Littleseed Canarygrass (Phalaris minor) Resistance to Isoproturon in India

The response of littleseed canarygrass biotypes to isoproturon, pendimethalin, and diclofop-methyl was evaluated in India, in pot studies and the field during the winters of 1991 to 1992 and 1992 to 1993. Some biotypes of littleseed canarygrass were resistant to isoproturon but cross-resistance to pendimethalin and diclofop-methyl was not confirmed. The resistant biotype required a higher dose of diclofop-methyl than the susceptible biotype. Variations in the response of littleseed canarygrass biotypes were not due to isoproturon formulation. Resistant biotypes required 2 to 8 times more isoproturon than a susceptible biotype for the same level of control. Diclofop-methyl at 1.0 kg ai/ha applied at the 2- to 3-leaf stage of littleseed canarygrass in pot experiments and PRE pendimethalin at 1.5 kg ai/ha in field trials controlled resistant biotypes. Field surveys of the affected areas revealed that resistance in littleseed canarygrass is more prevalent in rice-wheat rotations compared to other crop sequences. Control of littleseed canarygrass with isoproturon dropped from 78 to 21% from 1990 to 1993.