ABSTRACT
Ever since the widespread usage of phenoxy herbicides beginning 1946, weed scientists began pondering over the possibility of development of herbicide-resistant weed populations similar to those exhibited by insects against an inorganic insecticide in 1908 but documented in 1914, and plant pathogens against fungicides since 1940. In 1950, Blackman [1950] warned ‘‘… repeated spraying with one type of herbicide will sort out resistant strains within the weed population.’’ In 1954, McCall [1954] wondered whether weeds were becoming more resistant to herbicides. The same year saw a report from the U.K. suggesting that continuous application of 2,4-D has led to resistance of weed species normally susceptible to it. This was followed by two other reports against 2,4-D in 1957, one from Hawaii where biotypes of Commelina diffusa (spreading dayfl ower) in sugarcane fi elds [Hilton 1957], and another from Ontario, Canada, where biotypes of Daucus carota (wild carrot) in sections of highway weeds [Switzer 1957] exhibited resistance.
