Resistance to the Group I herbicide 2,4-D in sowthistle has been confirmed in South Australia.

The development is of great concern to the nation’s cropping industry as sowthistle, also known as milkweed or milk thistle (Sonchus oleraceus), has become an increasingly difficult weed to manage in no-till crops and fallow.

It is a problematic weed due to its ability to germinate on the soil surface and having populations already resistant to Group B herbicides and glyphosate.

A major weed of cropping systems north of Dubbo in New South Wales for more than 15 years, sowthistle is now considered a significant emerging weed in southern and western farming systems and will be a focus of the Grains Research and Development Corporation’s project on the evolution mechanisms and inheritance of herbicide resistance being led by Dr Chris Preston who is based at the University of Adelaide.

Dr Preston said recent testing of five sowthistle populations from SA had confirmed three populations resistant to solid rates of 2,4-D.

“These populations come from irrigation areas in South Australia’s South-East and are resistant to the equivalent of 2.2 L/ha 500 g/L 2,4-D,” said Dr Preston.

“This is another world first and highlights the need for growers to be using diverse weed management tactics to prevent ongoing development of resistance to our most important herbicides.”

Across Australia the majority of sowthistle populations are resistant to Group B herbicides such as chlorsulfuron and metsulfuron. Three cases of glyphosate resistance have been confirmed in northern NSW, with more suspected populations currently undergoing testing.

The three populations with resistance to 2,4-D showed typical 2,4-D symptoms within days of herbicide application and were severely damaged by the herbicide, but within two weeks of treatment grew new leaves that were normal in shape. More than half the plants survived treatment.

“The plants were treated at the 4-6 leaf stage and were quite small,” Dr Preston said. “As sowthistle grows larger it can be more difficult to control with 2,4-D. 2,4-D would be even less effective on large plants from the resistant populations.”

Dr Preston said mixtures with 2,4-D and other Group I herbicides were still likely to be effective at controlling these populations, provided there was a robust rate of the mixing partner and the herbicide was applied to small seedlings.

“Otherwise alternative herbicides will have to be used to control the resistant populations. Apart from Group B and Group I herbicides, there are few herbicides that can be used to control established sowthistle in crops, so attacking it early will be key.”

Rain in late winter and spring allows widespread establishment of sowthistle in winter crops and stubbles, often after the main weed spray timing. Plants can survive through summer, continually producing fertile seed as long as there is sufficient soil moisture.

Waiting for a post-winter crop harvest opportunity often reduces the level of weed control due to large plant size and moisture and temperature stress.

Seed of sowthistle can be spread by wind, stock fodder, seed grain and machinery.

For more information on better management of sowthistle go to the GRDC Integrated Weed Management Hub, and the GRDC Summer Fallow Weed Management Manual.



For information on herbicide sustainability, visit the WeedSmart information hub

ENDS

Sharon Watt, Porter Novelli

0409 675100

Caption: Resistant (left) and susceptible (right) populations of sowthistle following treatment with 2,4-D. Photo: C Preston

Caption: Sowthistle in canola stubble, southern Western Australia. Photo: AM Storrie

GRDC Project Codes: UA00124, UA00149, UA00158

GRDC Project Code UA00124, UA00149, UA00158

Region South, North