Dr. Hallett’s research interests are in the broad area of the ecology of plant pathogen interactions. His applied research targets the development of bioherbicides for weed control and studies the mechanisms of herbicide resistance while his basic research studies the ecology of the interactions between weeds and soil microbial communities in agricultural and natural systems.

Applied Research: Bioherbicide Development.
Microsphaeropsis amaranthi is a virulent pathogen of a number of important weedy Amaranthaceae, causing severe foliar and stem necroses that can lead to plant death. Dr Hallett’s group has studied a number of aspects of this pathogen with a view to developing it as a bioherbicide product for the control of common waterhemp (Amaranthus rudis) in midwestern corn and soybean production. A sequence of publications has been submitted examining the variable responses of common waterhemp to the herbicide glyphosate (Weed Technology), the climatic constraints on M. amaranthi (Phytopathology), optimal spray application parameters (Weed Technology), and the interactions between M. amaranthi and chemical herbicides (Proceeding of the BCPC International Congress: Crop Science and Technology; Weed Science). Field trials have been performed in Indiana and Illinois in collaboration with Prof. Gordon Rosskamp (Southern Illinois Univ.) and Dr. Loretta Ortiz-Ribbing (Univ. Illinois, Urbana Champaign). Dr. Hallett’s research with this system has developed a detailed understanding of the key opportunities and limitations for the development of a host restricted bioherbicide into a major field crop. Dr Hallett has been involved in bioherbicides research for 18 years and has published over 20 refereed journal articles in the field. His research, performed on four different continents, has consistently challenged existing theories and developed novel concepts. Dr. Hallett has become a leader in the international bioherbicides community. He is the author of the most recent comprehensive review of the subject, and is the chair of the USDA-CSREES Coordinating Committee on bioherbicides.

Applied Research: Soil Microbes and Glyphosate Resistance.
The role of the herbicide glyphosate in broad acre agriculture has increased enormously since the development of glyphosate resistant crops. It is a non-selective herbicide that can be used to target nearly all weed species in cropping systems and the advent of glyphosate resistant crops has enabled its use to be expanded to postemergence applications. The use of glyphosate in glyphosate tolerant crops has delivered significant improvements in efficiency in Midwestern farming, and it has been adopted by the majority of corn/soybean growers. In the long term, however, this cropping system is threatened by the evolution of glyphosate tolerant weeds that, should they become widespread, could seriously impede the effectiveness of glyphosate. A number of weeds have already evolved glyphosate – mostly at low levels – indicating that the threat is real. Importantly, however, except for a few exceptions, we do not know how this resistance has evolved. The resistance mechanisms that have been elucidated in weeds that have evolved resistance to other herbicides do not adequately explain resistance to glyphosate. Dr. Hallett, Dr. G. Johal (BTNY) and Dr. Johnson (BTNY) hypothesize that the role of plant defense mechanisms and soil microorganisms may have been overlooked. We know, from the research of Dr. Johal and others, that soil borne fungal plant pathogens play an important role in the activity of glyphosate. The site of action of glyphosate is an enzyme in the shikimic acid pathway that synthesizes the aromatic amino acids, and the conventional wisdom is that glyphosate kills plants by starving them of amino acid substrates for protein synthesis. Rather than being killed simply by the lack of aromatic amino acids, however, plants are actually killed, before protein starvation, by pathogens. Pathogens, then, should be considered an integral part of the mode of action of glyphosate. We thus hypothesize that this microbe mediated mechanism may be the source of glyphosate resistance. Resistant plants, rather than evolving a mechanism to resist glyphosate per se may evolve a mechanism to resist the pathogens that take advantage of glyphosate-weakened plants. Jessica Schafer has recently been hired as a graduate student to test this hypothesis, using, as models, populations of marestail (Conyza canadensis), lamsquarters (Chenopodium album) and giant ragweed (Ambrosia trifida) assayed as glyphosate resistant by Dr. Johnson’s lab.

Basic Research: Ecology of Agricultural Weeds and Soil Microbial Communities.
The recent adoption of a range of molecular techniques by soil ecologists has begun a long awaited overhaul of our understanding of the complex interactions in the plant rhizosphere. Dr. Hallett has been investigating the complex interactions between weeds and their soil biota using denaturing gradient gel electrophoresis of PCR-amplified RNA genes (PCR-DGGE). He has shown that weeds modify soil microbial communities in a species-specific way and that these modified microbial communities can have a range of impacts upon subsequent weed growth. He has also shown that weed seed decay in the soil seed bank is correlated with soil microbial communities that are promoted by specific crop management regimes. A study from a long term ecological trial in Iowa has shown that low-input weed management systems utilizing complex rotations and reduced herbicide inputs increased the diversity and seasonal stability of microbial communities. These findings are important to the development of low-input and organic farming practices and will lead to the recommendation of soil and crop management practices that promote weed suppressive soils. This research has been reported at a number of scientific meetings and published in Weed Science.

Parasitic weeds in the genus Striga cause devastating losses to resource-poor cereal farmers, particularly in sub-Saharan Africa. Dr. Hallett has shown that parasitism by Striga causes dramatic shifts in the composition of microbial communities in the rhizosphere of its cereal host. Dr. Hallett is a recognized expert in Striga biocontrol, and was invited to speak at the International Institute for Tropical Agriculture (IITA), Cotonou, Benin, at the Tanzanian Ministry of Agriculture, Ilonga, Tanzania, and at the American Phytopathologyical Society meetings in Austin, Texas (2005). A chapter in the recently published book Integrating New Technologies for Striga Control: Towards Ending the Witch-Hunt (J Gressel & G Ejeta eds) highlights some of his research in this area.

Basic Research: Dislocation of Plant Invaders from Coevolved Microbial Associations
In the last two years, Dr. Hallett has expanded his research efforts to study the mechanisms of plant invasion, and is seeking to expand the experimental foundation of new coevolution-based hypotheses to explain why some plants become ecologically transformed when they are geographically displaced. In a recent paper published in Weed Science, Dr. Hallett proposed that the key determinant of establishment, invasion and naturalization in plants is the dislocation from coevolved relationships that they experience following geographic displacement. A detailed understanding of the trade-offs between dislocation from coevolved mutualists, parasites and competitors will enable predictions of the outcomes of plant geographic displacements.

Dr. Hallett and co-workers have shown that garlic mustard interacts very differently with the soil biota of Europe and North America. When soils from Europe and North America were conditioned with garlic mustard, the perturbations to soil microbial communities were very different and had different impacts. The conditioning of North American soils with garlic mustard resulted in large perturbations to the soil biota and dramatic negative effects upon subsequent plantings of North American plants. The findings to date have been reported at the annual meeting of the North Central Weed Science Society (2004) and in papers published in PLoS Biology and Ecology.