A mesorhizotron and scanning system was modified to study the development of Russian thistle root systems during the 1996 and 1997 growing seasons at Lind, WA. Our imaging equipment combined the full profile images afforded by conventional rhizotrons with the portability of cylinder-based minirhizotron systems at a fraction of the cost of either system. Root development of Russian thistle in early spring was rapid and extensive compared with shoot growth. In 1996, 30 d after planting (DAP) Russian thistle roots were at least five times as long as the corresponding plant's shoots. During the next 20 d, shoots grew a maximum of 20 cm, whereas roots grew a maximum of 120-cm deep. Maximum root elongation rate reached 2 to 3 mm/cm2/d at the 70- to 120-cm depths 30 to 50 DAP in 1996 and 55 to 70 DAP in 1997. More than one (multiaxial grouping) Russian thistle root was often observed growing through the same soil channels. After the rapid early season growth, roots began to shrink or die back until shoots were clipped to simulate wheat harvest. Within 7 d after harvest, roots regenerated in old root channels. Our mesorhizotron system is a promising inexpensive tool for monitoring root morphological development of Russian thistle under field conditions.

Field studies in 1998 and 1999 determined the response of grain sorghum and sunflower to soil residues of MKH 6561 and MON 37500 in a failed winter wheat recropping situation. Averaged across years, MON 37500 at 30 g ai/ha reduced sunflower density by 39%. MKH 6561 at 30 or 45 g/ha did not reduce sunflower density, growth, or late-season biomass, whereas MON 37500 reduced late-season biomass in both years. Neither herbicide affected grain sorghum plant density in 1998, but MON 37500 decreased sorghum density 80% in 1999. Sorghum growth, biomass, and yield were limited more severely by MON 37500 in 1999 than in 1998. MKH 6561 had little or no effect on grain sorghum during either year. Differences in crop response to MON 37500 between years was likely because of wheat plant size at the time of application and greater interception and metabolism of the herbicide in 1998 crop year.

A survey of county extension agents was conducted in 1998 to determine the most troublesome weeds in corn, cotton, forages and pastures, peanut, small grains, soybean, tobacco, and vegetables in Georgia. The most troublesome weed statewide averaged over all crops was sicklepod. It was the most troublesome weed in cotton and soybean and among the four most troublesome weeds in corn, peanut, tobacco, and vegetables. Sicklepod was found in each of the nine climatological districts and in all the crops surveyed. Perennial nutsedge species were the second most troublesome weeds in Georgia. They ranked as the most troublesome weeds in tobacco and vegetables and were among the top five most troublesome weeds in corn, cotton, peanut, and soybean. Pigweed species were ranked third averaged over all the crops surveyed and were the second most troublesome weeds in cotton and vegetables and among the top five most troublesome species in corn, soybean, and tobacco. Morningglory species were listed as troublesome in six of the eight crops surveyed and ranked fourth overall. Similarly, Texas panicum was found in all districts and was the fifth most troublesome weed species. Texas panicum was the most troublesome weed in corn and among the top five most troublesome weeds in peanut, soybean, and tobacco. Florida beggarweed was the most troublesome weed in peanut, the second most troublesome weed in tobacco, and a top-10 weed species in corn, cotton, soybean, and vegetables, resulting in a ranking of sixth overall. Wild radish, large crabgrass, and tropic croton were the seventh through the ninth most troublesome weeds. Wild radish was the most troublesome weed of small grains and the sixth most troublesome weed of vegetables. Large crabgrass was the second most troublesome weed of forages and pastures and was reported in six other crops. Tropic croton was a troublesome weed in seven of the eight crops surveyed and was among the top five most troublesome weeds of cotton and peanut. The 10th most troublesome weed overall was bahiagrass, the most troublesome weed of forages and pastures.

The number of papers published on allelopathy was low until the 1970s but has accelerated rapidly in recent years. Alleged allelopathic activity has been suggested for numerous weed and crop species; however, many studies employed preliminary bioassays (leachates or extracts in the absence of soil). The importance of substratum and of abiotic and biotic stresses (nutrients, shade, herbides, plant diseases, herbivores), plant density, habitat, and climate has been realized in recent years. Laboratory bioassay, the first step to investigate probable involvement of allelopathy, should be designed to correspond to field conditions. Studies need to focus on the effects of (1) climatic, environmental, and habitat factors on allelopathic potential; (2) abiotic and biotic soil components on fate and biological activity of allelochemicals; and (3) allelochemicals on soil nutrient dynamics, microbial ecology, and other abiotic and biotic components. Recently, attempts to find crop cultivars with a competitive allelopathic edge have been made. Interaction of professionals from different disciplines is needed to understand the complexity of the ecosystem. The following are important topics for discussion: allelopathy and the transition to sustainable agriculture; physiological and biochemical studies; using transgenes to produce allelopathic crops; microbial allelochemicals and pathogens as weed biological agents; bioassays for allelopathy (problems and solutions); pollen allelopathy; and an ecological perspective of allelopathy.

