Commercial formulations of 33 herbicides, 12 fungicides, and 16 adjuvants were evaluated for their toxic effects on germination of Colletotrichum gloeosporioides f. sp. malvae (C.g.m.) spores. At the recommended rates, none of the herbicides for grass weed control (diclofop, difenzoquat, fenoxaprop-ethyl, flamprop-methyl, and sethoxydim) or formulated herbicides registered for both broadleaf and grass weed control [diclofop plus bromoxynil (23:8), propanil, and propanil plus MCPA (7:2)] were compatible with C.g.m. spores. C.g.m. spore germination and appressorial formation recorded 24 h after exposure were totally inhibited by these herbicides. At recommended rates, herbicides for broadleaf weed control (2,4-D ester, 2,4-D amine, benazolin, bentazon, clopyralid, cyanazine, cyanazine plus MCPA (1:2), dicamba, dicamba plus MCPA (1:4), dicamba plus mecoprop plus MCPA (1:1:4.4), dicamba plus 2,4-D plus mecoprop (4.2:11:3), MCPA amine, MCPA-K, MCPA-Na, and metribuzin), caused no more than a 20% reduction in C.g.m. spore germination, and appressorial formation was not significantly reduced except by benazolin and metribuzin. Cyanazine and dicamba at recommended rates increased appressorial formation without reducing germination compared to the control. At lower concentrations, the other herbicides recommended for broadleaf weed control (2,4-DB, bromoxynil, bromoxynil plus MCPA (1:1), dichlorprop plus 2,4-D (1:1), imazethapyr, linuron, and picloram) were less toxic to C.g.m. spores. The fungicide triadimefon at recommended rate had no effect on C.g.m. spores. Dicloran reduced germination more than 50% at recommended rate and growth was distorted. At recommended rates, spore germination was inhibited by more than 90% with benomyl, carbathiin, chlorothalonil, iprodione, mancozeb, and thiophanate-methyl although germination increased as concentration declined. Spore germination was totally inhibited at all concentrations with ferbam, thiram, and captan. Exposure to the adjuvants—Agral 90, Alkasurf-0P-10, Atplus-555, Citowett Plus, Enhance, Renex 36, Triton XR, and X-77–inhibited C.g.m. spore germination and reduced spore production compared to the control. Spore germination was significantly higher in suspensions containing ammonium sulfate, Assist, Bio-veg, CD-407, and Tween 20, as well as with starch, sucrose, and water (control) than with the other adjuvants. Spore production was higher in suspensions containing starch and Bio-veg than in those with water and the other adjuvants.

Herbicide resistance jeopardizes the usefulness of valuable chemical tools and, therefore, weed management in many crop systems. Models must be developed to evaluate management tactics that prevent, delay, or reduce resistance. The complexity of biological processes involved in herbicide resistance also requires models to focus research and to integrate experiments. A population model was developed that improves upon previous attempts to predict herbicide resistance dynamics. The model incorporates plant population demographics with the Hardy-Weinberg concept for gene segregation. The model simulates the evolution, spread, and subsequent dynamics of resistance in the presence and absence of a herbicide. Analysis of model simulations identified two sets of biological processes as key factors in the evolution and dynamics of herbicide-resistant weed populations. These are processes that influence ecological fitness and gene flow. Several options are suggested as examples for the management of resistant weed populations.

Immunochemical techniques offer many advantages over chromatographic methods used for pesticide trace analysis of substrates such as soil, water, plants, urine, and blood. These advantages include speed of processing samples, high specificity for detecting a pesticide, reduced amount of preparation and cleanup of the sample before analysis, and a dramatic increase in the number of samples that can be analyzed. Immunoassays are based on the principle that antibodies to pesticides can be prepared, in animals, that can recognize and attach with exquisite specificity to certain chemical configurations displayed on the surface of a molecule. Small molecules such as herbicides usually are not immunogenic but can be made so by chemically bonding them to a large immunogenic protein such as bovine serum albumin before injection into an animal. Development of herbicide-specific antibodies and their use in direct and indirect enzyme-linked immunosorbent assays (EIA or ELISA) as well as radioimmunoassays (RIA) are discussed. The principles behind monoclonal antibody production are outlined, and immunoassays using polyclonal and monoclonal antibodies are compared. Specific reference is made to the development and use of indirect ELISA and RIA procedures for trace analysis of 2,4-D and picloram.

