germination, and (2) determined base temperature (Tb) for germination. An outdoor pot trial was also set up to study emergence patterns. All species showed higher germination in complete darkness than they did with a light regime, irrespective of dormancy level, time of the year, and temperature regime under which germination was tested, illustrating better germination when seeds are buried. Tb ranged from −2.6 to 0 C, depending on the species, indicating low temperature requirements for germination. Given their higher germination in daily fluctuating, rather than constant temperatures, the three Papaveraceae species should have the capacity to form persistent seed banks. These species behaved as winter annuals (from November to February) in the pot experiment and had difficulties to emerge in spring. Given that they cannot avoid autumn–winter chemical treatments, this could partially explain their regression in arable fields. These results bring new information to develop management strategies for these Papaveraceae species in agroecosystems.

Weed suppression is one possible benefit of including cover crops in crop rotations. The late spring planting date of dry beans allows for more growth of cover crops in the spring. We assessed the influence of cover crops on weed dynamics in organic dry beans and weed seed persistence. Medium red clover, oilseed radish, and cereal rye were planted the year before dry beans; a no-cover-crop control was also included. After cover-crop incorporation, common lambsquarters, giant foxtail, and velvetleaf seeds were buried in the red clover, cereal rye, and no-cover control treatments and then retrieved 0, 1, 2, 4, 6, and 12 mo after cover-crop incorporation. Dry beans were planted in June and weed emergence and biomass measured. Eleven or more site-years of data were collected for each cover-crop treatment between 2011 and 2013, allowing for structural equation modeling (SEM), in addition to traditional analyses. Cereal rye residue increased giant foxtail and velvetleaf seed persistence by up to 12%; red clover decreased common lambsquarters seed persistence by 22% in 1 of 2 yr relative to the no-cover-crop control. Oilseed radish and incorporated cereal rye rarely reduced weed densities. When red clover biomass exceeded 5 Mg ha−1, soil inorganic N was often higher (5 of 6 site-years), as were weed density and biomass (5 and 4 of 12 main site sample times, respectively). Using SEM, we identified one causal relationship between cover-crop N content and weed biomass at the first flower stage (R1), as mediated through soil N at the time of dry bean planting and at the stage with two fully expanded trifoliates. Increasing cover-crop C : N ratios directly reduced weed biomass at R1, not mediated through changes in soil N. Cover crops that make a significant contribution to soil N may also stimulate weed emergence and growth.

Palmer amaranth is a highly invasive weed species causing huge economic losses in agricultural cropping systems under a broad range of environmental conditions. Sensitivity of this species to ozone (O3) air pollution and to soil water deficit, relative to native species or competing crops, may affect its competitiveness and invasive potential. In recent years, both high tropospheric O3 and soil water deficiency have become common in the San Joaquin Valley of California. Responses to these environmental parameters may help predict the invasiveness of this species and have implications for landscape hydrology. We assessed the impact of O3 and soil water deficit on Palmer amaranth. Five- to seven-leaf–stage potted plants were placed in continuous stirred tank reactor chambers and maintained for 30 to 35 d under 12-h mean daylight O3 exposures (0700–1900 hours) of 4, 59, or 114 ppb O3. Within the chambers the plants were either well-watered (WW) or exposed to regulated deficit irrigation (RDI) and grown for about 7 wk. Dry weights of the leaves, stems, roots, and leaf area were determined. Day- and nighttime stomatal conductances (gs) were measured at 1.5-h intervals. Nocturnal gs was about 16 to 29% of daytime gs; this suggests that the species could have substantial nighttime water loss, uncoupled from carbon gain in the weed, and could affect water availability for crops and reduce irrigation efficiency. Nocturnal gs was lower in the RDI than in the WW, but daytime gs was not affected by O3 or irrigation regime. Neither O3 nor irrigation regime affected root or shoot parameters. As O3 and drought are two key stressors in the San Joaquin Valley, to which potential competing species have been found to be sensitive, Palmer amaranth may proliferate and become more invasive in the future, potentially altering landscape hydrology and reducing irrigation efficiency.

