Natural products represent a vast repository of materials and compounds with evolved biological activity, including phytotoxicity. Some of these compounds can be used directly or as templates for herbicides. The molecular target sites of these compounds are often unique. Strategies for the discovery of these materials and compounds are outlined. Numerous examples of individual phytotoxins and crude preparations with weed management potential are provided. An example of research to find a natural product solution of a unique pest management problem (blue-green algae in aquaculture) is described. Finally, the problems associated with natural products for pest control are discussed.

Laboratory and field experiments were conducted to determine the effects of several environmental factors on seed germination and seedling emergence of the invasive weed, apple of Peru. The anatomy of the seed was also investigated in this study. The anatomy of the seed is similar to that of an eggplant (Solanum melongena L.) seed. The seed is albuminous, and consists of a seed coat, endosperm, and an embryo. The seed coat, which consists of both outer and inner integuments, is relatively thick. Fresh and dry stored seeds exhibited strong dormancy. Hot water treatment did not affect the seed germination in the light, but significantly (P < 0.05) reduced the germination rate in the dark. Dry-heat treatment had no significant effect on seed germination in the dark, but it significantly (P < 0.05) increased the germination rate in the light. Cold stratification alone did not influence seed germination under constant temperature (25 C) but affected it under alternating temperature (25–15 C). However, a combination of warm and cold stratification produced significant (P < 0.05) germination in the dark as well as in the light. In addition, the effect of warm stratification followed by cold stratification was more pronounced in the dark than in the light. Gibberellin A₃ (GA₃) (10^–3 and 10^–4 M) treatments significantly (P < 0.05) increased the seed germination rate compared with that of the control. Emergence rate was maximum for seeds placed on the soil surfaces; no seedling emergence occurred when seeds were placed at a depth of 5 cm.

Nomenclature: Apple of Peru, Nicandra physalodes (L.) Pers.

Greenhouse studies were conducted to examine the influence of temperature and relative humidity (RH) on the foliar activity of mesotrione on five weed species. Regression analysis was performed allowing for comparison of estimated GR₅₀ (herbicide dose to inhibit growth by 50%) values for each weed response at either temperature (18 or 32 C) or RH level (30 or 85%). Temperature and relative humidity did not influence the response of ivyleaf morningglory, common cocklebur, and velvetleaf to mesotrione markedly. An increase in temperature or relative humidity increased the efficacy of mesotrione on common cocklebur and velvetleaf up to threefold. Conversely, common waterhemp and large crabgrass were six- and sevenfold more susceptible at 18 C than at 32 C, respectively. Common waterhemp and large crabgrass were four- and twofold more susceptible to mesotrione at 85% compared with 30% RH, respectively. The influence of temperature and RH on the efficacy of mesotrione in foliar applications is species dependent and may be an important consideration for field applications.

Nomenclature: Mesotrione, 2-[4-(methylsulfonyl)-2-nitrobenzoyl]-1,3-cyclohexanedione; common cocklebur, Xanthium strumarium L. XANST; common waterhemp, Amaranthus rudis Sauer AMATA; large crabgrass, Digitaria sanguinalis L. Scop. DIGSA; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. IPOHE; velvetleaf, Abutilon theophrasti (L.) Medic. ABUTH.

Soybean and annual weed competition studies were conducted over 2 yr in Balcarce, Argentina, to define the critical period of weed control and to compare yield losses in soybean, when weeds competed in the total cropping area (row plus furrow) or only in the furrow. In the first year, broad-leaf weeds were more abundant than annual grasses, but in the second year, grasses were more abundant. Critical weed-free periods required to prevent yield losses of 2.5 and 10% were long and consistent for both years and for both types of competition, lasting between 50 and 61 d after crop emergence (DAE) or the V8 to V9 and R2 to R3 soybean growth stages, respectively. No differences in predicted yield for both places of competition was found when weeds grew after the V4 or later crop growth stages (35 DAE). The critical periods of weed removal to prevent equivalent yield losses were different in both years. Varying between 1 DAE in 1986–1987 and 30 DAE in 1988–1989, for 2.5% yield losses, corresponding to growth stages between V0 and V3 to V4, respectively. For yield losses lower than 10%, the critical periods of weed removal varied between 15 DAE in 1986–1987 and 35 DAE in 1988–1989, corresponding to the V2 and V4 crop growth stages, respectively. Nonsignificant differences in predicted yield were found between places of competition, if weeds were removed before V4 (35 DAE), maintaining yield losses lower than 20%.

Nomenclature: Soybean, Glycine max (L.) Merr. cv. ‘A3127’.

