ABSTRACT

Over 100 weed species throughout the world have been reported as resistant to a particular herbicide. Herbicide resistant weeds were once controlled by the herbicide in question but for some reason, usually selection pressure brought about by over-use of the herbicide, these weeds can no longer be controlled at the recommended use rate. Likely candidates for developing resistance are weeds that are prolific seed producers and are highly sensitive to the herbicide in question.

In most cases, poor weed control is not due to resistance, but can usually be traced to poor environmental conditions at the time of application, or incorrect timing of application. All factors which contribute to poor performance of the herbicide must be considered first before consideration is given to herbicide resistance.

Three field sites in Washington County, MS, which had received two or more applications of fluazifop-P (Fusilade 20009) without successful control of rhizome johnsongrass [Sorghum halepense (L.) Pers.] were investigated in August, 1991. The following research was conducted to investigate the possibility of fluazifop-P resistance in seedling johnsongrass biotypes from two of these sites.