Phytotoxic effects of metabolites from a naturally occurring rhizobacterial isolate, Pseudomonas syringae strain 3366, were determined on downy brome and ‘Hill 81’ winter wheat, along with 10 other weed and crop species. Centrifuged supernatant and concentrated ethyl acetate extracts from aerobic shake cultures of strain 3366 suppressed germination of seeds and reduced root and shoot growth in agar diffusion assays, soil assays, and under field conditions. Generally, root growth was inhibited more than shoot growth. Strain 3366 metabolites applied in soil inhibited all species tested. Crude ethyl acetate extracts in soil inhibited downy brome at concentrations that had little effect on winter wheat. Inhibitory activity was greater in Palouse silt loam (pH 5.8, 3.6% organic matter) than in Shano silt loam (pH 9.0, 0.8% organic matter). Activity of extracted metabolites decreased rapidly in wet soil but remained high in dry soil. Active metabolites were isolated and purified from the ethyl acetate extract using column chromatography, thin-layer chromatography, and crystallization. Chemical analysis revealed the presence of phenazine-1-carboxylic acid, 2-amino phenoxazone, and 2-amino phenol. Activity of these metabolites against downy brome was confirmed in agar assays. Phenazine-1-carboxylic acid, the major identifiable metabolite present in ethyl acetate extracts (20% by weight), inhibited downy brome root growth by 99% at concentrations of 5.7 mg L−1. Production of these metabolites in field soil by live bacteria of strain 3366 was confirmed with thin-layer chromatography.

The potential for use of compounds isolated from diverse sources, ranging from microbial to higher plants, has received increasing attention. These natural products represent a vast array of chemistries and mechanisms of action often having the requisite specificities to be ideal herbicides. Tentoxin, a cyclic tetrapeptide, has potent chlorosis activity on a variety of weeds of soybean and corn while not affecting these crops. Investigations into this toxin's mechanism suggest several enzyme target sites for the development of herbicidal analogues, each having the inherent specificity of the parent compound. Structure/activity relationships determined so far may permit a directed synthesis of efficacious analogues. New insights into biosynthesis of the toxin suggest possible means of increasing production to allow economical exploitation of the toxin itself as a herbicide.

Benzyl isothiocyanate (BITC) was extracted from mature papaya (Carica papaya L.) seeds and applied to imbibed velvetleaf (Abutilon theophrasti Medic. ♯3 ABUTH) seeds. Complete inhibition of germination occurred at a concentration of 6 × 10-4 M BITC, and only 28% (compared to controls) of the seeds germinated after 4 days exposure to the 4 × 10-4 M treatment. Corn (Zea mays L. ‘DeKalb XL-66′) was unaffected even at higher concentrations; however, 72% of imbibed soybeans [Glycine max (L.) Merr. ‘Tiger’] germinated in 6 × 10-4 M BITC. When this natural inhibitor was applied to etiolated velvetleaf seedlings at 4 × 10-4 M, 100% died in 2 days.

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.

Cogongrass is a troublesome, invasive weedy species with reported allelopathic properties. The phytotoxicity of different constituents isolated from roots and aerial parts of this species was evaluated on garden lettuce and creeping bentgrass. No significant phytotoxic activity was detected in the methylene chloride, methanol, or water extracts when tested at 1.0 mg ml−1. However, the total essential oil extract of cogongrass aerial parts was active. Bioactivity-guided fractionation of this extract using silica gel column chromatography led to the identification of megastigmatrienone, 4-(2-butenylidene)-3,5,5-trimethyl-2-cyclohexen-1-one (also called tabanone), as a mixture of four stereoisomers responsible for most of the activity. Tabanone inhibited growth of frond area of lesser duckweed, root growth of garden onion, and fresh weight gain of garden lettuce with 50% inhibition values of 0.094, 3.6, and 6.5 mM, respectively. The target site of tabanone is not known, but its mode of action results in rapid loss of membrane integrity and subsequent reduction in the rate of photosynthetic electron flow.

Myrothecium verrucaria, isolated from sicklepod, was investigated for bioherbicide potential against a wide range of economically important weed species from agronomic, pasture, and horticultural systems. A number of different weed species from a range of plant families were highly susceptible to sprays of crude preparations of the fungus. A small number of species, primarily monocots, were tolerant, showing no damage symptoms or insignificant effects on biomass. Symptoms developed very rapidly in susceptible hosts, suggesting the activity of toxins, several of which are known to be produced by Myrothecium spp. The activity of crude harvests of M. verrucaria was not diminished when they were filtered to remove fungal mycelium and spores. In contrast, washed conidia had relatively little impact on weed species. We conclude that the activity of M. verrucaria is primarily caused by the activity of metabolites produced by the fungus in culture and not due to infection by the fungus per se. Myrothecium verrucaria cultural preparations have extremely potent herbicidal properties, but given its ability to produce mammalian toxins, we caution against its use.

Invasive species and acid rain cause global environmental problems. Creeping daisy, an invasive exotic allelopathic weed, has caused great damage in southern China, where acid rain is prevalent. The impact of the acidity of simulated acid rain (SAR) on soil nutrients, the decomposition of creeping daisy litter, and on the allelopathic potential of the surrounding soils was investigated. Litter was treated with SAR at different acidity (pH 2.5, 4.0, 5.6) or with water (pH 7.0) as a control. After 70 d, the remaining amount of creeping daisy litter, nutrient contents, and allelopathic potentials in the surrounding soil were determined. The litter decomposition was commensurate to the increase in the acidity of the SAR. Total C and N contents, NO3 −-N and available P increased, levels of NH4 +-N, the ratio of C/N and soil pH values decreased, water contents increased and then decreased, whereas available K did not significantly change in the soil surrounding the litters in response to the increase in the acidity of the SAR. Bioassays showed that SAR promoted the allelopathic activity in the soil surrounding the litter, as measured by seedling growth of turnip and radish. In conclusion, our results indicated that SAR influenced soil nutrient status, accelerated creeping daisy litter decomposition, and enhanced the allelopathic potential of its litter in the surrounding soil, suggesting that acid rain may enhance the invasiveness of creeping daisy plants.

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.

Cover crop residues and other biologically based approaches often provide incomplete and inconsistent weed control. This research was conducted to evaluate interactions between hairy vetch residue on the surface of soil and the herbicide metolachlor. Herbicide was applied and incorporated with simulated rainfall before residue placement, residue was applied to the soil surface at precise rates, and potentially confounding variables such as nitrogen and soil moisture were controlled in a greenhouse experiment. Emphasis was placed on the use of suboptimal rates of both residue and metolachlor to explore the potential synergistic interactions between these factors. Deviation from a multiplicative model that included a quadratic response to hairy vetch residue and a log-logistic response to metolachlor was used to demonstrate the presence or absence of synergism. This model effectively showed that emergence of smooth pigweed, common lambsquarters, giant foxtail, and velvetleaf and early growth of smooth pigweed and common lambsquarters were reduced synergistically by the combination of hairy vetch residue and metolachlor. For example, smooth pigweed emergence was reduced 13% by 500 g m−2 of hairy vetch residue alone and was reduced 16% by 10 g ha−1 of metolachlor alone, but together, they reduced smooth pigweed emergence by 86%. This model could be used to determine synergistic interactions between any combination of a phytotoxin and a biologically based weed management approach that could be expressed in quantitative units.