Fungal Pathogens as Biological Control Agents for Herbicide Resistant Pigweeds and Waterhemp.

Common waterhemp (Amaranthus rudis) and pigweeds (Amaranthus spp.) have become major problems in pumpkin (Cucurbita moschata) and soybean (Glycine max) crops because they have developed resistance to several herbicide families.  The need for alternative weed control methods has increased since there are limited options for their control.  The goal of this project was to develop two fungal pathogens (Microsphaeropsis amaranthi) and (Phomopsis amaranthicola) as agents for biological control of herbicide resistant waterhemp and pigweeds.  Field trials were performed over two years, in Macomb and Havana Illinois, to evaluate the effectiveness of these organisms in irrigated and non-irrigated pumpkin and soybean plots.  The concentrations of spore suspensions were M. amaranthi (1.5×10⁶ conidia.ml⁻¹), P.  amaranthicola (4×10⁶ conidia.ml⁻¹), and a mixture of the two organisms (1.5×10⁶+4×10 conidia.ml⁻¹, M. amaranthi and P. amaranthicola, respectively).  The bioherbicide was applied in a lecithin and vegetable oil formulation at 163 L ha^-1 in year one and 326 L ha^-1 in year two. Treatments included a spore suspension of each fungal pathogen alone, a mixture of both pathogens, and a sequential treatment of either halosulfuron-methyl (Sandea® Herbicide) in pumpkin or glyphosate (Roundup® Herbicide) in soybean.  The effectiveness of the bioherbicides was evaluated at 6 and 14 days after application.  Effectiveness was determined by disease incidence, disease severity (on a scale of 0-5, 0 = no disease 5 = plant mortality), % weed control, and weed biomass reduction.  Preliminary results show a reduction in weed biomass when treated with one or both of the fungal organisms compared to the control treatment.  The development of these organisms as bioherbicides will improve crop yields by controlling problematic weeds.  These fungal organisms show potential for use as a biological control method for herbicide resistant weeds in the genus Amaranthus.