Recovery of common agricultural weeds after burial by soil was studied in four greenhouse and three field experiments. Species studied included velvetleaf, Powell amaranth, common lambsquarters, barnyardgrass, and giant foxtail. Seedlings were bent over before burial to simulate the effect of the impact of soil thrown by a cultivator. Altogether, more than 35,000 seedlings were marked and observed for recovery. No seedlings recovered from 4 cm of burial. Recovery from complete burial under 2 cm of soil ranged from 0 to 24% depending on the experiment, species, and watering treatment, but recovery greater than 5% was rare. Large-seeded species tended to recover from complete burial under 2 cm of soil better than small-seeded species. The study did not reveal a difference in recovery of grasses relative to broadleaf weeds. Overall, seedlings tended to recover best when water was applied daily after burial, worst when water was applied once on the day of burial, and to an intermediate extent when no water was applied. However, difference in recovery between the no-water and watering-once treatments were usually small. Also, many experiment by species combinations showed no significant differences among watering treatments. When even a small portion of the seedling was left exposed, recovery generally exceeded 50%. Organic weed management systems commonly use burial of weed seedlings with tine weeders and soil thrown by sweeps and hilling disks to control weeds in crop rows. Recovery from burial could pose a substantial weed management problem in some circumstances, particularly for large-seeded weed species. Maximizing burial depth is important for limiting recovery. Recovery from burial can be minimized by withholding irrigation for several days after hilling-up operations.

We used laboratory and field feeding trials to investigate adult carabid beetle preferences for three brassicaceous weed species (rapeseed, wild mustard, and field pennycress) that are pests in canola. All carabid species preferred seeds of rapeseed most and those of field pennycress least and showed intermediate preference for wild mustard seeds. Beetles highly preferred imbibed seeds of all three weed species. Activity–density of carabids and mean weed seed removal were highly correlated in field plots of canola, with activity–density accounting for 67% of the observed variation in seed removal. Our study indicates that seed consumption among carabids is influenced by several factors, including weed species, physiological state of seeds, and carabid activity–density. Carabid seed predation is significant in canola agroecosystems; therefore, understanding these influences has implications for ecological weed management.

A field study was undertaken to investigate the influence of different management strategies on rigid ryegrass plant density and seedbank dynamics over 4 yr. Even though weed seedbank declined by 86% after oaten hay in year 1, the residual seedbank enabled rigid ryegrass to reinfest field peas the next year, and the population rebounded sharply when weed control relied solely on PPI trifluralin. However, use of POST clethodim followed by crop-topping for seed-set prevention of rigid ryegrass in field pea was highly effective and caused a further decline in the weed seedbank. Integration of effective management tactics over 3 yr significantly reduced rigid ryegrass weed and spike density (90 and 81%) in the final year of the 4-yr cropping sequence. Use of oaten hay in year 1, followed by effective weed control in field pea and wheat crops, depleted the high initial seedbank (4,820 seeds m−2) to moderate levels (< 200 seeds m−2) within 3 yr. Effective weed-management treatments depleted the rigid ryegrass seedbank, reduced in-crop weed infestation, and returned higher grain yields and profitability. The results of this study clearly show that large rigid ryegrass populations can be managed effectively without reducing crop productivity and profitability provided multiyear weed-management programs are implemented effectively.

Palmer amaranth has greatly disrupted agricultural practices in the United States with its rapid growth and rapid evolution of herbicide resistance. This weed species is now suspected in Argentina. To document whether the suspected plant populations are indeed Palmer amaranth, molecular comparisons to known standards were conducted. Additionally, these same plant populations were screened for possible herbicide resistance to several acetolactate synthase (ALS)-inhibiting herbicides. Sequencing data confirmed that suspected populations (A2, A3, A4) were indeed Palmer amaranth. Another population (A1) was tested to determine whether hybridization had occurred between Palmer amaranth and mucronate amaranth the native amaranth species of the region. Tests confirmed that no hybridization had occurred and that A1 was simply a unique phenotype of mucronate amaranth. Each population was screened for resistance to imazapic, nicosulfuron, and diclosulam. All Palmer amaranth populations from Argentina were shown to be resistant to at least one ALS-inhibiting herbicide. The populations were then subjected to further testing to identify the mutation responsible for the observed ALS resistance. All mucronate amaranth populations exhibited a mutation previously documented to confer ALS resistance (S653N). No known resistance-conferring mutations were found in Palmer amaranth.

