Mark A. Weaver

Agronomic and environmental implications of enhanced s-triazine degradation.

Novel catabolic pathways enabling rapid detoxification of s-triazine herbicides have been elucidated and detected at a growing number of locations. The genes responsible for s-triazine mineralization, i.e. atzABCDEF and trzNDF, occur in at least four bacterial phyla and are implicated in the development of enhanced degradation in agricultural soils from all continents except Antarctica. Enhanced degradation occurs in at least nine crops and six crop rotation systems that rely on s-triazine herbicides for weed control, and, with the exception of acidic soil conditions and s-triazine application frequency, adaptation of the microbial population is independent of soil physiochemical properties and cultural management practices. From an agronomic perspective, residual weed control could be reduced tenfold in s-triazine-adapted relative to non-adapted soils. From an environmental standpoint, the off-site loss of total s-triazine residues could be overestimated 13-fold in adapted soils if altered persistence estimates and metabolic pathways are not reflected in fate and transport models. Empirical models requiring soil pH and s-triazine use history as input parameters predict atrazine persistence more accurately than historical estimates, thereby allowing practitioners to adjust weed control strategies and model input values when warranted.

Selection of Aspergillus flavus isolates for biological control of aflatoxins in corn

The fungus Aspergillus flavus is responsible for producing carcinogenic mycotoxins, the aflatoxins, on corn (maize) and other crops. An additional harmful toxin, cyclopiazonic acid, is produced by some isolates of A. flavus. Several A. flavus strains that do not produce one or both of these mycotoxins are being used in biological control to competitively exclude the toxin-producing strains from the agroecosystem, particularly from seeds, grain and other marketable commodities. Three well-studied non-aflatoxigenic strains, including two that are commercially available, have been compared in side-by-side field trials. The results of that study, together with a growing understanding of A. flavus ecology and new genetic insights, are guiding the selection of biocontrol strains and influencing crop management decisions for safe and sustainable production.

Identification of genetic defects in the atoxigenic biocontrol strain Aspergillus flavus K49 reveals the presence of a competitive recombinant group in field populations

Contamination of corn, cotton, peanuts and tree nuts by aflatoxins is a severe economic burden for growers. A current biocontrol strategy is to use non-aflatoxigenic Aspergillus flavus strains to competitively exclude field toxigenic Aspergillus species. A. flavus K49 does not produce aflatoxins and cyclopiazonic acid (CPA) and is currently being tested in corn-growing fields in Mississippi. We found that its lack of production of aflatoxins and CPA resulted from single nucleotide mutations in the polyketide synthase gene and hybrid polyketide-nonribosomal peptide synthase gene, respectively. Furthermore, based on single nucleotide polymorphisms of the aflatoxin biosynthesis omtA gene and the CPA biosynthesis dmaT gene, we conclude that K49, AF36 and previously characterized TX9-8 form a biocontrol group. These isolates appear to be derived from recombinants of typical large and small sclerotial morphotype strains. This finding provides an easy way to select future biocontrol strains from the reservoir of non-aflatoxigenic populations in agricultural fields.

Constructed wetlands as a component of the agricultural landscape: Mitigation of herbicides in simulated runoff from upland drainage areas

Constructed wetlands are a recommended practice for buffering pollutant source areas and receiving waters. A wetland consisting of a sediment trap and two treatment cells was constructed in a Mississippi Delta lake watershed. A 3-h simulated runoff event was initiated (2003) to evaluate fate and transport of atrazine and fluometuron through the wetland. Water samples were collected during a runoff simulation and then afterward at selected intervals for 21d, and analyzed for the herbicides. Breakthrough patterns for herbicide concentrations in water samples during the first 20h after simulated runoff showed peak concentrations in the first 6h, with gradual tailing as the herbicide pulse was diluted in the second, excavated (deeper) cell. Atrazine and fluometuron concentrations in the first (shallower, non-excavated) cell averaged 12- and 20-fold greater, respectively, than those in the second cell following simulated runoff, indicating entrapment in the first cell. Atrazine and fluometuron concentrations in the shallower cell decreased 32% and 22%, respectively, 9d following simulated runoff, indicating either degradation or sorption to soil or wetland flora. In the excavated cell, concentrations were even lower, and atrazine declined more rapidly than fluometuron. Results indicate constructed wetlands can improve downstream water quality though sequestration or processing of pollutants.

Implications of Bt Traits on Mycotoxin Contamination in Maize: Overview and Recent Experimental Results in Southern United States

Mycotoxin contamination levels in maize kernels are controlled by a complex set of factors including insect pressure, fungal inoculum potential, and environmental conditions that are difficult to predict. Methods are becoming available to control mycotoxin-producing fungi in preharvest crops, including Bt expression, biocontrol, and host plant resistance. Initial reports in the United States and other countries have associated Bt expression with reduced fumonisin, deoxynivalenol, and zearalenone contamination and, to a lesser extent, reduced aflatoxin contamination in harvested maize kernels. However, subsequent field results have been inconsistent, confirming that fumonisin contamination can be reduced by Bt expression, but the effect on aflatoxin is, at present, inconclusive. New maize hybrids have been introduced with increased spectra of insect control and higher levels of Bt expression that may provide important tools for mycotoxin reduction and increased yield due to reduced insect feeding, particularly if used together with biocontrol and host plant resistance.

