Barley (Hordeum vulgare L.) and brown mustard (Brassica juncea L.) are winter cover crops known to produce allelochemicals that suppress plant growth. Incorporating barley or brown mustard residues into the soil prior to planting a spring-seeded cash crop may suppress early season weeds in the cash crop; however, the comparative levels of weed suppression offered by barley and brown mustard cover crops incorporated into soil have not been determined. This study analyzed the relative capacities of barley and brown mustard cover crops to suppress early season weeds of spring-seeded chile pepper (Capsicum annuum L.). Reductions in weed density or hand hoeing time as a result of barley and/or brown mustard cover crop treatment were determined in two chile pepper fields in New Mexico over two growing seasons. For cover crop species that suppressed weeds in multiple site-years, a controlled environment study clarified possible growth stages adversely affected by determining the effects of cover crop-amended soil on the germination and seedling development of Palmer amaranth (Amaranthus palmeri S. Watson). Field study results indicated barley reduced early-season weed densities of chile pepper by up to 80% compared to the noncover control. Barley also reduced hoeing time in three of four site-years without affecting chile pepper fruit yield. Mustard cover crops reduced weed density in only one site-year (56% reduction relative to noncover control) and did not decrease hoeing time. The controlled environment study indicated that soil amended with barley slowed germination of A. palmeri without inhibiting seedling development. The results of this study indicate that a barley cover crop is more effective than brown mustard for early-season weed control of chile pepper in the southwestern United States.

Invasive plants can gain a foothold in new environments by manipulating soil conditions through allelopathy or through the disruption of associations between native plants and their mycorrhizal associates. The resulting changes in soil conditions can affect the recovery of habitats long after the invasive plant has been removed. We conducted a series of greenhouse experiments to examine the effects of soil conditioned by pale swallow-wort [Vincetoxicum rossicum (Kleopow) Barbarich; Apocynaceae], on the growth of native plants. Additionally, we tested the effects of aqueous extracts of common milkweed (Asclepias syriaca L.; Apocynaceae), a related plant with known allelopathic effects, on the regrowth of V. rossicum from transplanted root crowns. Soil from a 15-yr-old V. rossicum infestation reduced seedling emergence in A. syriaca as well as in V. rossicum itself. Conversely, the same soil had no effect on the growth of mature A. syriaca plants. Soil conditioned by V. rossicum growth in the greenhouse had no effect on the biomass and percentage cover generated by two restoration seed mixes. Soil conditioned by A. syriaca, however, yielded lower biomass and percentage cover from both seed mixes. In contrast to the allelopathic effects of A. syriaca on seedlings, aqueous extracts of A. syriaca increased aboveground plant growth in V. rossicum. Our results suggest that the effects of V. rossicum–conditioned soil on native plants are concentrated at the seedling establishment phase. Additionally, the use of diverse native seed mixes shows great potential for restoring productivity to ecosystems affected by V. rossicum.

Invasive species control management involves a large amount of plant material. The present work evaluated the allelopathic potential of the invasive species Ulex europaeus L. (Fabaceae) or ‘Gorse’ and its possible use as a bioherbicide, taking advantage of the extracted plant material after control measures, particularly needed in invaded areas. Specifically, we investigated the efficacy of dried plant material from U. europaeus in the control of the adventitious plants, Lolium multiflorum Lam. and Lolium rigidum Gaud., using the Avena sativa L. crop as a case study. We only used vegetative plant parts because it is essential to avoid the dispersion of U. europaeus with its use, especially in invaded areas. A greenhouse pot experiment was conducted, using activated carbon (AC). The target species (L. multiflorum, L. rigidum and A. sativa) were subjected to a mixture of organic substrate with U. europaeus mulch applied pre-emergence and a subsequent application of aqueous extracts from the mulch. Emergence, height and biomass of the target species were determined. After 2 months, we also tested a possible legacy effect of the substrate on the germination of the target species. We noticed a negative effect of U. europaeus mulch on the emergence of L. rigidum, which can be attributable to the allelopathic compounds released from U. europaeus mulch because the effect was non-significant in presence of AC. Conversely, no effect on L. multiflorum or A. sativa was produced by mulch treatments. Nevertheless, the combination of U. europaeus mulch and its extracts demonstrated a phytotoxic effect on the biomass of the crop species A. sativa, and a fertilizing effect on the weeds L. multiflorum and L. rigidum, which is why this use is discouraged. With our results we cannot recommend the use of U. europaeus as a bioherbicide in oat crops, but this study emphasizes the capability of U. europaeus to structure plant communities through the chemic- and bio-properties of its tissues that modifies the soil environment.

