Managing the delay of evolution of herbicide resistance in parasitic weeds

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Abstract
Acetolactate synthase (ALS)‐inhibiting imidazolinone and sulphonylurea herbicides have been found to be effective in selectively controlling the pernicious parasitic weeds Orobanche, Striga and Alectra spp. in some crops. This control could be effected both as whole field applications and as seed dressings. Weeds rapidly evolve resistance to this single‐target, high mutation frequency group of herbicides, which usually exert heavy selection pressure. This type of rapid evolution of resistant populations was previously predicted by models, and later validated in the field in other weed and cropping situations. The selection pressure of this herbicide group may be exceedingly strong with parasitic weeds, as they are controlled by very low dose rates and the doses used are in the ‘overkill’ range. A good management strategy with non‐parasitic weeds was to lower selection pressure, but this may be less effective with parasitic weeds. Many of the areas of the world where parasitic weeds are a problem do not use mechanical harvesters that rapidly spread weed seed. The inhomogeneous seed spread with hand harvesting necessitated developing new models. With mechanical‐harvester spread of parasitic weed seeds, the best strategy is to treat the resistant crop seed with high herbicide levels (that are low per hectare) and ensure the immediate removal of the rare resistant plants that appears, before they set seed, as well as to ensure that crop seed is free of resistant seed of the parasitic weeds. Modelling suggests that resistant parasitic weed populations will evolve within two to six seasons of use without resistance management, using mechanical crop harvesting. With hand harvesting of crop, delays of about eight seasons of effective herbicide use can be expected, much longer if the mitigating strategies of roguing resistant stalks and using parasite‐free crop seed are implemented.