Abstract
The development of sulfonylurea-herbicide (SU) resistant biotypes of Monochoria korsakowii Regel et Maack and Scirpus juncoides Roxb. var. ohwianus. T. Koyama as troublesome paddy weeds in Hokkaido region is a recent problem. The geographical distribution and level of infestation of these weeds were surveyed. Interpretation of the geographical survey data and SU resistance status suggest that SU resistant biotypes of these weed species originated in locales where weed infestation and selection pressure were severe. Once resistant biotypes appeared in these locales, they then quickly spread in seasonal succession to neighboring fields. Field experiments were conducted to investigate the use of combination-herbicides including non-sulfonylurea herbicides for controlling these two SU resistant weed biotypes. Combination herbicides were effective on the weeds and cost-effective ones, comprising of even a few actives, were suggested effective for paddy weed control in Hokkaido, including two SU resistant biotypes.
