Palmer Amaranth in California: Research and Forward Planning for Glyphosate Resistance Management

For the past several years glyphosate resistant (GR) Palmer amaranth has rapidly become one of the most troublesome weeds in most of the south eastern cotton growing region and is changing farming practices due largely to this weed. Growers in California’s San Joaquin Valley (SJV) have observed poor control of Palmer amaranth in glyphosate-tolerant cotton, corn, and alfalfa production systems. In addition, Palmer amaranth is rapidly advancing into permanent cropping systems and along roadsides and canals.  It is not known if these are cases of GR populations or just an issue of weed size with survivors of glyphosate applications treated on larger and more tolerant growth stage.  Seed was harvested from plants collected from Kern, Tulare, Kings and Fresno counties in the southern SJV. The suspected cases of GR Palmer amaranth plants from the SJV were compared to confirmed GR Palmer amaranth plants from other states.  The tolerance of the SJV Palmer amaranth plants to various rates of glyphosate at different growth stages was evaluated in several field, and greenhouses settings.   Alternative herbicides were also evaluated.  In these studies so far GR Palmer amaranth populations have not been confirmed in the populations that have been screened, however these will be ongoing studies. It was observed at some populations, it took increasing rates of glyphosate to control taller weeds. These greenhouse resistance studies will continue and alternative control methods will be evaluated as Palmer amaranth continues to spread in the Central Valley of California.