ORIGINAL ABSTRACT

N-(phosphonomethyl)glycine (glyphosate) resistance was previously reported in a horseweed [Conyza (=Erigeron) canadensis (L.) Cronq.] population from Houston, DE (P (0) (R) ). Recurrent selection was performed on P (0) (R) , since the population was composed of susceptible (5%) and resistant (95%) phenotypes. After two cycles of selection at 2.0 kg ae glyphosate ha(-1), similar glyphosate rates that reduced plant growth by 50%, glyphosate rates that inflicted 50% mortality in the population, and accumulations of half of the maximum detectable shikimic acid concentration were observed between the parental P (0) (R) and the first (RS(1)) and second (RS(2)) recurrent generations. In addition, RS(1) and RS(2) did not segregate for resistance to glyphosate. This suggested that the RS(2) population comprised a near-homozygous, glyphosate-resistant line. Whole-plant rate responses estimated a fourfold resistance increase to glyphosate between RS(2) and either a pristine Ames, IA (P (0) (P) ) or a susceptible C. canadensis population from Georgetown, DE (P (0) (S) ). The genetics of glyphosate resistance in C. canadensis was investigated by performing reciprocal crosses between RS(2) and either the P (0) (P) or P (0) (S) populations. Evaluations of the first (F(1)) and second (F(2)) filial generations suggested that glyphosate resistance was governed by an incompletely dominant, single-locus gene (R allele) located in the nuclear genome. The proposed genetic model was confirmed by back-crosses of the F(1) to plants that arose from achenes of the original RS(2), P (0) (P) , or P (0) (S) parents. The autogamous nature of C. canadensis, the simple inheritance model of glyphosate resistance, and the fact that heterozygous genotypes (F(1)) survived glyphosate rates well above those recommended by the manufacturer, predicted a rapid increase in frequency of the R allele under continuous glyphosate selection. The impact of genetics on C. canadensis resistance management is discussed.