Carrot is a high-value cash crop that is grown in Israel throughout the year. Egyptian broomrape is a chlorophyll-lacking, obligate, root holoparasite that parasitizes members of many botanical families, including the Apiaceae. At high infestation levels, Egyptian broomrape can cause total yield loss in carrot. A protocol has been developed for the control of Egyptian broomrape in carrot. Because carrots are grown in Israel under fall, winter, and spring conditions, information about the relations between the efficacy of control and temperature is important. Therefore, the objective of this study was to investigate the response of carrot and Egyptian broomrape to herbicides at different phenological stages under varying temperature regimes. This study was conducted under temperature-controlled conditions in a multiclimate greenhouse and in a net house. Applications of the imidazolinone herbicides imazapic and imazamox (each applied at 4.8 g ai ha−1) injured carrot plants and reduced yield and yield quality. Glyphosate effectively controlled Egyptian broomrape and did not negatively affect the carrot plants when applied three times at ≤ 108 g ae ha−1. High temperatures increased the carrot plants’ sensitivity to glyphosate. This study found that three applications of glyphosate at 108 g ae ha−1 can prevent Egyptian broomrape damage without causing any damage to the carrot crop. Our results indicate that weather conditions can affect herbicide phytotoxicity in carrot. The highest temperature at the time of herbicide application corresponded to the strongest observed phytotoxic effect. To summarize, effective Egyptian broomrape control can be achieved by three sequential foliar applications of glyphosate (108 g ae ha−1), beginning during the early parasitism stage (i.e., small tubercles). Moreover, applying glyphosate on carrot at high temperature (i.e., 28/22 C day/night temperatures) can injure carrot plants and reduce control efficacy.

Many Italian ryegrass populations in Oregon are resistant to more than one herbicide; therefore, the resistance patterns of these populations must be determined to identify alternative herbicides for management. Two suspected resistant Italian ryegrass populations (R2 and R4) survived flufenacet plus metribuzin applications under typical winter wheat production conditions. Populations R2 and R4 were resistant to clethodim, pinoxaden, quizalofop, mesosulfuron-methyl, flufenacet, but not to acetochlor, dimethenamid-p, metolachlor, pyroxasulfone, imazapyr, sulfometuron, or glyphosate. R4 was resistant to diuron, but R2 was not. The estimated flufenacet doses required for 50% growth reduction (GR50) were 438 g ai ha−1 (R2) and 308 g ai ha−1 (R4). Both populations were controlled by pyroxasulfone at rates greater than 15 g ai ha−1. An Asp-2078-Gly substitution in the ACCase gene was found in both populations, while an Ile-2041-Asn was found only in the R4 population. A Ser-264-Gly substitution in psbA gene was found in the R4 population. These mutations previously have been reported to provide resistance to ACCase and photosynthetic inhibitors, respectively. No resistance mutations were identified in the acetolactate synthase (ALS) gene of either population. The addition of the P450 inhibitor, chlorpyrifos, increased the injury resulting from mesosulfuron-methyl on both resistant populations providing indirect evidence that the ALS resistance may be metabolic. Multiple herbicide-resistant Italian ryegrass populations were identified in this study with both target site and nontarget site based mechanisms likely involved. However, several herbicides were identified including pyroxasulfone, a herbicide in the same group as flufenacet, which could be used to control these two populations.

