Seed of three weed species collected from the grain bins of combines while standing hard red winter wheat was harvested germinated better than hand-harvested seed. Combine-harvested curly dock seed germinated from 4 to 24% more than hand-harvested seed. Curly dock seed harvested with a commercial-type combine germinated better than those harvested with a small-plot combine. Harvesting slimleaf lambsquarters and Venice mallow seed with a commercial-type combine also enhanced germination compared to hand-harvested seed.

Field studies were conducted in 1982 to 1984 to determine the effects of common lambsquarters on growth, yield, and nutrient concentration of transplanted tomato. Common lambsquarters densities ranged from 16 to 64 plants/m tomato row and fresh weight ranged from 26 360 kg/ha at 16 plants/m to 46 000 kg/ha at 64 plants/m row. Common lambsquarters did not affect tomato shoot dry weight at the vegetative stage but decreased the weight at the early fruit stage. Season-long interference of common lambsquarters reduced marketable tomato fruit number and also, marketable fruit weight ranging from 17% at 16 plants/m to 36% to 64 plants/m row. Concentrations of N in tomato leaves were unaltered at vegetative and flowering stages but decreased regardless of common lambsquarters density at early fruit and harvest stages. Weed density did not alter concentrations of P, K, and Ca in tomato leaves.

Allelochemicals representing numerous chemical groups have been isolated from over 30 families of terrestrial and aquatic plants. Some of the compounds also have been isolated from soil in quantities sufficient to reduce plant growth. Although selected allelochemicals are believed to influence plant densities and distributions, none isolated from higher plants have been considered active enough for development as commercial herbicidal products. Almost all herbicidal allelochemicals exist in plants in nontoxic, conjugated forms. The toxic moiety may be released upon exposure to stress or upon death of the tissue. The most successful use of allelochemicals in weed control has been management of selectively toxic plant residues. For example, rye residues have controlled weeds effectively in a variety of cropping systems. Several weed species may interfere with crop growth through chemicals released from their residues. A number of noxious perennial species appear to exploit allelochemicals in their interference processes. This review focuses on the more recent chemical discoveries and how they might be exploited for weed control.

A rationale for the study of phytotoxins from fungal pathogens of plants is presented. Structural chemistries and biological data are given for numerous, recently discovered phytotoxins in such diverse chemical classes as polyketides, terpenoids, diketopiperazines, and isocoumarins. The biological activities of these compounds range from broadly toxic (curvulin) to host specific (maculosin-1). Phytotoxicology offers a viable supplement to organic synthesis as a means of developing and implementing new, biorational, and economical herbicides.

Biologically active natural products of microbial origin are attractive candidates for possible use in agriculture. They may be obtained by fermentation, used in their natural state, or subjected to synthetic modification for specific uses. These natural products are characterized by high specific activity and high selectivity, and they are biodegradable. The structures are extremely diverse and represent many classes of compounds ranging from very complex to simple. Cyclocarbamide A and B, from Streptoverticillium sp., have marked preemergence herbicidal activity. Nigerazine A and B, from Aspergillus niger van Tieghem, also inhibit root growth in certain plants. Citreoviridin, from Penicillium charlesii Smith, preferentially controls the growth of monocotyledonous plants, as does a synthetic derivative of cladosporin, from Aspergillus repens DeBary, which bleaches chloroplasts. The 12-membered fungal macrolides (macrocyclic lactones) also inhibit root growth in many test plants and offer templates for further synthetic work. Herbicidins, from Streptomyces saganonensis, are particularly effective against barnyardgrass, goosegrass, tufted mannagrass, and green panicum. Alternaria eichorniae Nag Raj et Ponnappa produces a toxin that is active against waterhyacinth and represents one of the more exotic structures. The macrocyclic trichothecenes are a significant class of natural products that tend to concentrate against a gradient in seeds of certain plants, which resist these microbially derived metabolites thereby producing seed with “built-in” natural herbicides.

Mitotic disrupters from higher plants have been studied for their effects on animal cells, but their effects on plant cells have been studied less. The plant-derived compounds have counterparts among herbicides with respect to cellular effects and, in some cases, mode of action. Cellular effects in common are arrested prometaphases (colchicine, dinitroanilines), multipolar divisions (Vinca alkaloids, carbamates), and production of binucleate cells (caffeine, dichlobenil). The potential of these natural compounds, either directly or as the basis of new chemistries for herbicides, has remained largely untapped.

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.

Many natural and synthetic plant compounds become toxic when irradiated by ultraviolet or visible light. These compounds may be considered for use as herbicides, remembering the requirement for light. Concurrently, methods for blocking metabolic pathways so that phototoxic intermediates accumulate and thus kill the plant also have been studied. The mechanisms of action of potentially useful plant phototoxins are described.

Common lambsquarters or fat hen (Chenopodium album L. # CHEAL) was classified by Linneaus in 1753. The generic name is from the Greek chen, a goose, and pous or podos, a foot; the leaf shape of plants in this genus are reminiscent of goose feet. The goosefoot family, Chenopodiaceae, includes many vegetables: table beets, sugar beets, spinach, and mangold. The name ‘fat hen’, used for several plants of the Goosefoot family, was first published in English in 1795.