The determination of host range is an important component in developing a plant pathogen for use as a bioherbicide. The safety of non-target economic and wild plants must be assured before experimental release and commercial use. In contrast to other methods of weed control, the genetic variability and genetic stability of both the weed and the biological control agent must be considered. Schemes to determine host range generally assume a close phylogenetic relationship between the weed host and its co-evolved pathogens. Therefore, testing generally is based on inoculation of genetically related plant species and progresses to more distantly related species until the host range is circumscribed. Several potential weaknesses in these schemes will be illustrated with examples using specific biological control agents. Future tests must place greater emphasis on testing taxa representing the full range of genetic diversity within the biogeographic area of intended use.

In full-season soybean experiments, adding glyphosate, paraquat, or HOE-0661 to chlorimuron, chlorimuron plus linuron, chlorimuron plus metribuzin, or imazaquin was required for effective weed control, especially eastern black nightshade. In double-crop soybean experiments, chlorimuron plus linuron provided similar weed control and yields with or without either glyphosate, paraquat, or HOE-0661. In both full-season and double-crop soybeans, metolachlor combined with chlorimuron mixtures did not enhance weed control or increase yield. These studies indicate a potential for reducing the use of nonselective herbicides or metolachlor when chlorimuron or imazaquin are components of tank mixes, particularly in double-crop no-till soybeans.

Colletotrichum gloeosporioides f. sp. malvae (C.g.m.) was evaluated under greenhouse conditions for round-leaved mallow control in combination with the following herbicides: bromoxynil plus MCPA (1:1), diclofop, imazethapyr, metribuzin, sethoxydim, and 2,4-DB. Treatments consisted of C.g.m. and herbicide applied alone, tank mixed, and C.g.m. applied 48 h before, and 1, 24, and 72 h after herbicide. Split applications with bromoxynil plus MCPA, imazethapyr, metribuzin, (C.g.m. before or after) and sethoxydim (C.g.m. before) enhanced round-leaved mallow control over C.g.m. applied alone. However, diclofop or 2,4-DB tank mixed or applied in a split application with C.g.m. did not improve round-leaved mallow control over C.g.m. applied alone. Bromoxynil plus MCPA, metribuzin, 2,4-DB, and imazethapyr may be tank mixed with C.g.m. without significantly reducing round-leaved mallow control, unlike sethoxydim or diclofop which inhibited C.g.m. development. Dew period did not influence the effectiveness of bromoxynil plus MCPA applied 30 h before C.g.m., but round-leaved mallow control increased with increasing dew period for metribuzin applied 30 h before C.g.m.

Picloram was applied to leafy spurge using flexible and rigid ropewick applicators for 3 yr at each of two upland pasture sites. The ropewick applicators were a cost-effective way to apply picloram to control leafy spurge compared to broadcast application, but minimal advantage was demonstrated for the segmented, flexible unit compared to the rigid applicator. Picloram applied as an 80 g ae/L solution through ropewick applicators controlled leafy spurge equal to broadcast picloram at 1.1 kg/ha while using 36% as much picloram.

Few plant pathogens are both lethal and specific enough to be effective weed control agents. In short, highly specific organisms seldom kill. Two genetic approaches to overcome this problem are to delimit the host range of lethal pathogens or to enhance the efficacy of host-specific ones. Narrowing the virulence or survival of a deadly pathogen seems more plausible than imparting new characters to a nonlethal organism. Our approach has been to genetically restrict the host range or to decrease the survival and/or spread of Sclerotinia sclerotiorum (Lib.) de Bary, a highly virulent and aggressive pathogen of several weeds. Working with this fungus, three classes of induced mutants which meet criteria for delimitation were obtained: auxotrophic mutants that only attack plants when applied concomitantly with an exogenous source of the required nutrient; mutants unable to form sclerotia, structures required for long-term survival and precursors to fruiting bodies; and mutants with reduced virulence and/or host ranges. These studies demonstrate the validity of genetically improving bioherbicides and greatly expanding the number of fungi that may be useful as bioherbicides.

Field experiments were conducted to determine factors that predispose winter wheat to injury by sulfonylurea herbicides. Wheat was injured occasionally when herbicides were applied postemergence in November or when tank mixed with malathion. CGA 131036 at 28 or 56 g ha-1 was less injurious than chlorsulfuron or DPX-G8311 at 26 or 53 g ha-1, and preemergence treatments of chlorsulfuron and DPX-G8311 were less injurious than preplant incorporated or postemergence treatments. Wheat growth stage, minimum post-treatment daily temperature, and summed diurnal temperature fluctuations after treatment and after first post-treatment rainfall were correlated with wheat injury. Grazing and cultivar selection did not affect injury.

