A SEEDLING BIOASSAY FOR ASSESSING THE RESPONSE OF WILD OAT (AVENA-FATUA) POPULATIONS TO TRIALLATE

Germinated seeds of wild oat populations that were susceptible (S) or resistant (R) to triallate at the recommended soil-applied rate (1.7 k g/ha) were treated with six triallate concentrations on filter paper i n petri dishes. Measurement of shoot length 8 d after treatment provid ed an accurate indication of differences among populations, and was mo re reliable than determining shoot fresh weight. ED(50) values (herbic ide concentrations that reduced shoot length by 50% relative to untrea ted controls), derived from nonlinear regression analysis, indicated f our and five levels of response to triallate among eight S and seven R populations, respectively, The ED(50) values varied from 0.11 to 11 p pm a.i. triallate for the most susceptible to the most resistant popul ations, respectively. Routine testing of wild oat samples suspected of resistance, at triallate concentrations of 0.5 or 1 ppm in the petri dish bioassay, effectively identified populations that had become resi stant to the recommended soil-applied rate.