Comparison of Molecularly Identified Resistant and Susceptible Johnsongrass ( L.) Populations at Gene, in the Absence and Presence of Field Crops.

Background/objectives: Johnsongrass () is an erect tetraploid, perennial, C4 grass weed species categorized among the world's most noxious weeds due to its high competitive ability against crops and the increased number of field-evolved herbicide-resistant populations. The aim of the present study was to assess the growth rate and performance of resistant (R) johnsongrass genotypes hosting Trp574Leu target-site cross-resistance at gene, inhibiting various herbicides, compared to susceptible (S) conspecific weeds, in the absence and presence of corn or sunflower antagonism.

Methods: The aboveground biomass, tiller, and rhizome production ability of one S and one R johnsongrass population with a Trp574-Leu substitution conferring cross-resistance to ALS-inhibiting herbicides were compared under non-competitive conditions.

Mechanisms of Lolium rigidum multiple resistance to ALS- and ACCase-inhibiting herbicides and their impact on plant fitness.

Three putative resistant (R1, R2, R3) and one susceptible (S) Lolium rigidum populations originating from Greece were studied for resistance to ALS and ACCase inhibiting herbicides, using whole plant, sequencing of als and accase gene, and in vitro ALS activity assays. The S and two R (R1, R2) populations were also evaluated for fitness in competition with wheat. The whole plant assay indicated unsatisfactory control of the R populations with mesosulfuron-methyl + iodosulfuron-methyl or pinoxaden application, whereas sequencing of the als gene revealed that all ALS-resistant individuals had a Pro-197 substitution by Leu, Glu, Ser, Ala, Thr, or Gln.

Resistance of Rapistrum rugosum to tribenuron and imazamox due to Trp574 or Pro197 substitution in the acetolactate synthase.

Ten putative resistant and two susceptible Rapistrum rugosum populations originating from Greece were studied for resistance to acetolactate synthase (ALS)-inhibiting herbicides, using dose-response assays, sequencing of als gene and in vitro ALS activity assays. The dose-response assays showed that one (P1) out of ten putative resistant populations was cross-resistant to tribenuron and imazamox, while another population (P4) was resistant to tribenuron only. All populations were susceptible to MCPA at the recommended rate.

Trp574 substitution in the acetolactate synthase of Sinapis arvensis confers cross-resistance to tribenuron and imazamox.

Rate-response experiments with nine putative resistant wild mustard (Sinapis arvensis) populations from Greece showed cross-resistance to tribenuron and imazamox. The calculated GR values [herbicide rate (gaiha) required for 50% reduction of fresh weight] of the nine resistant (R) populations ranged from 51.8 to 555.