Extensive AHRI trial probes multiple resistant wild radish

Reduced sensitivity to HPPD (hydroxyphenylpyruvate dioxygenase) inhibitor herbicides in wild radish was reported earlier this year and followed previous concerns over several populations in Western Australia’s northern agricultural region.

“Wild radish is a real champion in evolving resistance to herbicides. Currently it is resistant to four herbicide modes of action,’’ said Weed Scientist with AHRI, Roberto Busi, based at the University of Western Australia.

“In optimal winter conditions in a greenhouse trial, we wanted to confirm the putative resistance to the important HPPD group of herbicides and assess the level of resistance to different herbicides, including new products in development that are expected to be available to growers in the next few years.’’

The trial compared responses to herbicides from a known susceptible wild radish population, as well as a suspected multiple resistant population from the northern WA wheatbelt. This population is resistant to Group B, I and H herbicides.

“It was a comprehensive trial looking at the efficacy of 55 different herbicide treatments in a stand-alone use pattern and in herbicide mixtures with up to four different active ingredients. It comprised most herbicide groups currently available for the control of wild radish, including Groups I, F and H,’’ Roberto said.

“We looked at how the resistance expressed at three distinct weed application stages, including pre-emergent, two-leaf and four to five-leaf wild radish. We also ran a dose response assay to understand the response of weeds to different dosages at the different weed stages.

“After a period of time when there was maximum efficacy, we assessed the survival of the weeds and the biomass produced.’’

He said overall, the trial showed one of the keys to effective wild radish control was applying herbicides when plants were small, at the two to three-leaf stage.

“This is when we see the efficacy is fully expressed for many herbicides – even herbicides that wild radish has a low level of resistance to.’’

“At four to five-leaf, wild radish is difficult to kill and when it’s a multiple resistant population, it is tough to kill. It is where resistance is expressed the strongest.’’

 

The Group H plus Group C herbicide, Velocity^® from Bayer, showed a strong benefit of applying when wild radish is small, at the two-leaf stage.

 

“Even at the lower rate of 500 mL/ha on both the susceptible and resistant populations, it provided virtually perfect control. At 670 mL/ha and 1 L/ha, the control was 100 per cent,’’ Roberto said.

 

“When Velocity was applied to four to five-leaf wild radish at 1 L/ha, control dropped from 100 per cent in the susceptible population to 70 per cent in the resistant population, yet the biomass (remaining growth) was still highly suppressed up to 90 per cent.

 

“Velocity has been around for a decade and has been the standard for control of wild radish. Applying early is the key. In perfect conditions, the recommended dose can fully control the resistant population.’’

 

Group H herbicide, Callisto®, was applied pre-emergent in the trial at the full rate of 200 mL/ha and while it was effective on the susceptible wild radish population, there was greater plant survival in the resistant population, suggesting a cross-resistance issue. There was 11 per cent survival and 25 per cent biomass recorded in the resistant population treatment.

 

One of the developmental products included in the trial was Mateno^® Complete from Bayer. Providing a new herbicide mode of action to Australian cereal growers, aclonifen, an SPS (solanesyl di-phosphate synthase) inhibitor, in a synergistic co-formulation with pyroxasulfone (Group K) and diflufenican (Group F) herbicides, its registration has been submitted to the Australian Pesticides and Veterinary Medicines Authority (APVMA) for review. At the time of publication, Mateno Complete is not registered.

 

“Applied on susceptible and resistant small radish at two to three-leaf, it provided 100 per cent control,’’ Roberto said.

 

“We also checked Mateno Complete in mixes to see how it performed. With MCPA LVE it worked well on susceptible and resistant populations, even if the resistant population was resistant to MCPA.

 

“With the new mode of action, we can see it can provide an additional tool to assist the control of difficult to kill, multiple resistant radish. It’s very promising. It’s good news really, given that resistance is never good news.’’

 

The trial also investigated whether cross-resistance exists within Group H herbicides, which included an application of Frequency^® at 200 mL/ha in combination with Bromicide^® 200 at 900 mL/ha.

 

“The mix was fully effective on susceptible wild radish at four to five-leaf, but on the resistant population, while plants were highly damaged, some 40 per cent were clearly surviving.’’

 

It was a similar result when Frequency at 200 mL/ha was applied in a mix with 440 mL/ha of MCPA LVE on susceptible and resistant populations of wild radish at four to five-leaf.

 

“While it was fully effective on susceptible wild radish, the resistant population is resistant to MCPA, so it was quite expected that some radish was able to survive,’’ Roberto said.