Multiple herbicide-resistant populations of horseweed [Conyza canadensis (L.) Cronquist] continue to spread rapidly throughout Ontario, notably in areas where no-till soybean [Glycine max (L.) Merr.] is grown. The occurrence of multiple herbicide resistance within these populations suggests that the future role of herbicide tank mixtures as a means of control will be limited. An integrated weed management strategy utilizing complementary selection pressures is needed to reduce the selection intensity of relying solely on herbicides for control. Field studies were conducted in 2018 and 2019 to test the hypothesis: if fall-seeded cereal rye (Secale cereale L.) can reduce C. canadensis seedling density and suppress seedling growth, then the interaction(s) of complementary selection pressures of tillage, cereal rye, and herbicides would improve the level of C. canadensis control. Laboratory studies were conducted to determine whether the allelopathic compound 2-benzoxazolinone (BOA) affected the root development of C. canadensis seedlings. The interactions observed among multiple selection pressures of tillage, cereal rye, and herbicides were inconsistent between the 2 yr of study. A monoculture of cereal rye seeded in the fall, however, did reduce seedling height and biomass of C. canadensis consistently, but not density. This reduction in seedling height and biomass was likely caused by the allelopathic compound BOA, which reduced seedling root development. Control of C. canadensis seedlings in the spring required the higher registered rates of dicamba or saflufenacil. The addition of shallow fall tillage and the presence of cereal rye did not improve the variability in control observed notably with 2,4-D or the lower rates of saflufenacil or dicamba. With the implementation of complementary weed management strategies, environmental variables in any given year will likely have a direct influence on whether these interactions are additive or synergistic.

Artemisia adamsii is a weed with low palatability to livestock and is thus recognized as an indicator of rangeland degradation in Mongolia. We investigated the germination characteristics of this species, phytotoxic suppression of its germination by its own residue, and the applicability of such germination behaviour to the control of this species. We also discussed the ecological consequence of these factors regarding the vegetation dynamics of the Mongolian steppe. Germination of A. adamsii was tested at different light and temperature conditions. Germination was also tested for A. adamsii and four native grassland species in the presence or absence of A. adamsii residue. Germination of A. adamsii was light-demanding and temperature-dependent. Artemisia adamsii residue showed autotoxic but sub-fatal suppression of germination, and the chemicals causing that suppression were shown to be aqueous and volatile. Phytotoxicity of A. adamsii residue on the germination of grassland species was low, except for in Artemisia frigida. The applicability of the observed sub-fatal autotoxicity for controlling this species was likely to be low, but the elucidated germination characteristics could contribute to developing a strategy for controlling this species. The autotoxicity in A. adamsii germination was suggested to have an ecological consequence that mediates species transition from A. adamsii to other species in degraded land occupied by A. adamsii. As low-palatability A. adamsii can act as a nurse plant of palatable species under grazing conditions, the invasion of A. adamsii into disturbed grasslands may facilitate the recovery of such grasslands in terms of improving pasture quality.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is one of the most troublesome and competitive weed species in rice (Oryza sativa L.) grown under conventional flood (FLD) irrigation and is expected to be similarly damaging under alternate-wetting-and-drying (AWD) irrigation. Several indica rice genotypes have been shown to suppress E. crus-galli under FLD management, but very little is known about the nature and potential of weed suppression using AWD irrigation. In this 3-yr field study, we evaluated seven diverse rice genotypes for their weed suppression and grain yield potential under FLD and AWD irrigation. The E. crus-galli dry biomass at midseason was 11% less under AWD compared with FLD. Overall, plots of an indica variety and a hybrid resulted in less E. crus-galli dry biomass under AWD compared with FLD. Grain yield in weed-free AWD plots averaged 12% less than in weed-free FLD plots. Grain yield of the tropical japonica (TRJ) type, ‘Bengal’, was 32% lower under AWD than FLD, whereas grain yields of the two indica genotypes, PI 312777 and ‘Rondo’, and a hybrid were similar in both irrigation systems. Grain yield reduction in E. crus-galli–infested AWD plots averaged greater than 90%. Thus, E. crus-galli greatly reduced the grain yield of all rice genotypes tested in both FLD and AWD systems in this study. Grain yield of the indica and hybrid genotypes was relatively less affected by the AWD treatments compared with the TRJ genotypes, particularly with Bengal, suggesting that the weed-suppressive genotypes, PI 312777 and Rondo, would be better suited to AWD irrigation systems and for inclusion in weed-suppression rice-breeding programs in the southern United States.

