RESEARCHERS from the Australian Herbicide Resistance Initiative (AHRI) are concerned at findings of wild radish populations with signs of resistance to Group H herbicides.
Reduced sensitivity to Group H 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.
Combined with its small seed that can lie dormant for years, it is a package that makes the brassica weed one of Australia's most problematic.
Group H herbicides, such as Bayer's Balance and Velocity, have been an important means of ensuring reliable wild radish control.
However, in good news for growers, the AHRI work found that with timely application, even resistant wild radish was better controlled.
"Wild radish is a real champion in evolving resistance to herbicides," said Weed Scientist with AHRI, Roberto Busi, based at the University of Western Australia.
"Currently it is resistant to four herbicide modes of action,'' Dr Busi said.
AHRI has been testing wild radish populations to see the extent of the resistance and how it impacted plants.
"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," he said.
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 resistant 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," Dr Busi said.
"It comprised most herbicide groups currently available for the control of wild radish, including Groups I, F and H."
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," he said.
"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 herbicide Velocity which features both Group H and Group C modes of action, showed strong performances when applied when wild radish was smaller.
"Even at the lower rate of 500 mL/ha on both the susceptible and resistant populations, it provided virtually perfect control," Dr Busi said.
"At 670 mL/ha and 1 L/ha, the control was 100 per cent.''
This compares to Velocity, which, when 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."
One of the developmental products included in the trial was Mateno, a Bayer product featuring a new mode of action, aclonifen, an SPS (solanesyl di-phosphate synthase) inhibitor, in a synergistic co-formulation with pyroxasulfone (Group K) and diflufenican (Group F) herbicides.
Materno has been submitted to the Australian Pesticides and Veterinary Medicines Authority (APVMA) for review, but is not yet registered.
Dr Busi said trial results were promising.
"Applied on susceptible and resistant small radish at two to three-leaf, it provided 100 per cent control,'' he said.
"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 and good news given that resistance is never good news.''
AHRI worked in partnership with Bayer on the project.
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