Field studies were conducted in northern Greece, in 1997 and 1998, to investigate the effect of nitrogen fertilization and red rice density on interference between red rice and two rice cultivars (Thaibonnet, Ariette). Interference between rice and red rice began 3 wk after rice emergence, but was not affected by the increasing nitrogen rate from 100 to 150 kg N ha^–1. Dry weight of both rice cultivars was proportionally reduced with increasing red rice interference duration and density, but dry weight of Thaibonnet was reduced more than that of Ariette. At harvest, grain yield of Thaibonnet was reduced by 58% because of the occurrence of 40 red rice plants m^–2, whereas that of Ariette was reduced by 46%. Red rice interference affected panicle number more than 1,000 grain weight in both rice cultivars. The reduction of all yield components was greater in Thaibonnet than in Ariette. Dry weight and stem or panicle number of red rice plants grown with either of the two rice cultivars increased with increasing red rice density and were greater most of the time when grown with Thaibonnet than with Ariette. Ten weeks after rice emergence, red rice plants were 14 and 35 cm taller than the Ariette and Thaibonnet plants, respectively. Shattering of red rice plants ranged from 63 to 79% and was greater grown with Thaibonnet than with Ariette, but it was not affected by nitrogen fertilization and red rice density.

Nomenclature: Red rice, Oryza sativa L. ORYSA; rice, Oryza sativa L. ‘Ariette’, ‘Thaibonnet’.

Integration of two or more methods in a weed control strategy may produce a positive interaction. In this study, sequential applications of sublethal rates of 2,4-D and the plant pathogen Sclerotinia minor were assessed for integrated control of common dandelion. S. minor was prepared as a granular treatment of fungal-colonized barley grits. Treatments of 2,4-D (25 or 50% of the recommended field rate) and S. minor treatments (20, 40, or 60 g m^–2 rate) were applied alone or sequentially with a 3 wk interval. Fourteen days after inoculation (DAI), sequential applications of either rate of 2,4-D with 40 or 60 g m^–2 of S. minor caused greater damage than either treatment alone (P = 0.05). By 21 and 28 DAI, control from 60 g m^–2 of S. minor alone was equivalent to any of the sequential treatments (P = 0.05). At all assessment times, the combination of either rate of 2,4-D and 20 or 40 g m^–2 of S. minor caused damage equivalent to or greater than that caused by 60 g m^–2 of S. minor alone (P = 0.05). According to Colby's test for interactions, 19 of 24 assessments of the sequential treatments were synergistic. Therefore, sequential treatments of sublethal rates of 2,4-D and S. minor can interact positively to increase damage. This synergistic interaction may reduce the rate of either component required for adequate levels of control, possibly decreasing the cost or volume of use of herbicides in traditional weed control strategies.

Nomenclature: 2,4-D; barley, Hordeum vulgare; common dandelion, Taraxacum officinale Weber TAROF.

The exotic plant, swallow-wort, a twining perennial of the Milkweed family, has become increasingly invasive in natural areas, successional old fields, and tree plantations on calcareous soils of the lower Great Lakes basin. Because mechanical control strategies are inadequate, swallow-wort's response in natural areas to foliar spray and cut-stem applications of glyphosate and triclopyr was evaluated. Foliar spray applications of glyphosate (3.1 and 7.8 kg ae ha^–1) and triclopyr (1.9 kg ae ha^–1) were more effective than cut-stem applications of the herbicides (3.1 kg ae ha^–1 glyphosate, 1.4 kg ae ha^–1 triclopyr). There were no statistical differences in effect among the foliar spray applications. Response to cut-stem application of glyphosate (3.1 and 6.2 kg ha^–1) and triclopyr (1.4, 2.8, and 5.6 kg ha^–1) in the following year indicated glyphosate to be more effective than triclopyr at all concentrations tested. To effect long-lasting control, all treatments require additional herbicide application.

Nomenclature: Glyphosate; triclopyr; swallow-wort, Vincetoxicum rossicum (Kleo.) Barb., syn. Cynanchum rossicum Kleo.

Today, the aim of weed management is to keep the weed community at an acceptable level rather than to keep the crop totally free of weeds. Satisfactory control of weeds may often be obtained when herbicides are used at lower doses than normally recommended. To facilitate the decision of what is an adequate dose in a specific field, the farmer needs support. In 1988 and 1989, a total of 10 field trials in spring cereals were initiated in Sweden with the objective of studying long-term effects of herbicide application according to recommendations from guidelines: the guidelines were developed at the Swedish University of Agricultural Sciences and consisted of printed cards designed for in-field use. Treatments also included a full and a half dose and an untreated control. As an average over the experimental time, i.e., until 1997, the dose used in the guideline treatment varied at different sites between 20 and 70% of a full dose. In 1998, i.e., 1 yr after the last herbicide application, the plant densities of annual weeds in the guideline treatment, the half and the full doses were 51, 57, and 67% lower, respectively, than in the untreated control when averaged over sites. At two and four sites, the half and full doses resulted in significantly lower weed densities than where guidelines had been used. Compared with the control, the full and half doses increased the proportion of difficult-to-control weed species significantly at five and four sites by 21 and 24%, respectively. In the guideline treatment the proportion of difficult-to-control weeds was increased at one site. In 1998, weed counts were higher where guidelines had been used than in the full dose for common lambsquarters and common chickweed at three sites each and for wallflower mustard, catchweed bedstraw, field violet, Galeopsis spp., and Lamium spp. at one site each. At three sites, no significant treatment effects on crop yields were found, whereas yields at the remaining seven sites were higher where guidelines had been used than in other treatments in several years. It is concluded that application of dose rates according to recommendations from guidelines can be a fruitful way to reduce herbicide use.

Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; common chickweed, Stellaria media (L.) Vill. STEME; wallflower mustard, Erysimum cheiranthoides L. ERYCH; catchweed bedstraw, Galium aparine L. GALAP; field violet, Viola arvensis Murr. VIOAR.

Different herbicide-application strategies may serve as part of an integrated weed management (IWM) system. In 1987 and 1988, ten field trials were initiated in the south-central part of Sweden, with the objective of studying the long-term effects of herbicide-application strategies on the development of weed populations and productivity of spring-sown cereals. Each year until 1997, herbicides were applied at 25, 50, 75, or 100% of the full recommended dose. Treatments included herbicide exclusion every second year and herbicide application at 25 or 50% of a full dose during two out of three years, with a full dose applied in the third year. An untreated control was included. In 1998, weed densities in 25, 50, 75, and 100% of a full dose were reduced 43, 58, 64, and 67% compared with the control. At seven sites, the exclusion of herbicides every second year resulted in 43 to 178% higher weed densities than in 50% of a full dose, although the same amount of herbicide was applied when summarized over 2 yr. At four sites, herbicide application each year at 50, 75, or 100% of a full dose increased the density of difficult-to-control species by 24% compared with the untreated control. Averaged over sites, the untreated control contained 30% more weed species than herbicide application at 75 or 100% of a full dose. At three sites in 1996, the full dose resulted in 520 kg ha^–1 (pooled over sites) higher barley yields than in the control. Neither in 1996 nor in 1997, was there any difference in crop yield between herbicide application at 25 and 100% of a full dose. This study demonstrates the potential of reducing the input of agrochemicals for weed control by using herbicides at reduced rates in competitive crops.

Nomenclature: Spring barley, Hordeum distichon L.

In citrus, weedy grasses compete for moisture, nutrients, and light and can inhibit the growth of young trees and delay fruit production. These weeds are difficult to control, either because of their tolerance to available herbicides or due to growth habits that enable them to resist other control practices. Control of seven such weedy grasses (southern sandbur, large crabgrass, crowfootgrass, guineagrass, Texas panicum, johnsongrass, and yellow foxtail) with a mixture of three fungal pathogens, termed the multiple-pathogen strategy, was field tested in 1996 and 1998. Three fungi indigenous to Florida, Drechslera gigantea, Exserohilum longirostratum, and E. rostratum, isolated from large crabgrass, crowfootgrass, and johnsongrass respectively, were used. Two separate field studies were conducted: one study with seven grasses transplanted and grown within each plot (grass mixture field trial) and another study on a population of guineagrass alone present in a naturally infested field (guineagrass field trial). The objectives of this study were to (1) evaluate the field performance of D. gigantea, E. longirostratum, and E. rostratum individually and in a mixture to control the seven transplanted weedy grasses (grass mixture) and a population of guineagrass in a naturally infested field, respectively, and (2) compare the effectiveness of three carriers (water, Metamucil^®, and an invert emulsion) on the bioherbicidal efficacy under field conditions. The fungi were applied as foliar sprays, each pathogen alone or in a mixture of the three fungi (1:1:1, v/v/v, for a total of 5 × 10⁵ spores ml ^–1) in water, 0.5% aqueous Metamucil^®, or an emulsion containing Sunspray^® 6E. During the 14-wk experimental period, one or two additional sprays of all treatments were applied. Disease severity was recorded weekly for 4 to 6 wk after the initial spray (WAI). Maximum disease severities were obtained in emulsion-inoculum treatments, and were higher than those in the water-inoculum and the Metamucil-inoculum treatments. The pathogen mixture was equally effective as the individual pathogens in controlling the weeds tested. In the 1996 trial, 6 WAI, disease severity on grasses inoculated with D. gigantea spore suspensions in emulsion ranged from 78 to 100%, with E. longirostratum 90 to 100%, E. rostratum 79 to 100%, and the mixture 74 to 100%. In the 1998 trial, 4 WAI, disease severity on grasses inoculated with D. gigantea spore suspensions in emulsion ranged from 45 to 98%, with E. longirostratum 45 to 98%, E. rostratum 34 to 98%, and the mixture 32 to 98.5%. Thus, it was possible to manage all seven weedy grasses under field conditions using an emulsion-based inoculum preparation with the individual pathogens as well as the mixture of pathogens. The same three fungal pathogens were field tested for their ability to manage populations of guineagrass in a naturally infested field. The experimental design and treatments were identical to the field testing with the seven transplanted grasses. Two applications of an emulsion-based inoculum preparation of each pathogen or the mixture of pathogens effectively controlled guineagrass for up to 10 wk, with no regrowth.

Nomenclature: Bromacil; diuron; fluazifop-P; glyphosate; norflurazon; oryzalin; oxyfluorfen; paraquat; sethoxydim; simazine; thiazopyr; Drechslera gigantea (Heald & Wolf) Ito; Exserohilum longirostratum