It is challenging to predict the changes in weed flora that may occur because of changes in global climate. Limited data are available on the effect of climate change and drought conditions on weed flora and their competitiveness in Southern Europe. Future predictions by scientists indicate reduced and untimely rainfall, along with increased temperatures in this region. Weeds possess a variety of developmental and physiological mechanisms, including senescing, increased leaf cuticular wax deposition, well-developed palisade parenchyma in the leaves, high root/shoot ratio, stomatal closure, peroxidase accumulation and symbiosis with endophytes that enable them to adapt to drought and high temperatures. Because of high adaptability of weeds to adverse environmental conditions, it can be assumed that under future warmer and drier environmental conditions, their growth will be favoured, while the competitiveness of vegetable crops against weeds will be decreased. It is important to highlight that the predicted decrease in overall rainfall levels throughout the year may lead to increased problems of herbicide residues (carryover effects) to following crops. The current paper provides an up-to-date overview of the adaptation mechanisms of weed species commonly found in Southern Europe, in order to expand the available knowledge regarding their response to drought and elevated temperatures. Emphasis is placed on revealing the effects of drought and increased temperatures on vegetable–weed competition and, most importantly, its effect on vegetable crop yield.

Vinasse, a liquid fermentation residual of bio-ethanol production that also contains solid particles in suspension, is commonly used as a soil amendment. Previous studies reported vinasse reduced seed germination and seedling establishment, suggesting herbicidal activity. Laboratory experiments were conducted to determine whether vinasse herbicidal activity is present in the liquid or solid phase, and whether it affects plants during seed early germination (i.e., imbibition), late germination (i.e., embryo growth and radicle protrusion), or seedling growth. Most of the herbicidal activity was associated with the liquid phase, and for most species, seed viability was predominantly affected after the imbibition phase. Susceptibility to vinasse was species dependent. Lettuce (Lactuca sativa L.) germination was <8% when seeds were imbibed and germinated in vinasse solutions or imbibed in water and germinated in vinasse. Conversely, imbibing lettuce seeds in vinasse solutions and germinating them in water did not change their germination in comparison with seeds imbibed and germinated in water (>80% germination). Wheat (Triticum aestivum L.) and sicklepod [Senna obtusifolia (L.) H. S. Irwin & Barneby] germination decreased 10% and 35% when seeds were imbibed and germinated in vinasse, respectively, while Palmer amaranth (Amaranthus palmeri S. Watson) and southern crabgrass [Digitaria ciliaris (Retz.) Koeler] germination decreased >90%. All evaluated species reduced radicle growth as vinasse concentration increased. Filtered liquid vinasse with reduced concentration of salt and ionic compounds inhibited radicle growth similarly to unfiltered vinasse, indicating that the herbicidal activity was not due to osmotic effects and was likely present in the organic liquid phase. Amaranthus palmeri, S. obtusifolia, and D. ciliaris increased the proportion of dormant seed more than 2-fold when they were imbibed or imbibed and germinated in vinasse solutions. Vinasse might be useful for weed management to reduce germinable weed seedbanks by increasing seedling mortality and seed dormancy either by properly timing of its application as a soil amendment or by purifying herbicidal compounds and using them directly for weed control.