Allelopathy has been suggested as a mechanism of interference in several weed species. Allelochemicals released from certain weed species influence the growth and yield of crop species. Several laboratory studies present circumstantial evidence of the occurrence of allelopathy as a causative agent in weed–crop agroecosystems. Field evidence, however, is still lacking. In this paper, the significance of field studies is argued in terms of a multifaceted approach to allelopathy, and mugwort is used as an example. Previous research demonstrated the allelopathic potential of mugwort; however, experiments were not carried out in a natural environment. Inderjit and Foy (1999) have demonstrated that chemical characteristics (pH, inorganic ions, and phenolics) of soil amended with mugwort leaf leachate were altered when compared to unamended soil. We have analyzed the mugwort-infested field soil and compared its chemical characteristics with those of amended soils. No definite trend, in terms of influence of mugwort on soil chemistry, was observed. Results indicate the importance of field studies in order to obtain ecologically relevant data from laboratory studies. Field situations are often complex in terms of the presence of interfering flora. Cyanobacteria, for example, play an important role in weed–crop interactions in rice paddy soils. Allelochemicals released from weed species present in the paddy field may influence nitrogen-fixing cyanobacteria in addition to their phytotoxic effects to the paddy crop. Significance of phytotoxic effects of weed species on crop growth, and N2-fixing potential of cyanobacteria in paddy soils is discussed.

The involvement of allelopathy in various crop–weed competition studies has been suggested by several authors, but its significance has been demonstrated with varying success. It is extremely difficult to unambiguously demonstrate allelopathy in nature because of the complexity of plant interference and its relationship to soil chemistry. However, with an increased understanding of the chemical processes occurring in the agroecosystem, genetic mapping of quantitative traits, and the ability to identify allelochemicals, an effort should now be directed toward understanding the mechanisms for allelopathy, as well as trying to optimize an allelopathic effect to produce more competitive crops. The approach used for rice allelopathy research can be used as a general framework for understanding how genetically encoded traits affect the competitive ability of plants. This framework requires the involvement of a range of scientists from multidisciplinary research areas with the overall objective of optimizing competitive ability in crops. Such research efforts could reduce dependency on herbicides and thus increase the sustainability of weed management practices. This paper aims to illustrate the importance of allelopathy for crop competitive ability and to identify a framework suitable for result-oriented collaborative research toward breeding for competitive ability in crops.

The interaction between wheat and perennial ryegrass seed density and seedlings of different ages was studied under controlled conditions. Root length of perennial ryegrass, after sowing, was suppressed by wheat and was dependent on the density of wheat seeds. Shoot growth of perennial ryegrass, however, was unaffected by the presence of wheat. Perennial ryegrass density had no effect on the first 2 wk of wheat seedling growth. The age of wheat seedlings had no appreciable influence on either root or shoot growth of perennial ryegrass. The present study highlights the need for an unbiased two-way experimental design to identify the dominant competitor.

The root exudates produced by sorghums contain a biologically active constituent known as sorgoleone. Seven sorghum accessions were evaluated for their exudate components. Except for johnsongrass, which yielded 14.8 mg root exudate/g fresh root wt, sorghum accessions consistently yielded approximately 2 mg root exudate/g fresh root wt. Exudates contained four to six major components, with sorgoleone being the major component (> 85%). Three-dimensional structure analysis was performed to further characterize sorgoleone's mode of action. These studies indicated that sorgoleone required about half the amount of free energy (493.8 kcal/mol) compared to plastoquinone (895.3 kcal/mol) to dock into the QB-binding site of the photosystem II complex of higher plants. Light, cryo-scanning, and transmission electron microscopy were utilized in an attempt to identify the cellular location of root exudate production. From the ultrastructure analysis, it is clear that exudate is being produced in the root hairs and being deposited between the plasmalemma and cell wall. The exact manufacturing and transport mechanism of the root exudate is still unclear. Studies were also conducted on sorgoleone's soil persistence and soil activity. Soil impregnated with sorgoleone had activity against a number of plant species. Recovery rates of sorgoleone-impregnated soil ranged from 85% after 1 h to 45% after 24 h. Growth reduction of 9 14-d-old weed species was observed with foliar applications of sorgoleone.

