Response of Horseweed (Erigeron canadensis) from New York Vineyards and Orchards to Paraquat and Diquat

Horseweed ( Erigeron canadensis L.) is a troublesome species in specialty crops, including orchards and vineyards. Some growers in New York have adopted paraquat [photosystem I (PSI) electron diverter] for in-season weed control as an alternative to glyphosate. This change was facilitated by concerns about possible crop injury and herbicide resistance to glyphosate. In response to weed control failures following paraquat applications in a vineyard (NY-Gr) and apple orchard (NY-Ap), whole-plant dose–response assays were conducted to confirm putative resistance. The paraquat rates required to reduce NY-Gr and NY-Ap biomass by 50% ( GR 50 ) were 0.63 and 0.56 kg a.i./ha, respectively; these values were 31- and 28-fold greater than the mean estimated GR 50 value (0.02 kg a.i./ha) for the paraquat-susceptible checks from a roadside (NY-Ro) and a soybean field (NY-So). The diquat rates required to reduce the biomass of the NY-Gr and NY-Ap populations by 50% were 0.019 and 0.052 kg a.i./ha, respectively. Conversely, the mean rate required to reduce the biomass of the NY-Ro and NY-So populations was ≤0.004 kg a.i./ha. The photosynthetic efficiency (CO 2 assimilation and chlorophyll a fluorescence) of resistant individuals remained unaltered, suggesting that the resistance mechanisms may not be directly related to the target site. All horseweed populations were effectively controlled by glufosinate and saflufenacil applied at 0.05 and 0.98 kg a.i./ha, respectively. These findings confirm that paraquat resistance is present in two New York horseweed populations. The NY-Gr and NY-Ap populations were less sensitive to diquat than the NY-Ro and NY-So populations, which could suggest emerging resistance to another PSI active ingredient. The extent of paraquat resistance in New York state is not known and requires additional investigation. Given the wind-dispersed nature of horseweed seeds, growers should be vigilant for paraquat escapes to prevent the spread of resistant biotypes. This should include the use of effective, alternative chemistries when resistance is suspected.