Australia has an impressive track record with biological control of environmental and pasture weeds.
In recent years, there has been a new focus on difficult-to-control weeds, such as flaxleaf fleabane, in grain production systems, with a promising fungus identified as having potential in controlling the damaging weed.
Ben Gooden, weed ecologist at CSIRO, Australia's national science agency, said flaxleaf fleabane was selected as a target for biological control due to its tolerance of and resistance to herbicide controls and its spread through the grain growing regions in response to minimum tillage farming practices, which have made it easier for the weed to establish.
"Introducing a biological control agent into Australia involves a rigorous process and a long lead time to ensure that the introduced agent is effective on the target and does not threaten other plant species in the Australian environment," Dr Gooden said.
"CSIRO started work on identifying potential agents against fleabane in 2017 and soon identified a rust fungus, Puccinia cnici-oleracei (ex. Conyza), native to Colombia, South America as a strong candidate for controlling flaxleaf fleabane in Australia."
In 2021, the fungus was approved for release from quarantine and a pilot field release trial was conducted in 2022 and 2023.
"When flaxleaf fleabane plants are infected with this rust pathogen, the plants produce 50 to 60 per cent fewer flower heads and, therefore, less seed than healthy plants."
"If the pathogen can be established under field conditions in Australia, it will take several years before there is a significant impact on flaxleaf fleabane populations.
"Other examples of successful biological control have typically involved multiple agents, so we are pursuing insects with biocontrol potential to help control fleabane in addition to the fungus."
Dr Gooden said biocontrol would play a key role in keeping overall fleabane populations down.
He said controlling flaxleaf fleabane populations in marginal areas adjacent to cropping paddocks with a non-herbicide tactic like biocontrol will potentially reduce the amount of seed produced and available to infest adjacent cropping areas.
While the fungus has shown promise there are some matters to work through to ensure maximum efficacy.
The pilot release program in 2022-23 highlighted some practical issues that need to be overcome to ensure a successful establishment and succession of Puccinia cnici-oleracei in various seasonal and environmental contexts in Australia.
Like other rust pathogens, P. cnici-oleracei infects host plants readily under humid conditions during sporulation, or the period when it develops spores.
Research has also shown that young seedlings are more susceptible to infection than mature plants.
When monitoring infection levels, it is necessary to observe plants in the release area within the first two to four weeks, before the host leaves die off and are shed from the plant.
The fungus is also specific to flaxleaf fleabane and does not infect related species such as tall fleabane or Canadian fleabane.
As well as the fungus, researchers are looking for other biocontrols.
To date, they have identified three insect candidates - a stem-boring weevil (Lixus caudiger), a leaf-galling midge (Asteromyia modesta), and a stem-gall-forming tephritid fly (Trupanea bonariensis).
Colonies of these insects have been established, and biology studies and preliminary host specificity testing have been undertaken.
These insect colonies are ready for quarantine host testing in Australia if resources become available for ongoing research.
Flaxleaf fleabane is not the only problem weed suited to biocontrol.
Saffron thistle, a grain contaminant, that can reduce grain yields by 10 per cent or more may be susceptible to a rosette crown feeding fly (Botanophila turcica) as a potential biocontrol agent.
The grain industry also continues to benefit from the successful biocontrol of the weed parthenium, nine insects and two rust pathogens, and skeleton weed, a rust pathogen and two insects.
The cost-benefit analysis of the skeleton weed biocontrol program estimated a $112 return for every $1 spent on research.
Both of these biocontrol programs have been sustained since the 1970s and clearly demonstrate the long-term nature of biocontrol, along with the impact of long periods of drought on insect populations and pathogen inoculum in the environment.
The pursuit of a non-chemical strategy to reduce weed numbers fits in with the work done by WeedSmart.
WeedSmart is a Grains Research and Development Corporation (GRDC) backed industry body dedicated to delivering science-backed weed control solutions to enhance on-farm practices and promote the long term, sustainable use of herbicides in Australian agriculture.
The WeedSmart 'Big 6' strategy embraces non-herbicide weed control tactics and diverse cropping systems to keep weed numbers low in cropping systems.
