Management Options for Multiple Resistant (glyphosate and paraquat) Hairy Fleabane (Conyza bonariensis) Populations In San Joaquin Valley.

Perennial cropping systems of the San Joaquin Valley (SJV) of California are commonly treated with glyphosate and paraquat for post-emergence weed control. Due to repeated use of these herbicides, glyphosate and paraquat resistant populations of hairy fleabane were reported in the SJV in 2007 and 2009, respectively. These hairy fleabane populations showed an 8-16 fold resistance to either glyphosate, paraquat, or in some cases to both herbicides when treated at 5 to 8 leaf stage. A short-term strategy to prevent a rapid population increase of the multiple-resistant biotype is to find chemical alternatives while long-term management options are being developed. A greenhouse study is being conducted to evaluate the efficacy of alternate herbicide treatments for the control of the multiple-resistant populations. Post-emergence treatments include glufosinate, 2,4-D, carfentrazone, saflufenacil and double knock down with glyphosate and paraquat. As pre-emergence treatments rimsulfuron, pexosulam and flumioxazin are being tested. Preliminary results indicate that glufosinate and saflufenacil have satisfactory control over all tested biotypes; tests are ongoing with the other herbicides.