Fleabane now a national challenge
Integrated weed management is the key to managing flaxleaf fleabane, which is emerging as a problem weed across southern Australia
Flaxleaf fleabane is a major weed in dryland crops in southern Queensland and northern New South Wales, and is emerging as a problem weed across the entire cereal-cropping belt of southern Australia.
Previously, fleabane was found mainly on roadsides, particularly where council use of glyphosate created bare ground on which the weed could flourish without competition. However, the weed is highly mobile and soon found its way into adjacent cropping systems.
With the move to minimum tillage and the increasing use of glyphosate, the scene was set for an expansion of the troublesome weed. Wet summers in southern grain regions over the past two years have aided the weed’s spread.
Fallow weed control costs have increased markedly because of fleabane, with some zero-till growers having to reintroduce cultivation as a last-resort control tactic.
Disturbingly, populations of fleabane have recently been confirmed as resistant to eight times the normal rate of glyphosate – earning fleabane the title of Australia’s first glyphosate-resistant broadleaf weed.
The resistant populations were found in zero-tilled or minimum-tilled paddocks in southern Queensland and northern NSW. In addition, glyphosate-resistant populations were identified along roadsides near these cropping paddocks.
On the other side of the continent, a survey in Western Australia initially found fleabane following the typical pattern of occupying roadsides and fencelines, except for some incursion into crops in the Esperance district. But now, GRDC-funded researcher Dr Sally Peltzer, from the Department of Agriculture and Food, WA, says fleabane is becoming an increasing problem across the wheatbelt.
In other countries, fleabane has developed resistance to herbicides from Groups B, C, L and M, with some populations resistant to multiple chemicals.
Herbicide resistance has the potential to spread due to the weed’s highly mobile seed. Research in WA found that fleabane seed could travel on the wind up to 800 metres from its parent plant.
While fleabane presents a serious and costly weed challenge, GRDC-funded research has shown that a strategic approach using integrated weed management (IWM) can significantly reduce the weed’s impact on crop production.
University of Queensland researcher Dr Steve Walker says the key to getting on top of fleabane is to attack all parts of the weed’s life cycle to keep the seedbank low. Adopting an IWM strategy, which includes chemical and non-chemical tactics, will result in substantially fewer fleabane problems and resistant populations in subsequent seasons (www.qaafi.uq.edu.au/content/Documents/weeds/IWM-Fleabane-guide.pdf).
With the capacity to produce two or three generations each year and 110,000 seeds per plant, controlling fleabane before it sets seed is critical.
In southern and western Australia, fleabane often germinates under crops during spring or at harvest. Following harvest, a lack of crop competition combined with summer rain can cause rapid weed growth. By the time there is a window for control, the fleabane plants are often mature, with a large root system, a reduced leaf area and a high tolerance to most herbicides.
In northern NSW and southern Queensland, fleabane is a major weed of winter and summer cropping. It germinates either just before or after the crop is sown, competing strongly if left uncontrolled.
Research across Australia indicates that hitting the weed with herbicide while it is young and actively growing is the best approach. Conversely, delaying herbicide application until the weed is mature and water-stressed can result in poor control.
The ‘double-knock’ approach, with glyphosate followed by paraquat, has proved a critical component of a fleabane IWM program.
This approach, coupled with the use of competitive crops and pastures and strategic cultivations to bury ‘blow-outs’ of seed production, can reduce the weed’s seedbank to manageable levels within a few seasons. It is also important to target fencelines and roadsides.
New GRDC-funded research in NSW, led by Dr Hanwen Wu, from the NSW Department of Primary Industries, aims to identify residual herbicides for fleabane control in wheat and canola to overcome the common problem of growers tackling mature fleabane plants during the fallow.
Alternative herbicide chemistries for controlling fleabane in fallows are also being investigated to allow more options for a double-knock approach (www.qaafi.uq.edu.au/content/Documents/weeds/Controlling-flaxleaf-fleabane-2.pdf).
This would reduce the pressure on the current approach, which heavily relies on glyphosate and paraquat.
Dr Michael Widderick, Senior research scientist weeds, Agri-Science Queensland,
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