A bioassay, based on seedling shoot inhibition with difenzoquat at 10 mg/L, provides a rapid means of predicting hard red spring wheat (HRSW) cultivar response to foliar-applied difenzoquat. The tolerance of several cultivars to difenzoquat was determined by shoot length inhibition in the laboratory, and by visible crop injury and shoot dry weight in the greenhouse. Of the 19 cultivars tested, one cultivar that was susceptible in the laboratory assay was intermediate in tolerance to difenzoquat in the greenhouse assay; two cultivars that were tolerant in the laboratory bioassay were of intermediate tolerance in the greenhouse assay. Results from laboratory and greenhouse studies both identified cultivars distinctly susceptible to difenzoquat. Shoot inhibition indicated that HRSW cultivar susceptibility to difenzoquat involves continuous variation from tolerant to susceptible. This may explain why some cultivars show differential responses to difenzoquat under different environmental conditions.

Nomenclature: Difenzoquat; hard red spring wheat, Triticum aestivum L.

Additional index words: Herbicide tolerance, root length, shoot length.

Abbreviation: HRSW, hard red spring wheat.