Glyphosate-Resistant Weed Species in the Perennial Cropping Systems of California and the Costs of Alternative Herbicide Treatments.

Glyphosate-resistant (GR) weed species in orchards and vineyards of California include horseweed (Conyza canadensis), hairy fleabane (C. bonariensis), rigid ryegrass (Lolium rigidum), Italian ryegrass (L. multiflorum), and junglerice (Echinochloa colona).  Of these species, the Conyzas have been particularly problematic in the San Joaquin Valley (SJV) crop and non-crop areas because of large-scale prevalence of GR biotypes.  Alternative pre- and post-emergence herbicides are being evaluated in greenhouse and field studies as potential immediate short-term measures to control the GR Conyza biotypes.  Pre-emergence herbicides being evaluated include flumioxazin, rimsulfuron, penoxsulam+oxyfluorfen, and indaziflam.  Post-emergence herbicides include glufosinate, saflufenacil, paraquat, carfentrazone, and 2,4-D.  Costs of these alternative treatments are being calculated and extension events are being held to promote herbicide resistance management strategies in these perennial cropping systems.