Biofumigation is practiced to control soilborne pests and weeds in agronomic fields. The objectives of this research were to assess the dose response of weed seeds to Indian mustard biofumigation and associate responses to seed dormancy state, initial dormancy, and seed parameters. A petri dish biofumigation methodology was developed to expose seeds of common lambsquarters, bird vetch, wild carrot, common ragweed, green foxtail, velvetleaf, hairy galinsoga, and red clover to allelochemicals produced after rehydrating 0 (control), 1.94, 2.90, 5.81, 11.61, and 17.41 mg cm−2 of dried mustard powder. Weed species expressed specific dose responses, estimated ED50, LD50, and maximal mortality. Hairy galinsoga and wild carrot were consistently the most affected by biofumigation, with maximal mortality reaching 97% and 95%, ED50 values for germination were 1.91 and 2.68 mg cm−2 , and LD50 values were 3.31 and 3.69 mg cm−2 of dried mustard tissue, respectively. Initial dormancy was assessed by germination and tetrazolium tests. Seed parameters such as testa thickness, relative weight of the testa, and seed size were measured directly by manual dissection, weighing seed structures, and stereomicroscopic imaging software measurements. Linear regression analyses revealed initial dormancy to be positively related to ED50 and LD50 values with a significant interaction with seed surface and seed width, respectively. Exposure to 5.81 mg cm−2 of dried mustard powder increased common ragweed seed mortality for after-ripened seeds by 293% and by 58% for primary dormant seeds compared with untreated seeds. Mortality of common lambsquarters secondary and primary dormant seeds increased by 730% and 106%, respectively, and for wild carrot by 1,193 and 156%, respectively. Results underline the potential to incorporate biofumigation into weed management programs for repression of susceptible weed species.

Laboratory experiments were undertaken to investigate the allelopathic activity of Mexican sunflower in soil as well as to determine the effect of several soil factors on activity. There was a decrease in shoot and root growth of the test plant species when grown in soil previously planted with Mexican sunflower. However, seed germination was not affected. Soil-water separated from the same soil also inhibited shoot and root growth. A water extract from Mexican sunflower leaves applied to soil also reduced shoot and root growth, but the degree of inhibition varied among test plant species. Incorporation of dry Mexican sunflower leaves into soil at the rate of 1 and 2% (w/w) inhibited the growth of rice seedlings. The phytotoxic activity of incorporated leaf residue diminished 4 wk after treatment of soil. The inhibitory effect of a leaf water extract in nonautoclaved soil was less than in autoclaved soil and the phytotoxic effect to rice seedling grown in autoclaved soil was less than in autoclaved sand. It is suggested that the decrease in allelopathic activity of Mexican sunflower extract and its residue in soil is due to the action of soil microorganisms and the soil adsorption of the active components.

The relative sensitivity of various crops and weeds to celery root residue incorporated into soil was determined in greenhouse experiments. A range of concentrations was used to determine the percentage of celery residue needed to cause a 50% reduction in plant growth (I50). The most sensitive weed was spiny amaranth and the least sensitive was common purslane. The most sensitive crop was radish and the least sensitive was celery. Selectivity was calculated as crop I50/weed I50 and varied from a maximum of 13 for celery and spiny amaranth to a low of 0.4 for radish and common purslane. Selective weed suppression might be obtained when celery residue is present.

The allelopathic potential of germinating seed of velvetleaf (Abutilon theophrasti Medic.) was established in a laboratory study with germinating seed of turnip (Brassica rapa L. ‘Purple Top’). Two-day-old velvetleaf seedlings inhibited radicle growth of germinating turnip by 17% at 24 h after both were placed in the same petri plate. Alcoholic and aqueous extracts of velvetleaf seed, however, seriously inhibited germination of turnip seed. The extract of velvetleaf seed was fractionated by ion-exchange chromatography into acid, basic, and neutral fractions. Most of the allelopathic activity was located in the acid fraction, which includes the plant phenolics. The basic fraction (amino acids) inhibited radicle growth of turnip by 18% after 48 h. The amino acid profile of velvetleaf seed extracts is presented and discussed, because this fraction has previously been implicated as the allelopathic fraction.

