CROSS-RESISTANCE OF SEVERAL DICLOFOP-RESISTANT WILD OAT (AVENA-FATUA)BIOTYPES FROM THE WILLAMETTE VALLEY OF OREGON

The first occurrences of wild oat resistance to diclofop in the Willam ette Valley of Oregon were reported in 1990. Among eight resistant bio types, GR50 values for diclofop were 3 to 64 times greater than the GR 50 for a susceptible wild oat biotype. GR50 Values for other aryloxyph enoxypropionate herbicides varied from 1 to over 100 times greater tha n a susceptible biotype. Only one resistant biotype was resistant to c yclohexanedione herbicides, and this was only a three-fold increase in GR50. Except for one biotype that had a low level of resistance to pr onamide, none of the wild oat biotypes were cross-resistant to any oth er commonly used wild oat herbicide. Levels of resistance and cross-re sistance did not follow a consistent pattern among biotypes in this st udy, suggesting more than one resistance trait. There were significant differences in the light use efficiency, height, dry weight, leaf are a, and extent and timing of tillering and flowering of four wild oat b iotypes studied. These physiological and morphological differences sug gest that these resistant biotypes were selected independently. The di versity of resistance patterns and the coevolution of resistance at se veral locations will add to the difficulty of resistance management.