7 Bioherbicides for Weed Control

Management of weeds is a necessary but expensive challenge. Chemical weed control accounts for over

Compatibility of a Biological Control Agent With Herbicides for Control of Invasive Plant Species

14 billion spent annually (Kiely et al., 2004), excluding immense indirect costs to producers, consumers and the environment, and resulting also in the development of resistant weed biotypes. While chemical herbicides effectively control unwanted vegetation, many herbicides are no longer available due to lack of re-registration, competition from other products, and development of numerous genetically modified crops with resistance to broad-spectrum herbicides, namely glyphosate and gluphosinate. The implementation of conservation tillage practices to promote soil quality, to minimize erosion, or to simplify crop management has increased reliance on ‘burn-down’ herbicides and placed additional selection pressure on weeds to develop resistance. After years of applying herbicides, often in the presence of high weed pressure, 180 species of herbicide-resistant weeds have been identified (WeedScience, 2006). The majority of herbicide usage is for agronomic areas or turf, but few herbicides are registered for, or are being developed for, smaller markets or niche weed problems, such as invasive weeds in noncropland areas. Furthermore, chemical weed control is not an option in organic cropping systems and near to sensitive natural habitats. The high costs involved in developing and registering chemical herbicides, and recent trends in environmental awareness concerning pesticides in general, have prompted researchers to develop additional weed control tools, such as biological weed control using plant pathogens. A review of pathogen-based weed control prospects by Charles Wilson (1969) noted that ‘the idea of using plant pathogens to control weeds is almost as old as the science of plant pathology itself ’, but that the ‘seeds of the idea ... have lain dormant since their sowing’. Since that review, almost 40 years ago, numerous pathogens for weed control have been identified and a few have enjoyed limited commercial success (Hoagland, 1990, 2001). Classical pathogen-mediated biocontrol of weeds generally employs an exotic pest to manage a weed population. This is an effective weed management strategy in many systems (Bedi et al., 2002; also see Blossey, Chapter 6, this volume). An alternative method is to overwhelm the target weed with direct pathogen application, or multiple applications of a pathogen. Because this tactic uses biological agents in an application similar to chemical herbicidal applications, it is often called the ‘bioherbicidal’ approach. When the plant pathogens are fungi, these bioherbicides are often called ‘mycoherbicides’.

Aflatoxin and Fumonisin in Corn (Zea mays) Infected by Common Smut Ustilago maydis

Abstract Kudzu, Pueraria montana var. lobata, is an exotic invasive weed that is difficult to control with available products and management practices. The fungal pathogen, Myrothecium verrucaria, is being developed as a bioherbicide for kudzu and other invasive vines. This biological control agent might be applied with conventional herbicides to improve the efficacy or spectrum of weed control. The survival of M. verrucaria was measured over time in simulated tank-mixes of commercial formulations of the herbicides: amniopyralid (Milestone*), metsulfuron (Escort XP), and fluroxypyr (Vista). The fungus was also grown in vitro in the presence of these herbicides to evaluate any growth inhibition. M. verrucaria was highly tolerant to all concentrations of amniopyralid and metsulfuron for up to two days in simulated tank-mixes, while mixtures with fluroxypyr resulted in a gradual loss of spore viability. The fungus grew on media supplemented with amniopyralid and metsulfuron with only small effects on the growth rate, but fluroxypyr caused growth inhibition. These studies provide insight for developing effective, integrated control strategies for kudzu.

Macrocyclic trichothecene production and sporulation by a biological control strain of Myrothecium verrucaria is regulated by cultural conditions

Corn infected with Ustilago maydis (common smut) produces galls that are valued as a delicacy in some cultures. During a 4-year period, aflatoxin levels in asymptomatic kernels of smutted ears were, on average, 45-fold higher than in kernels harvested from smut-free control ears and 99-fold higher than in smut galls. Aflatoxin levels in smut galls were lower than in kernels of smut-free control corn in all years combined. Fumonisin levels in asymptomatic kernels harvested from smutted ears were 5.2-fold higher than in kernels from smut-free control ears and 4.0-fold higher than in smut galls. Fumonisin levels in smut galls were not significantly different than in kernels of smut-free control corn. These studies indicate that, although corn smut was relatively free of the mycotoxins studied, the asymptomatic kernels of those ears contained mycotoxins at levels much higher than usually considered safe for direct human consumption.

Enzyme‐Linked Immunosorbent Assay Detection of Trichothecenes Produced by the Bioherbicide Myrothecium verrucaria in Cell Cultures, Extracts, and Plant Tissues

Myrothecium verrucaria is a pathogen of several invasive weed species, including kudzu, and is currently being evaluated for use as a bioherbicide. However, the fungus also produces macrocyclic trichothecene mycotoxins. The safety of this biological control agent during production and handling would be improved if an inoculum could be produced without concomitant accumulation of macrocyclic trichothecenes. Sporulation and trichothecene production by M. verrucaria was evaluated on standard potato dextrose agar (PDA) and a series of complex and defined media. Sporulation on PDA and on agar media with nitrogen as ammonium nitrate or potassium nitrate was more than ten-fold greater then sporulation on the medium with ammonium sulphate as the nitrogen source. Accumulation of macrocyclic trichothecenes was strongly affected by the media composition, with higher levels often associated with higher carbon content in the media. Overall, incubation in continuous darkness resulted in higher macrocyclic trichothecene...