Wild radish resistance of concern

Wild radish resistance of concern

Cropping
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GLOBALLY, resistance to the world’s oldest herbicide is relatively rare.

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Group I resistance has also been recorded in wild radish in SA, Vic and NSW.

GLOBALLY, resistance to the world’s oldest herbicide is relatively rare.

Unfortunately, one of the economically significant examples of auxin (Group I – eg. 2,4-D, dicamba and MCPA) resistance is in wild radish populations in WA.

INSIGHT: AHRI's Roberto Busi says researchers are working to better understand the way auxin herbicides work and the mechanisms plants use to evolve resistance.

INSIGHT: AHRI's Roberto Busi says researchers are working to better understand the way auxin herbicides work and the mechanisms plants use to evolve resistance.

Australian Herbicide Resistance Initiative research fellow Roberto Busi says the recent and future release of new crops with resistance to synthetic auxins (United States only) will heighten the risk unless stewardship measures are followed.  

“The synthetic auxin herbicides have been in widespread use since their discovery in the 1940s,” he said.

“According to a 2014 Dow AgroSciences report, herbicides such as 2,4-D and dicamba are used on about 200 million hectares worldwide.

“They mimic the activity of natural plant hormones and seem to have multiple sites of action along with both physiological and biochemical effects that lead to impaired plant growth and death. The rate of resistance evolution has been lower than other herbicide modes of action.” 

Dr Busi said 2,4-D-resistant wild radish was first documented in WA in 1999.

“Since then, surveys have revealed some populations with high-level 2,4-D resistance and a total of 60 per cent of randomly surveyed samples in 2003 that has levelled out at about 74pc in both 2010 and 2015,” he said.

Multiple resistance to at least two herbicides is common in wild radish populations in WA, generally to chlorsulfuron and 2,4-D, but the last survey also showed alarming levels of resistance to diflufenican.

Resistance to atrazine, glyphosate or Group H herbicides remains rare.

“WA farmers have been able to find other ways to reduce the overall number of wild radish plants in their crops by running down the soil seedbank and this is the correct way to overcome resistance,” he said.

“If auxin herbicide use were to increase or be used more frequently in Australian cropping rotations growers would need to implement non-herbicidal strategies that reduce the risk of herbicide resistance.”

Group I resistance has also been recorded in wild radish in SA, Vic and NSW and in other species including Indian hedge mustard, sowthistle and capeweed.

Why did wild radish in WA evolve 2,4-D resistance when it is so rare globally?

Short answer: In the northern WA wheatbelt, the repeated use of Group B and 2,4-D in wild radish-infested paddocks rapidly increased the levels of resistance.

Longer answer: Growers and agronomists are well aware that simply swapping from one herbicide to another does not help manage herbicide resistance.

This lesson was learnt the hard way in WA where 2,4-D initially provided excellent control of Group B-resistant wild radish. Failure ensued because 2,4-D was applied frequently to dense populations of wild radish – a species that is a prolific seeder and the seed can persist in the soil for many years.

In two study populations, it was found that 2,4-D resistance was clearly inherited as a single, nuclear (pollen transmitted) dominant or near-dominant gene trait. This contributed to the speed of resistance in wild radish.

In other species, the traits conferring 2,4-D resistance may be less frequent or be less genetically dominant, and that slows the evolution of resistance.

Are 2,4-D resistant plants less fit than susceptible plants?

Short answer: Following a 2,4-D application, surviving weeds are less competitive.

Longer answer: True fitness penalty has not been established for 2,4-D resistant weeds. A ‘fitness penalty’ is when a resistant plant, growing in the absence of the herbicide is less fit than a susceptible plant.

AHRI research has demonstrated that when 2,4-D resistant plants are sprayed with 2,4-D they are suppressed, show strong symptoms of herbicide damage, but do not die.

But if these plants are sprayed when they are small and grow in competition with a crop, the resistant weeds may die or may produce far less seed than it would in a non-competitive situation.

Crop competition can therefore be used as a weapon against 2,4-D resistant weeds, while continuing to use 2,4-D or MCPA as part of a tank mix. 

How can tank mixes extend the use of herbicides that weeds are resistant to?

Short answer: Multiple resistance to auxin herbicides and other MOA is rare in individual plants. Focus on low seedbanks, use herbicide mixes and mix it up with non-herbicide strategies to keep ahead of resistance.

Longer answer: The most important objective is to aim for low weed numbers.

In this situation, while a population of wild radish in a paddock may be resistant to a number of herbicide MOA, the chance of an individual plant being resistant to more than one MOA is very low.

In plants such as wild radish that are cross-pollinated, there is mixing of genetic material so stacking of resistant traits is initially uncommon, particularly for resistance traits that are recessive.

Consequently, if a tank mix of three compatible herbicides with different MOA is applied, with each herbicide at robust label rates, an individual plant is likely to be susceptible to at least one of the herbicides applied and so overall plant numbers are reduced through the use of the tank mix.  

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