Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2018
Publication Date: 10/29/2018
Citation: Hoagland, R.E., Boyette, C.D., Jordan, R.H., Stetina, K.C. 2018. Interaction of the bioherbicide Myrothecium verrucaria with technical-grade glyphosate on glyphosate-susceptible and -resistant Palmer amaranth. American Journal of Plant Sciences. 9:2306-2319.

Interpretive Summary: Combinations of bioherbicides and herbicides have been examined to discover possible synergistic interactions to improve weed control. Scientists from the Crop Production Systems Research Unit and the Biological Control of Pests Research Unit, conducted studies to assess possible interactions of the herbicide glyphosate with Myrothecium verrucaria (MV), a bioherbicide that can control several economically important weeds including Palmer amaranth. Glyphosate inhibits a key enzyme responsible for the biosynthesis of aromatic amino acids and compounds involved in plant defense, but the mode of action of MV is unknown. Previous reports from these researchers showed that MV, when combined with some commercial formulations of glyphosate, resulted in synergistic interactions on weed control, but some commercial glyphosate products were toxic to the bioherbicide. Greenhouse and laboratory studies were conducted to determine the toxicity of high purity glyphosate on MV and the effects of combining MV with this herbicide on glyphosate-susceptible and –resistant Palmer amaranth biotypes. Results showed that high purity glyphosate (without adjuvants/surfactants) was not inhibitory to MV growth and sporulation at concentrations up to 2.0 mM. Plants of both biotypes were injured by MV and MV plus glyphosate treatments at 20 h after application. Injury effects increased through the 6-day time course, and at 120 h after treatment the MV plus glyphosate treatment had killed all glyphosate-susceptible and -resistant plants. The interaction of glyphosate plus MV was found to be synergistic for Palmer amaranth control. Data suggest that glyphosate (not formulation additives) is responsible for the synergy found when this bioherbicide is combined with some commercial formulations of glyphosate.

Technical Abstract: Previously we found that a strain of Myrothecium verrucaria (MV) exhibited bioherbicidal activity against several important weeds, and that some commercial formulations of glyphosate applied with MV resulted in synergistic interactions that improved weed control efficacy. We also found that MV had bioherbicidal activity against glyphosate-resistant Palmer amaranth. We have also reported that some commercial formulations are inhibitory to MV. To test the effect of unformulated glyphosate we tested high purity, technical-grade glyphosate alone and in combination with MV for bioherbicidal activity on glyphosate-susceptible and -resistant Palmer amaranth biotypes under greenhouse conditions. High purity glyphosate (without adjuvants/surfactants) was not toxic to MV growth and sporulation at concentrations up to 2.0 mM when grown on agar supplemented with the herbicide. Both biotypes were injured by MV and MV plus glyphosate treatments as early as 19 h after application (3 h after a dew period of 16 h). These injury effects increased and were more evident through the 6-day time course, when after 120 h the MV plus glyphosate treatment had killed all glyphosate-susceptible and -resistant plants. The interaction of glyphosate plus MV was synergistic toward the control of Palmer amaranth. Data strongly suggest that the active ingredient is responsible for the synergy previously found when this bioherbicide was combined with some commercial formulations of glyphosate. Results demonstrated that MV can control both glyphosate-resistant and -susceptible Palmer amaranth seedlings and act synergistically with high-purity glyphosate to provide improved weed control.