Rigid ryegrass (Lolium rigidum Gaudin) is the most problematic weed in Australia, with evolved resistance to multiple herbicide sites of action. Selection pressure by cinmethylin (Group 30, a fatty-acid thioesterase inhibitor) has been limited, because few populations have been exposed to the herbicide since its introduction in 2019. In this study, we examined the sensitivity of L. rigidum populations to this new herbicide. From a screening of almost 500 field populations in 2020, 28 potentially resistant populations were further investigated in a dose–response experiment. Seedlings from five populations surviving treatments of 250 or 375 g ai ha−1 cinmethylin were grown to maturity and seeds were harvested. The level of resistance found among the five putative-resistant parental populations of L. rigidum was negligible. In one population, one round of selection with cinmethylin resulted in a 2-fold increase in the lethal dose causing 50% mortality in the progeny population, although this dose was still only one-sixth of the recommended field rate of cinmethylin. Having a unique site of action, cinmethylin is a viable preemergence herbicide option to control existing multiple-resistance populations of L. rigidum. Comprehensive field monitoring and recurrent selection studies under controlled environmental conditions are needed to better ascertain the risk of L. rigidum evolving a high level of resistance to cinmethylin, although current data suggest that this risk is relatively low.

The susceptibility to glyphosate and genetic diversity based on intersimple sequence repeat markers were characterized for 17 tropical sprangletop populations collected from two separate regions mainly in Persian lime groves in Veracruz, Mexico. The whole-plant dose response together with shikimic acid assays indicated different levels of glyphosate resistance in those populations. Genetic diversity values (h) estimated using POPGENE ranged from 0.119 to 0.198 and 0.117 to 0.214 within susceptible and resistant populations, respectively. The average genetic diversity (HS) within the susceptible populations was 0.157, and the total genetic diversity (HT) was 0.218. The HS of the resistant populations was 0.144, and the HT was 0.186. The analysis of molecular variance based on the response to glyphosate indicated that most of the genetic variation was found within groups of susceptible and resistant populations (90% of the genetic variation), whereas 10% or less was among groups. The high level of genetic diversity between glyphosate-resistant tropical sprangletop populations from distant and adjacent locations is likely due to both short- and long-distance seed dispersal and independent evolutionary events in tropical sprangletop populations among Persian lime groves in Veracruz.

This study used statistical methods to investigate linkages in phenotypic resistance profiles in a population sample of 321 Salmonella Typhimurium isolates from sporadic salmonellosis cases in Lower Saxony, Germany, collected during 2008–2010. A resistance index was applied to calculate the conditional probability of resistance to one antimicrobial agent given the resistance to one or more other antimicrobial agent(s). A susceptibility index was defined analogously. A contingency plot, which visualizes the association between resistances to two antimicrobial agents, facilitated the interpretation. Linkages between minimum inhibitory concentrations (MIC) were analysed using Spearman's rank correlation coefficient and jittered scatter plots. Applying these methods provided a compact description of multi-resistance and linkages between resistance properties in large datasets. Moreover, this approach will improve monitoring of antimicrobial resistance dynamics of bacteria in human or animal populations by identifying linked resistance to antimicrobial agents (cross- or co-resistance) with a non-molecular method.

A quantitative index of resistance to ticks, which had been derived from results of artificial infestation of East African zebu cattle with a Rusinga Island (Lake Victoria) stock of Rhipicephalus appendiculatus, was tested for consistency using three additional tick stocks—from a laboratory (Muguga), Intona (Trans Mara) and Ukunda (Kenya coast). Results from all the stocks gave similar indices when compared with corresponding values for the Rusinga stock, confirming both the validity of the mathematical function and of the use of any local stock of the tick species for assessing the resistance of cattle to R. appendiculatus.