Crop allelopathy has seldom been used effectively by farmers in weed management. Traditional breeding methods have not been successful in producing highly allelopathic crops with good yields. Genetic engineering may have the potential for overcoming this impasse. Crops have been made resistant to insects, pathogens, and herbicides with transgenes, but biotechnology has not produced crops that control weeds with allelochemicals. The strategies for producing allelopathic crops by biotechnology are relatively complex, usually involving multiple genes. One can choose to enhance production of allelochemicals already present in a crop or to impart the production of new compounds. The first strategy involves identification of the allelochemical(s), determination of the enzymes and genes encoding them, and the use of genetic engineering to enhance their production. The latter strategy employs altering existing biochemical pathways by insertions of transgenes to produce new allelochemicals. With either strategy, there are potential problems with tissue-specific promoters, autotoxicity, metabolic imbalances, and proper movement of the allelopathic compound to the rhizosphere.

The initiative to use plant pathogens and allelochemicals from pathogens and other microorganisms as biological weed control agents (bioherbicides) began about 30 yr ago. Since then, numerous plant pathogens (bacteria and fungi) and microbial allelochemicals have been isolated, identified, and tested for their bioherbicidal potential. Pathogens (and in some cases microbial phytotoxins) may be used directly on target weed species, or such allelochemicals may provide unique chemical templates for the synthesis of new herbicide classes with novel molecular modes of action. To date, the most successful microbial products that have led to the development of commercial herbicides are bialaphos (commercially available in Japan) and glufosinate (marketed worldwide). Glufosinate is the ammonium salt of phosphinothricin, which is the active ingredient of bialaphos derived from a nonphytopathogenic Streptomyces species. This overview will examine selected advances in the isolation and identification of novel plant pathogens that have weed hosts, and some microbial allelochemicals with phytotoxic properties. Perspectives on the use of these bioherbicides in weed control, relative to their allelopathic interactions with plants will be discussed.

Kalmia angustifolia is a problematic understory shrub in boreal forests of eastern Canada. In this article, we discuss the nature of its interference with conifer regeneration, and control measures using herbicides. Although laboratory studies suggest the leaching of water-soluble phenolic compounds from Kalmia, allelopathic interference cannot be invoked as a cause of conifer regeneration failure because of lack of field evidences. Because of severe inhibitory effects of Kalmia on conifer growth and its role in long-term habitat degradation, the development of strategies for Kalmia management is a priority research area. Neither prescribed burning, plowing, scarification, nor 2,4-D application was proven effective in Kalmia control. Two studies were carried out using transplanted Kalmia to determine the efficacy of several herbicides and a herbicide plus burning treatment to control Kalmia. In one experiment, eight treatments were applied. They were control, glyphosate (3.36 kg/ha), glyphosate (2.88 kg/ha) plus surfactant Tween 20 (0.2%), glyphosate (2.88 kg/ha) plus surfactant Triton XR (0.2%), fosamine ammonium (5.38 kg/ha), triclopyr (4 kg/ha), hexazinone (2.2 kg/ha), and hexazinone (5 kg/ha). Triclopyr (4 kg/ha) provided the best control with significant reduction in the number and dry weight of sprouts, rhizomes, and current-year foliage of Kalmia compared with control. In another experiment, triclopyr (4 kg/ha), hexazinone (5 kg/ha), and hexazinone (2.2 kg/ha) plus burning treatments were applied. To determine the response of black spruce seedlings to the herbicide treatments, black spruce seedlings were planted in all the Kalmia pots except the ones that received the hexazinone (2.2 kg/ha) plus burning treatment. Triclopyr (4 kg/ha) caused the maximum damage to Kalmia, affecting all the above- and below-ground components of the plant. We recommend that an experimental field trial be established using triclopyr to test its efficacy for Kalmia control under field conditions.

Pollen allelopathy results when pollen releases toxins that inhibit seed germination, seedling emergence, sporophytic growth, or sexual reproduction. Of the six known pollen-allelopathic species, two are crops (timothy and corn and four are weeds (orange hawkweed, parthenium, yellow hawkweed, and yellow-devil hawkweed). Allelopathic pollen in weeds could pose threats to crops, especially if both are wind pollinated. Even if it is the crop that is pollen-allelopathic, other crops could be threatened, or more likely, weeds might adapt to pollen allelopathy and pose a greater problem. Nonetheless, pollen allelopathy could be a useful approach to biological control because allelochemicals are packaged in a natural targeting system (pollen grains) and are biologically active at low doses (<10 grains/mm2 on stigmas). If it is to be an effective biological control agent, pollen allelopathy must be examined within the wider context of farming systems management and used as one method of varying selection pressures to prevent weeds from adapting to any one particular management technique or suite of techniques.

