ABSTRACT

Many weeds that were previously controlled by specific herbicides have evolved biotypes with increased levels of tolerance. Some of these biotypes can have a seemingly absolute resistance; they can be treated with saturated solutions of herbicides that have little or no effect on the weed. Herbicide resistance has appeared only when there were long periods where these herbicides were used, year after year with no other herbicide in rotation. Triazine-resistance has occurred in corn fields, orchards, vineyards, groves and tree plantations, and along roadsides and railroad rights-of-way. Resistance is usually inherited on one or at most two major nuclear genes in species where newly resistant biotypes or varieties have appeared. Many effects in different biochemical pathways often accompany a mutation. Some of the pleiotropic effects will seem secondary in the case of triazine resistance. There is one useful clue to help understand the possible multiplicity of binding sites from the genetics of triazine resistance: resistance is maternally inherited.