The distribution pattern of s-triazine-resistant biotypes of common lambsquarters (Chenopodium album L. #3 CHEAL) and smooth pigweed (Amaranthus hybridus L. # AMACH) in Virginia was determined. Seeds were col- lected from suspected triazine-resistant biotypes of both species. Triazine resistance was confirmed by measuring chlorophyll fluorescence in the presence of atrazine (6- chloro - N-ethyl - N' - (1 - methylethyl) - 1,3,5 - triazine - 2,4 - di- aminel. Greenhouse bioassay with whole-plant material and a sinking leaf disc assay were also performed as further confirmation of triazine resistance. Triazine-resistant smooth pigweed was confirmed in 19 counties and common lambs- quarters in eight counties in Virginia. Triazine-resistant smooth pigweed and common lambsquarters were located mostly in the northern and southwestern highlands of the state where there has been a long history of triazine use in no-till corn (Zea mays L.) production. S-triazine-resistant biotypes were also cross-resistant to other representative s-triazine and as-triazine herbicides but susceptible to the substituted urea herbicide diuron (N'- (3,4-dichlorophenyl)- N,N-dimethylureal.
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