A phytotoxin that inhibited seedling root growth was isolated and purified from rhizomes of quackgrass [Agropyron repens (L.) Beauv.]. Aqueous solutions of the phytotoxin (0.1%, w/v) significantly inhibited the seedling root growth of corn (Zea mays L. ‘New Jersey 8’), oat (Avena sativa L. ‘Clintland 60’), cucumber (Cucumis sativus L. ‘Straight Eight’), and alfalfa (Medicago sativa L. ‘Vernal’). Growth inhibition was not due to extremes of pH or osmotic potential of the inhibitor solutions. The phytotoxin was isolated and purified from a methanol:water extract of dried and ground rhizomes by adsorption and elution from charcoal. A golden-brown, crystalline substance was obtained that was soluble in water, partly soluble in semipolar solvents, and insoluble in nonpolar solvents. Thin layer chromatography (TLC) and high pressure liquid chromatography (HPLC) suggested that a single compound was present. Elemental analysis showed that it contained 42.17% carbon, 5.06% hydrogen, 38.28% oxygen, 4.02% nitrogen, 2.03% sulfur, 0.03% chlorine, and 0.03% phosphorus. The compound was tentatively identified as a glycoside with a molecular weight of 460. Its ultraviolet (UV) absorption spectrum showed an intense maximum at 307 nm.

Emergence of sugarbeets (Beta vulgaris L. ‘MonoHy D2’) from soil was reduced by germinating kochia [Kochia scoparia (L.) Schrad. ♯ KCHSC] seed at densities greater than 1 seed/cm2 when the fungus Rhizopus sp. was present. Water-soluble exudates from germinated kochia seed did not influence Rhizopus growth in vitro or sugarbeet germination. Cell-free culture extracts of Rhizopus reduced sugarbeet germination and radicle elongation. There appeared to be an interaction between kochia and Rhizopus, but the responsible compounds were not identified.

Greenhouse experiments that used capillary mat subirrigation to maintain constant soil moisture and to supply fertilizer continuously were conducted to evaluate the effects of sorghum or rye residue on early growth of barnyardgrass and velvetleaf. The separate effects of root residue and of shoot residue were compared to the combined effects of root plus shoot residues and to an uncovered soil control. Residues included as nontoxic controls were leached shoot tissue and poplar excelsior. Shoot residue, leached shoot tissue, and poplar excelsior were surface-applied on an equal light transmittance basis such that mass of poplar excelsior > shoot residue > leached shoot tissue. The presence of rye root residue delayed emergence of barnyardgrass. Surface-applied residues tended to decrease barnyardgrass height, but velvetleaf stem length was greater in treatments with surface residue. Although cover crop shoot residues had little effect on weed growth after 18 d, weed growth decreased in the presence of cover crop root residues and poplar excelsior.

Research was conducted to determine the potential for allelopathy to occur in pastures infested with bitter sneezeweed. Aqueous extracts of bitter sneezeweed leaves reduced alfalfa and Italian ryegrass seedling growth as much as 50% at concentrations of 0.5% (w/v). Leaf extracts were more phytotoxic than either stem or root extracts and seedling growth was reduced more than seed germination. Bitter sneezeweed tissue mixed in potting soil at concentrations as low as 0.3% w/w reduced alfalfa seedling numbers by 43%, plant height by 26%, and foliage dry matter production by 54% compared to plants cultured in soil without bitter sneezeweed leaf tissue. The potential concentration of bitter sneezeweed leaf material in soil in the pasture ecosystem was determined to be 0.5% w/v in the liquid phase and 0.2% w/w in the solid phase. Alfalfa seedling growth was reduced by 70% when germinating seed were placed under a bell jar with a potted mature bitter sneezeweed plant compared to control seedlings. A potential exists for bitter sneezeweed interference with developing alfalfa and Italian ryegrass seedlings following fall interseeding into pastures infested with bitter sneezeweed.

