Johnsongrass [Sorghum halepense (L.) Pers.] is a problematic, perennial weed species that is increasingly difficult to control across a range of agroecosystems; especially under intensifying pressures from climate change and herbicide resistance. Glyphosate has historically been effective at controlling S. halepense; however, repeated and widespread use has imposed strong selection pressure resulting in resistant biotypes. A suspected glyphosate resistant biotype was identified in Texas, and this study was performed to characterize potential target and non-target site resistance mechanism(s) in this biotype. Dose-response assays revealed an observed ED50 of 1036 g ae ha-1, a 2.3-fold resistance ratio compared to a glyphosate sensitive biotype. Target-site resistance mechanisms were not detected; no amino acid substitutions or increased copy number of the EPSPS gene were observed. Liquid chromatography mass spectrometry was used to quantify glyphosate. At 7 and 10 days after application, glyphosate concentrations in aboveground tissue were 48 and 50% lower, respectively, in the resistant compared to the susceptible biotype; however, there were no decreases in belowground glyphosate concentrations. Collectively, these results provide putative evidence for altered glyphosate fate as a resistance mechanism in S. halepense. Additionally, this study describes the development of novel molecular markers for EPSPS amplification and copy number variation, addressing a key limitation in resistance diagnostics for this polyploid weed species.

Indaziflam is a long-term residual weed control option for Georgia pecan [Carya illinoinensis (Wangenh.) K. Koch] growers. As a nonselective cellulose biosynthesis inhibitor, indaziflam has a niche for broad-spectrum weed control with long residual activity in various perennial cropping systems. Indaziflam’s soil persistence and chemical behavior at various temperatures have not been fully evaluated; therefore, the objectives of these experiments were to: (1) quantify indaziflam soil dissipation under field conditions in two common Georgia soils and (2) evaluate indaziflam molecular stability as affected by temperature and time using laboratory techniques. Indaziflam soil dissipation followed first-order kinetics and was adequately described by the exponential decay equation. Indaziflam half-life in Greenville sandy clay loam and Faceville loamy sand was 96 and 78 d, respectively. Indaziflam half-life and soil clay content had a direct relationship, while indaziflam half-life and microbial biomass had an inverse relationship. Aqueous solutions of indaziflam were exposed to temperatures that ranged from 20 to 70 C for up to 672 h, with results indicating that temperature had no influence on indaziflam’s molecular stability.