The extensive use of propanil for weed control in Oryza sativa over the past 30 yr has led to the development of propanil-resistant Echinochloa crus-galli. This problem was first identified in one Arkansas county in 1990. Since that time, 171 resistant populations have been identified in 20 counties. Random amplified polymorphic DNA (RAPD) analysis was used to assess genetic variation among populations of propanil-resistant and -susceptible E. crus-galli in Arkansas and to elucidate the origin of resistance and its dispersal. Sixteen E. crus-galli populations from seven Arkansas counties and one population from Mississippi were analyzed using 13 primers, which amplified a total of 159 repeatable fragments. Genetic distance was calculated among the populations, and the populations were clustered. The analysis revealed two distinct clusters with an average between-cluster genetic distance of 0.436 and average within-cluster distances of 0.012 and 0.029. These clusters may represent different Echinochloa species. Each cluster contained at least one resistant population and populations from different regions of the state. In addition, several resistant populations from different regions of the state were genetically nearly identical. Thus, it is likely that resistant biotypes have spread in Arkansas by seed dispersal and by independent mutation events. This suggests that there are multiple origins of the resistance, and simple control of weed seed dispersal will not stop the spread of resistance.

Nomenclature: Propanil; Echinochloa crus-galli (L.) Beauv. ECHCG, barnyardgrass; Oryza sativa L., rice.