A survey of more than 900 Iowa waterhemp (<i>Amaranthus tuberculatus</i>) populations demonstrated that multiple herbicide resistances is the norm; evolved resistance to three herbicide groups was found in 69% of the populations and resistance to four and five herbicide groups  was found in 15% and 3% of the populations screened. These estimates are predictive at the 95% confidence interval. Representative 3-way, 4-way, 5-way herbicide-resistant and a herbicide-susceptible waterhemp populations were included in herbicide-free field experiments to determine if multiple herbicide-resistant waterhemp incurred a fitness penalty attributable to the mutation(s) that conferred herbicide resistance. Relative growth rate, flowering, biomass and seed production were measured.  No significant differences between the multiple herbicide-resistant waterhemp populations and the herbicide-susceptible population were observed and supports the conclusion that multiple herbicide-resistant waterhemp are not at an ecological disadvantage.  A population of HPPD-resistant and an HPPD-sensitive waterhemp population were studied to assess the mechanism by which resistance occurred.  Our results indicated that the response of HPPD-resistant and susceptible waterhemp populations to mesotrione were very rapid and measureable as soon as three hours after treatment. Furthermore, little overlap was found among the differentially expressed transcripts expressed by each genotype. We also identified the possibility of overlapping gene networks in response to other herbicides.  The results of these studies indicate that multiple herbicide resistances in waterhemp is likely to increase in frequency given the lack of fitness penalty found in the resistant populations and the fact that the resistances may be the result of non-target site mechanisms.  Clearly, the management of waterhemp will be increasingly difficult given the frequency of multiple herbicide resistances and the lack of herbicides with novel mechanisms of action to which no resistances have evolved.<br/>

Multiple herbicide resistance in Iowa waterhemp is the norm: Implications of multiple resistances on fitness, resistance mechanisms and future management

256th National Meeting (2018)

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