Herbicide resistance in black nightshade and Onehunga weed.

When black nightshade (Solanum nigrum) plants were poorly controlled by cyanazine in a Manawatu pea crop, seeds were collected from these plants and susceptibility of the resulting seedlings to various herbicides was compared in pot experiments with other black nightshade plants. The biotype was resistant to cyanazine, terbuthylazine, atrazine, prometryn and possibly also terbutryn. No damage occurred even when a 30-fold increase in the recommended application rate of terbuthylazine was applied. Pot trials were also conducted at Massey University to study the poor control of Onehunga weed (Soliva sessilis) by herbicides noticed in turf at Helensville. This trial showed the Helensville ecotype to be resistant to pyridine herbicides, resulting in no control by clopyralid, triclopyr, picloram/2,4-D and picloram/triclopyr mixtures. Strategies to control both resistant ecotypes are discussed.