MANAGING THE DELAY OF EVOLUTION OF HERBICIDE RESISTANCE IN PARASITIC WEEDS

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 applic ations and as seed dressings. Weeds rapidly evolve resistance to this single-target, high mutation frequency group of herbicides, which usua lly exert heavy selection pressure. This type of rapid evolution of re sistant populations was previously predicted by models, and later vali dated in the field in other weed and cropping situations. The selectio n pressure of this herbicide group may be exceedingly strong with para sitic weeds, as they are controlled by very low dose rates and the dos es used are in the 'overkill' range. A good management strategy with n on-parasitic weeds was to lower selection pressure, but this may be le ss effective with parasitic weeds. Many of the areas of the world wher e parasitic weeds are a problem do not use mechanical harvesters that rapidly spread weed seed. The inhomogeneous seed spread with hand harv esting necessitated developing new models. With mechanical-harvester s pread of parasitic weed seeds, the best strategy is to treat the resis tant crop seed with high herbicide levels (that are low per hectare) a nd ensure the immediate removal of the rare resistant plants that appe ars, before they set seed, as well as to ensure that crop seed is free of resistant seed of the parasitic weeds. Modelling suggests that res istant parasitic weed populations will evolve within two to six season s of use without resistance management, using mechanical crop harvesti ng. With hand harvesting of crop, delays of about eight seasons of eff ective herbicide use can be expected, much longer ii the mitigating st rategies of roguing resistant stalks and using parasite-free crop seed are implemented.