Resistance to glyphosate in hairy fleabane and horseweed is a problem in orchards and vineyards in California. Population genetic analyses suggest that glyphosate resistance evolved multiple times in both species, but it is unknown if resistance to other herbicides is also present. Two approaches of research were undertaken to further evaluate herbicide resistance in Conyza sp. in the perennial crop systems of California. In the initial study, the distribution of Conyza sp. in the Central Valley, using a semistructured field survey, was coupled with evaluation of the presence and level of glyphosate resistance in plants grown from field-collected seed. In a subsequent study, single-seed descendants representing distinct genetic groups were self-pollinated in the greenhouse and these accessions were evaluated for response to glyphosate and paraquat. Conyza sp. were commonly found throughout the Central Valley and glyphosate-resistant individuals were confirmed in all field collections of both species. The level of glyphosate resistance among field collections varied from 5- to 21-fold compared with 50% glyphosate resistance (GR50) of the susceptible, with exception of one region with a GR50 similar to the susceptible. When self-pollinated accessions from different genetic groups were screened, the level of glyphosate resistance, on the basis of GR50 values, ranged from 1.7- to 42.5-fold in hairy fleabane, and 5.9- to 40.3-fold in horseweed. Three accessions of hairy fleabane from different genetic groups were also resistant to paraquat (40.1- to 352.5-fold). One glyphosate-resistant horseweed accession was resistant to paraquat (322.8-fold), which is the first confirmed case in California. All paraquat-resistant accessions of Conyza sp. identified so far have also been resistant to glyphosate, probably because glyphosate resistance is already widespread in the state. Because glyphosate and paraquat resistances are found across a wide geographical range and in accessions from distinct genetic groups, multiple resistant Conyza sp. likely developed independently several times in California.

The tomato mutant line HRT was obtained by ethyl methanesulfonate seed mutagenesis of the commercial tomato line M82. Greenhouse studies were conducted to determine whole-plant response to the imidazolinone herbicides imazamox, imazapic, and imazapyr; pyrithiobac-sodium (a herbicide from the pyrimidinylthiobenzoic acid group); and propoxycarbazone sodium (sulfonylaminocarbonyltriazolinone group). The mutant was highly resistant to imazamox, imazapic, and imazapyr, but did not differ from M82 in its response to the sulfonylurea herbicides Envoke (trifloxysulfuron), Monitor (sulfosulfuron), and Glean (chlorsulfuron). Equip (foramsulfuron), a sulfonylurea herbicide, was toxic to M82 but less so to HRT plants. Under field conditions, HRT showed high resistance to imazapic and imazapyr. The herbicides at a rate of 144 g ai ha−1 did not cause any reduction in HRT plant vigor, development, or yield. Results of greenhouse and field experiments demonstrated high Egyptian broomrape–control efficacy with the imidazolinone herbicides imazapic and imazapyr. Two imazapic applications of 9.6 or 14.4 g ai ha−1 and three applications of 4.8 g ai ha−1 in pot experiments completely prevented appearance of broomrape shoots aboveground. Three and four applications of the same herbicides in the field at a rate of 12 or 24 g ai ha−1 completely prevented shoot appearance without any yield losses. Single imazapic application as high as 144 g ai ha−1 did not damage the plants or reduce HRT yield.

Giant ragweed has been increasing as a major weed of row crops in the last 30 yr, but quantitative data regarding its pattern and mechanisms of spread in crop fields are lacking. To address this gap, we conducted a Web-based survey of certified crop advisors in the U.S. Corn Belt and Ontario, Canada. Participants were asked questions regarding giant ragweed and crop production practices for the county of their choice. Responses were mapped and correlation analyses were conducted among the responses to determine factors associated with giant ragweed populations. Respondents rated giant ragweed as the most or one of the most difficult weeds to manage in 45% of 421 U.S. counties responding, and 57% of responding counties reported giant ragweed populations with herbicide resistance to acetolactate synthase inhibitors, glyphosate, or both herbicides. Results suggest that giant ragweed is increasing in crop fields outward from the east-central U.S. Corn Belt in most directions. Crop production practices associated with giant ragweed populations included minimum tillage, continuous soybean, and multiple-application herbicide programs; ecological factors included giant ragweed presence in noncrop edge habitats, early and prolonged emergence, and presence of the seed-burying common earthworm in crop fields. Managing giant ragweed in noncrop areas could reduce giant ragweed migration from noncrop habitats into crop fields and slow its spread. Where giant ragweed is already established in crop fields, including a more diverse combination of crop species, tillage practices, and herbicide sites of action will be critical to reduce populations, disrupt emergence patterns, and select against herbicide-resistant giant ragweed genotypes. Incorporation of a cereal grain into the crop rotation may help suppress early giant ragweed emergence and provide chemical or mechanical control options for late-emerging giant ragweed.