Products were collected from rhizospheres of a hydroponically grown sorghum-sudangrass hybrid [Sorghum bicolor (L.) Moench. × Sorghum sudanense (P.) Stapf. ‘DeKalb SX17+’] (SSH) and tested for effects on seedling root growth of selected species. Phytotoxic activity was recovered in CH2Cl2 extracts of aqueous solutions of the rhizosphere products. Significant inhibition of annual ryegrass (Lolium multiflorum L.) and alfalfa (Medicago sativa L. was produced by ca. 200 ppmw (H2O weight basis) of the most phytotoxic fraction in filter-paper-disk bioassays. In soils, 70 ppmw (soil weight basis) inhibited alfalfa but not ryegrass. Among several bioassay species, alfalfa was the most sensitive and curly dock (Rumex crispus L. ♯ RUMCR) the least. Treatment of SSH plants with herbicides increased the phytotoxicity but not the amounts of materials collected. Sethoxydim {2-[1-(ethoxyimino)-butyl]-5-[2-ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} was detected in rhizosphere products from plants treated with the herbicide. Amounts of products collected from rhizospheres of common rye (Secale cereale L. ‘Wheeler’), johnsongrass [Sorghum halepense (L.) Pers. ♯ SORHA], and cucumber (Cucumis sativus L. ‘PI 169391’) were similar to those from SSH, but their phytotoxicities differed. Johnsongrass products were most phytotoxic, those from common rye and SSH were similar in phytotoxicity, and those from cucumber were least phytotoxic.

Aqueous extracts of quackgrass [Agropyron repens (L.) Beauv. # AGRRE] shoots and rhizomes inhibited seed germination and root growth of alfalfa (Medicago sativa L. ‘Vernal’), soybean (Glycine max (L.) Merr. ‘Corsoy 79’], navybean (Phaseolus vulgaris L. ‘Seafarer’), and curly cress (Lepidium sativum L.) at concentrations of less than 2.5 mg dried extract/ml. Extracts of quackgrass shoots were generally more inhibitory than extracts of rhizomes. Root and shoot dry weights of snapbeans (Phaseolus vulgaris L. ‘Bush Blue Lake’) grown under sterile conditions were reduced by aqueous extracts of shoots. Root systems were stunted and necrotic and lacked root hairs. The growth of Rhizobium species was not influenced by the presence of 40 or 80 mg/ml concentrations of extracts of shoots or rhizomes. Quackgrass may inhibit indirectly the legume-Rhizobium symbiosis by inhibiting root hair formation rather than directly inhibiting Rhizobium growth. The presence of soil microorganisms was not necessary for the development of quackgrass toxicity in soil or agar. Soil microorganisms reduced toxicity of quackgrass residues in soil.

Soil- and foliar-applied herbicide treatments were evaluated for the control of seedling johnsongrass in the interim between row formation and the planting of fallowed sugarcane fields approximately 90 d later. Soil-surface applications of metribuzin at 1,680 g ai/ha, pendimethalin plus atrazine each at 2,240 g ai/ha, terbacil at 1,680 g ai/ha, and sulfometuron at 35 and 70 g ai/ha and an incorporated application of trifluralin at 2,240 g ai/ha followed by a surface application of atrazine at 2,240 g/ha did not consistently control seedling johnsongrass until the crop was planted. Rhizome johnsongrass populations originating from seedling johnsongrass that escaped the fallow treatments were lowest in the newly planted crop when sulfometuron at 140 to 280 g/ha was applied to the soil surface and when glyphosate was applied POST at 2,240 g ai/ha, particularly as a sequential treatment alone or as a spot treatment at 2% by volume following applications of metribuzin, terbacil, and pendimethalin or trifluralin with atrazine. Sugarcane shoot populations in the fall after planting and sugar yields at the end of the crop's first growing season were highest where fallow treatments minimized johnsongrass development. These treatments also provided broad spectrum control of other seedling weeds, the residues of which influenced crop development.

Spotted spurge (Euphorbia maculata L.) and prostrate spurge (E. supina Raf.), both in subgenus Chamesyce, were examined by light and electron microscopy using a caffeine - fixation technique to sequester the phenolic pools intercellularly. Both species have typical dicotyledon-type Kranz anatomy. Sequestered phenolic pools were located in vacuoles in epidermal and mesophyll cells. Only in spotted spurge, however, were additional phenolic pools formed in bundle - sheath cells. This study was undertaken because allelopathy has been demonstrated in prostrate spurge and because phenolic compounds have been implicated in allelopathy. These results would indicate that spotted spurge should also be allelopathic.