Plant and Soil Science

Faculty Advisor

Bryan G. Young


The present experiment quantified the selection for PPO-resistant waterhemp following a soil-residual application of fomesafen applied alone (1.32, 13.2, and 132g ai ha-1) and in combination with dicamba (0.77, 7.7, 77 g ai ha-1), respectively. The logistic rate structure aimed to simulate the degradation of herbicide in the soil, with the highest rate being one third of a full use rate. Tissue samples were taken from the first 20 emerging waterhemp plants in each treatment, including the non-treated control, and genotyped using an allele specific TaqMan assay to detect the codon deletion responsible for PPO resistance in waterhemp. Results indicated that applications of fomesafen alone at 132 g ai ha-1 increased the frequency of PPO resistance in the emerging population, with 90% of the sample population having resistance compared to 25% of population in the untreated control. The addition of dicamba to fomesafen reduced the frequency of resistant waterhemp to 70% of the population. While this research demonstrates that the addition of dicamba may not fully reduce the selection for PPO-resistant biotypes, fomesafen and dicamba applied together at the highest rate provided considerable residual control of the resistant waterhemp. Therefore, these results further emphasize the importance of proactive herbicide resistance management by employing full use rates of soil-residual herbicides and the combination of multiple herbicide modes of action.