The Role of Gene Flow In the Spread of Chenopodium album Resistance to Herbicides.

Common lambsquarters (Chenopodium album L.) is a highly competitive weed species in many cropping systems that has demonstrated resistance to several herbicide modes of action.  However, predicting the spread of resistance is difficult due to limited information about the roles of weed genetics and gene flow.  The objective of our research was to determine the potential for flow of resistance alleles in common lambsquarters in the field environment.  In 2009 an accession with an incompletely dominant magenta leaf axil phenotypic marker (magenta phenotype) was used as a pollen parent in gene flow experiments.  A wild-type with no magenta coloration was used as a seed parent.  If resistance to herbicides in common lambsquarters is conferred by a nuclear dominant or incompletely dominant allele, as has been shown in other resistant species, pollen-mediated flow of resistance alleles would be expected to be similar to that of the magenta phenotypic marker.  Therefore, wild type common lambsquarters plants were grown in a soybean field in concentric circles at 2, 3, 4, 6, 10, and 15 m from a center which contained 24 magenta phenotype plants.  The concentric circles were divided into eight sectors (cardinal directions S, SW, W, NW, N, NE, E, SE).  Pollen movement was analyzed by sampling seed from wild-type plants. The percent of progeny with the magenta phenotype from those plants was determined for each distance and sector. Average cross-pollination across sectors was greatest (3.0%) at 2 m and decreased to near zero (0.16%) at 15 m from the center, consistent with observations in other primarily self-pollinated species.  In addition, cross-pollination was greatest (P < 0.05) in the west and southwest sectors.  Based on these results, changes in susceptible and resistant allelic frequencies within common lambsquarters populations would likely be affected more by seed movement than by cross-pollination.