A quicktest using seeds for detecting dicamba resistance in fathen (Chenopodium album)

A quick test was developed using seeds in petri dishes to detect resistance to dicamba in fathen (Chenopodium album L.) which has developed in some maize fields in Waikato, New Zealand. Seeds were collected from four Waikato maize fields (populations A, B, L and M) where dicamba has been applied for many years, and also three sites (populations C, P and Y) where dicamba was unlikely to have been used. Following a dormancy-breaking procedure, seeds of each population were germinated in petri dishes containing a range of dicamba concentrations from 0.02 to 0.32 mg L-1. The length of seedling hypocotyls and radicles was measured 14 days later, and these were found to be significantly shorter at most dicamba concentrations for five of the populations (A, B, C, P and Y) compared with the seedlings from populations L and M at the same concentrations. Dose response curves estimated populations L and M were 22 and 48 times more resistant, respectively, than the other five populations. The seven populations were also grown in pots in two separate greenhouse experiments, sprayed with a range of dicamba rates after 1 month then shoot weight was measured 7 weeks later. The greenhouse experiments confirmed that populations L and M were the only two dicamba resistant populations. The petri dish test was much quicker and has seldom been used previously for auxinic herbicides but over-estimated the levels of resistance which were approximately 7-fold and 19-fold for populations L and M respectively in the greenhouse experiments.