Harry Strek

Characterization of glyphosate resistance in Amaranthus tuberculatus populations.

The evolution of glyphosate-resistant weeds has recently increased dramatically. Six suspected glyphosate-resistant Amaranthus tuberculatus populations were studied to confirm resistance and determine the resistance mechanism. Resistance was confirmed in greenhouse for all six populations with glyphosate resistance factors (R/S) between 5.2 and 7.5. No difference in glyphosate absorption or translocation was observed between resistant and susceptible individuals. No mutation at amino acid positions G101, T102, or P106 was detected in the EPSPS gene coding sequence, the target enzyme of glyphosate. Analysis of EPSPS gene copy number revealed that all glyphosate-resistant populations possessed increased EPSPS gene copy number, and this correlated with increased expression at both RNA and protein levels. EPSPS Vmax and Kcat values were more than doubled in resistant plants, indicating higher levels of catalytically active expressed EPSPS protein. EPSPS gene amplification is the main mechanism contributing to glyphosate resistance in the A. tuberculatus populations analyzed.

Management of an ACCase-inhibitor-resistant Lolium rigidum population based on the use of ALS inhibitors: weed population evolution observed over a 7 year field-scale investigation

BACKGROUND A 7 year experiment was set up in 2002 to evaluate the long-term effects of weed management strategies based on graminicidal sulfonylureas (SUs) on the evolution of a Lolium rigidum population resistant to ACCase inhibitors in a continuous wheat cropping system. The strategies included the continued use of ALS inhibitors, the continued application of ACCase inhibitors and a simple resistance management strategy based on a biennial rotation of herbicide mode of action (MoA). RESULTS The efficacy of the tested SUs in the field decreased significantly, starting from the fourth treatment in all control strategies. Regardless of control strategy, the few survivors of the ALS treatment in the third season produced a significant number of ACCase- and ALS-resistant (multiple-resistant) progeny. Continuous ALS and biennial rotation of herbicides reduced weed densities, but L. rigidum conserved its ACCase resistance trait. Enhanced metabolism was detected in ALS-resistant plants, whereas target site was primarily involved in the ACCase-resistant individuals. CONCLUSION At the end of the experiment, multiple-resistant individuals were found in all samples coming from the control strategies investigated. The biennial rotation between ALS and other MoA appeared to delay the development of resistance to SUs over continuous treatments, but additional measures will likely need to be taken in order to make this sustainable in the long term, whereas the field efficacy of SUs remained relatively high until the end of the experiment. Integrated weed management with more diversity should be introduced in oversimplified cropping systems in order to sustainably manage resistant L. rigidum populations.

Herbicides as weed control agents – state of the art. II. Recent achievements

Herbicide discovery has faced significant challenges over the past few decades, and weed control innovations are urgently required. In response to changing market dynamics, the discovery of new herbicides has declined significantly over the past few decades and has only seen a modest upsurge in recent years. Nevertheless, the few introductions have proven to be interesting and have brought useful innovation to the market. In addition, herbicide-tolerant or herbicide-resistant crop technologies have allowed the use of existing nonselective herbicides to be extended into crops. An increasing and now major challenge is being posed by the inexorable increase in biotypes of weeds that are resistant to herbicides. This problem is now at a level that threatens future agricultural productivity and needs to be better understood. If herbicides are to remain sustainable, then it is a must that we adopt diversity in crop rotation and herbicide use as well as increase the use of nonchemical measures to control weeds. Nevertheless, despite the difficulties posed by resistant weeds and increased regulatory hurdles, new screening tools promise to provide an upsurge of potential herbicide leads. Our industry urgently needs to supply agriculture with new, effective resistance-breaking herbicides along with strategies to sustain their utility.

Herbicide discovery in light of rapidly spreading resistance and ever increasing regulatory hurdles

According to the United Nations, the global population is expected to grow to almost 10 billion people in 2050. This means that the demand for food, feed and fiber will double while at the same time, agriculture is being challenged by a scarcity of water, global warming, less land available for farming, protection of natural habitats, a demand for biodiversity on farms and other factors. In addition, crop protection products are under pressure from rapidly spreading resistance and increasing regulatory requirements. Many regulatory bodies are also moving away from a risk assessment approach to a more hazard-based approach to grant registrations. Nevertheless, chemical crop protection compounds remain attractive and necessary to combat pests, particularly weeds. Industry has increased its efforts to find new molecules that are highly biologically effective on target species, including resistant populations, but safe for non-target organisms. To manage resistance in the future, a diverse toolbox is needed that includes herbicides with a variety of different chemistries and modes of action, combined with non-chemical measures in integrated systems. However, discovering a herbicide and getting it registered and to the market is an extremely complex endeavor full of risk, much of it incalculable.

Herbicide resistance-what have we learned from other disciplines?

Herbicide resistance is a growing threat to agriculture and has parallels to resistances to fungicides and insecticides. However, there are many reasons to treat the resistance to herbicides differently. To highlight these similarities and differences, three pests, a weed, an insect, and a disease that have shown the ability to rapidly develop resistance to a variety of products and product classes were used as illustrations. The situation in herbicide resistance is approaching a point already experienced by the other pest control disciplines, and thus, it is worthwhile to revisit their experiences.