Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract only
Publication Acceptance Date: 9/28/2006
Publication Date: 2/6/2007
Citation: Hanson, B.D., Shrestha, A., Pelham, K., Shaner, D.L. 2007. An enzyme assay and GIS as tools to characterize and determine the spatial distribution of glyphosate-resistant horseweed in the San Joaquin Valley of California. Weed Science Society of America Meeting Abstracts, February 5-8, 2007. Interpretive Summary:
Technical Abstract: Glyphosate-resistant (GR) horseweed (Conyza canadensis) was first identified in the San Joaquin Valley (SJV) of California in 2005 on irrigation canal banks with a history of repeated glyphosate applications. Irrigation canals often are adjacent to perennial and annual crop fields that also rely on glyphosate for control of annual and perennial weeds. For several years, glyphosate has successfully provided economic and effective weed control in crop and non-crop areas of the SJV. Further, environmental regulations in the SJV have identified glyphosate as a reduced-risk material, thus increasing the reliance on glyphosate. Although horseweed is not a major problem in SJV annual cropping systems due to tillage operations, in recent years it has become more prevalent in orchards, vineyards, canal banks, roadsides, and fallow lands. It is not known if the increasing frequency of horseweed in these areas is due to glyphosate resistance or other factors. There is a need to rapidly determine the presence of GR horseweed to develop management strategies and herbicide stewardship programs in the SJV. We used an EPSPS enzyme assay and GIS as tools to rapidly characterize and map the spatial distribution of GR horseweed. Horseweed leaf tissue samples were collected from various locations in Kern, Kings, Tulare, Fresno, Merced, Madera, and Stanislaus counties and the geographical coordinates of these locations were entered into a GIS system. EPSPS activity was determined by measuring invivo shikimate accumulation by leaf tissue exposed to glyphosate. Approximately 8 times more glyphosate was required to increase the amount of shikimate present in GR tissue compared to glyphosate-susceptible (GS) leaf tissue although at high concentrations shikimate accumulated in both biotypes. This suggests that glyphosate resistance in this horseweed biotype may be at least partly due to differences in translocation. The assay and spatial distribution results suggested that the vast majority (84%) of horseweed present in late summer in the SJV were resistant to glyphosate. The presence of GR or GS plants was not associated with collections made in crop or non-crop sites which may be due to long distance seed dispersal. Research is planned to determine the whole plant and enzyme level response to glyphosate at different horseweed growth stages and to determine the relative competitive ability of the two biotypes.