Pronamide Resistant Annual Bluegrass Confirmed in Georgia.

A biotype of annual bluegrass with suspected resistance to pronamide was identified on a golf course in Georgia.  The objectives of this research were to determine the level of resistance to pronamide and the mechanisms associated with resistance.  From postemergence applications, the pronamide rate that reduced shoot biomass 50% from the nontreated measured >10-times higher for the resistant (R)-biotype as compared to susceptible (S)-biotypes.  The R-biotype was not controlled by preemergence applications of dithiopyr or prodiamine, but was controlled >92% by PRE applications of pronamide at 0.56 and 1.68 kg ha^-1.  Mature plants (3 to 5-tiller) of the R-biotype absorbed 32% less ¹⁴C-pronamide than the S-biotype after 72 h in hydroponic culture and accumulated 39% less Bq g^-1 of dry shoot mass.  The R-biotype metabolized ¹⁴C-pronamide similar to the S-biotype, averaging 16% of the extracted radioactivity.  The resistance to POST pronamide applications in the R-biotype is associated with reduced absorption and translocation from the S-biotype.  This is the first report of pronamide-resistant annual bluegrass.