Several weed species have been reported to have allelopathic activities. However, most of these studies indicate the probable involvement of allelochemicals but are not conducted in field settings. In addition to their adverse effects on growth and yield of many crop species, many troublesome weeds such as mugwort and lantana influence biodiversity. More studies on the ecological, physiological, and molecular aspects of weed allelopathy should be conducted in order to better understand community structure and declining biodiversity.

Field studies were conducted from 1997 to 1999 in Westminster, MD, to evaluate a variety of preemergence (PRE) and postemergence (POST) herbicide programs on crop injury and control of triazine-resistant common lambsquarters (TR-CHEAL) in no-till corn. In 1997 PRE studies, combinations of metolachlor with flumetsulam or halosulfuron and high rates of rimsulfuron + thifensulfuron (0.02 + 0.009 kg ai/ha) injured corn most 4 weeks after treatment (WAT), averaging 11 to 15%. In 1998 and 1999, metolachlor plus a high rate of halosulfuron (0.07 kg/ha) injured corn most 4 WAT, averaging 13 and 10%, respectively. High rates of rimsulfuron + thifensulfuron also provided a higher level of corn injury in 1998 and 1999 in comparison with many of the other treatments. However, for all three years of the study, no injury was observed from any PRE treatment 8 WAT. In 1997 and 1998, at 8 WAT, combinations of metolachlor with flumetsulam or halosulfuron provided greater TR-CHEAL control than many of the other treatments, averaging 98 and 100%, respectively. In 1999, however, control of TR-CHEAL with these same treatments did not vary in comparison with most of the other treatments. At 8 WAT, there was a trend for increased TR-CHEAL control as the rates of RPA-201772 and rimsulfuron + thifensulfuron increased. Control of TR-CHEAL with metolachlor + atrazine + pendimethalin varied across years 8 WAT. Similar observations were made 16 WAT. In 1997, POST applications of dicamba, SAN 1269H at 0.3 lb ai/ha, primisulfuron + dicamba, and primisulfuron + CGA 152005 + dicamba provided the highest level of TR-CHEAL control 8 WAT, averaging 93, 93, 95, and 93%, respectively. In 1998, with the exception of carfentrazone + atrazine, all POST treatments provided 90% control of TR-CHEAL or better 8 WAT. In 1999, POST applications of SAN 1269H at 0.3 kg/ha, pyridate + atrazine, and primisulfuron + CGA 152005 + pyridate provided the highest level of TR-CHEAL control, averaging 80, 90, and 96%, respectively, 8 WAT. With the exception of carfentrazone + atrazine, control of TR-CHEAL with the other POST treatments varied in 1999 from 60 to 74% 8 WAT. Carfentrazone + atrazine applied POST provided the lowest level of TR-CHEAL control 8 WAT averaging 28, 37, and 17% for 1997 to 1999, respectively.

The competitive ability of five cool-season grasses relative to Dalmatian toadflax, musk thistle, and downy brome was assessed in two field studies. In 1994, Bozoisky Russian wildrye and four wheatgrass varieties (Critana thickspike, Hycrest crested, Luna pubescent, and Sodar streambank wheatgrass) were seeded into populations of downy brome and musk thistle at Riverside, WY. The same grasses were seeded into populations of Dalmatian toadflax at Cheyenne, WY, in 1995. In 1997 and 1998, weed populations at both study sites were reduced in areas seeded with the five grasses relative to unseeded controls. Hycrest crested and Luna pubescent wheatgrasses were the most competitive against the three weed species. Bozoisky Russian wildrye was more competitive against Dalmatian toadflax than against the other weeds. Sodar streambank wheatgrass suppressed musk thistle and downy brome but was not competitive against Dalmatian toadflax. Seeded grasses, such as Hycrest crested and Luna pubescent wheatgrass, appeared to limit the re-establishment of these weeds. Economic model predictions of the net present values and the internal rates of return suggest that Hycrest crested and Luna pubescent wheatgrass can provide financially feasible long-term weed control only if desired grass yields are maintained for more than 15 yr.

… on finding a root of shepherd's purse (Capsella bursa-pastoris) open a seed-vessel. If the seed is yellow you will be rich, but if green you will be poor.

Yorkshire Folklore, W.M.E. Fowler, 1909 (Vickery 1995)