Experiments were conducted in 1987 and 1988 at two locations each year to determine how two weed management strategies and three planting dates affected the cost and amount of herbicide needed to control weeds in peanuts. Weed management strategies used for each planting date included preventive, which duplicated a standard grower program, or an integrated weed management system. The least expensive strategy depended upon weed species composition, weed populations and planting date. In all cases, the integrated weed management strategy required less herbicide (acid equivalent or active ingredient) than the preventive strategy.

Field studies were conducted from 1985 to 1987 to evaluate postemergence herbicide systems with preemergence systems to control Texas panicum, Florida beggarweed, sicklepod, and pitted morningglory in peanuts. Adding paraquat at 0.14 kg ai/ha to postemergence herbicide systems reduced fresh weight of Florida beggarweed 92% (18% increase over the same systems without paraquat), sicklepod 95% (21% increase), and pitted morningglory 95% (11% increase). Herbicide systems containing paraquat improved peanut yields by 230 kg/ha and net returns by $52/ha over herbicide systems not containing paraquat. Fluazifop-P and sethoxydim systems reduced Texas panicum fresh weight (at least 96%) more than a preemergence system (92% reduction) that used benefin applied preplant incorporated and alachlor plus naptalam and dinoseb applied at cracking (GC) or a postemergence system that used alachlor and naptalam plus dinoseb GC and paraquat applied early postemergence (86% reduction). Systems containing fluazifop-P provided greater yields (4190 kg/ha) and net returns ($383/ha) than systems containing sethoxydim (4010 kg/ha, $305/ha) when averaged across all rates of application.

Field studies were conducted over a four-year period to determine the weed control efficacy of ICIA-0051 and SC-0774 in no-tillage and conventional corn. Treatments consisted of both pre- and postemergence applications of ICIA-0051, alone and in combination with atrazine, and preemergence applications in combination with metolachlor. SC-0774 was applied preemergence, alone and in combination with atrazine. Results indicated good control of triazine-resistant (TR) smooth pigweed and other annual broadleaf weeds present with no adverse effects on crop vigor and yield with ICIA-0051. Generally, the addition of atrazine to ICIA-0051 improved weed control. SC-0774 provided good control of fall panicum, but generally poorer and less consistent control of giant ragweed, common ragweed, common lambsquarters, and ivyleaf and tall morningglory species.

Field studies were conducted to determine the effect of several herbicides on soybean thrips numbers in soybean. Adult soybean thrips numbers in the preplant-incorporated and preemergent herbicide treatments were observed to be either greater than or equal to, but never less than, numbers in the control plots. Larval soybean thrips numbers were not observed to be affected by the preplant-incorporated or preemergent treatments. The postemergence herbicide treatment of naptalam plus dinoseb consistently reduced both adult and larval soybean thrips numbers. The other postemergent treatments did not affect numbers consistently.

Barnyardgrass, large crabgrass, and Texas panicum were evaluated in field experiments over 3 yr to measure their duration of interference and density on grain sorghum yield. When grain yield data were converted to a percentage of the weed-free control, linear regression predicted a 3.6% yield loss for each week of weed interference regardless of year or grass species. Grain sorghum grown in a narrow (61-cm) row spacing was affected little by full-season interference; however, in wide (91-cm) rows, interference increased as grass density increased. Data from the wide-row spacing were described by linear regression following conversion of grain yield to percentages and weed density to log10. A separate nonlinear model also was derived which could predict the effect of weed density on grain sorghum yield.

Greenhouse and field experiments were conducted in east-central Texas to evaluate the influence of simulated rainfall on the availability of foliar-applied herbicides for huisache and honey mesquite control. The foliar activity of the amine salts of glyphosate, dicamba, picloram, clopyralid, and triclopyr was decreased on greenhouse-grown huisache when simulated rainfall was applied up to 240 min after herbicide treatment. The effectiveness of the butoxyethyl ester of triclopyr or 2,4,5-T was not reduced by rainfall washoff within 15 min after application. In natural huisache stands, injury from the ester of triclopyr or amine salts of picloram or clopyralid was not reduced by simulated rainfall at 60 min after herbicide treatment. In the greenhouse and field, honey mesquite leaves rapidly absorbed most herbicides, and triclopyr, 2,4,5-T, picloram, and clopyralid were highly phytotoxic even when simulated rainfall was applied within 15 min after herbicide treatment.