Diminished control of goosegrass was observed in tomato fields located in Manatee County, FL, after years of repeated paraquat use. Tolerance of the Manatee biotype to paraquat was confirmed by its comparison in greenhouse studies with a susceptible biotype from the Alachua County, FL. A 30-fold increase in paraquat rate was required to reach the 50% growth reduction level of the resistant biotype over the susceptible biotype. The Manatee biotype was not tolerant to clethodim, metribuzin, or sethoxydim. These herbicides provided adequate control of all the goosegrass biotypes tested.
Nomenclature: Clethodim; metribuzin; paraquat; sethoxydim; goosegrass, Eleusine indica L. #3 ELEIN; tomato, Lycopersicou esculentum Mill.
Additional index words: Additive, antagonism, herbicide resistance, synergism.
Abbreviations: GR50, herbicide concentration required to inhibit goosegrass aboveground biomass by 50%; WAT, weeks after treatment.
