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Eradication of red witchweed on track

A red witchweed (Striga asiatica) plant.
Photo: Plant Health Australia

Early detection, a comprehensive surveillance and eradication program, and willingness to experiment with new technologies means the red witchweed eradication program is on track to eradicate this invasive plant from the only known infestation in Australia, near Mackay in Queensland.

Red witchweed (Striga asiatica) is a root parasitic weed that causes significant yield losses in sorghum, maize, rice, sugarcane and several other tropical grasses. Under the right conditions, the microscopic seeds germinate and attach to the roots of the host plant, emerging from below the ground to flower. It can produce more than one million seeds per square metre and the seeds are able to survive underground for up to 15 years.

If red witchweed was to spread from Mackay to other Australian cropping areas, it could result in significant market access issues for grain producers participating in lucrative overseas markets.

A containment and eradication plan was endorsed by the National Management Group (NMG) in June 2015. The Australian, Queensland, New South Wales and Northern Territory governments and industry partners including Grain Producers Australia, Meat & Livestock Australia and Canegrowers funded the initial response until 2025.

The initial plan was independently reviewed in April 2019 and, although it was concluded that eradication of red witchweed remained technically feasible, it was found that the proposed timeframe was too short. The NMG approved the review panel’s recommendations for an extended treatment period, redesigned the post-treatment phase and increased resourcing.

“The combination of a dedicated surveillance team, innovative research and targeted eradication and fumigation were invaluable to the program’s success. Employing false host plants and using the latest weather prediction technologies in the form of (GRDC co-invested ) Forewarned is Forearmed (FWFA) further increased the program’s effectiveness. This puts us in the box seat for eradicating red witchweed in Australia,” says senior project officer Tom Bowditch, from the Queensland Department of Agriculture and Fisheries (DAF).

If red witchweed is eradicated from Australia, it will be the first country in the world to achieve this. The US has spent more than $260 million over 64 years trying to eradicate the weed, while in Africa it is estimated that losses to farmers from Striga species amount to $7 billion annually.

Intensive surveillance

Red witchweed was first detected on a sugarcane property near Mackay in July 2013. After extensive delimitation surveillance, eight properties were placed under quarantine orders. The properties incorporated 538 hectares, of which 100ha was quarantined, with all cane removed and intensive surveillance and treatment activities conducted under a response plan.

Due to the surveillance, seedling detection and destruction, and treatment activities undertaken by the program team, the first of the quarantined paddocks within the 100ha area were released from quarantine in June 2020, and the first entire property released from quarantine in July 2021.

Successful eradication relies on plants being controlled prior to reaching reproductive maturity; however, red witchweed is extremely difficult to detect without flowers and it only takes two weeks between flowering and setting seed.

Regular surveillance to detect and destroy the weed prior to seeding is an extensive undertaking. It involves inspecting hundreds of hectares every eight to 10  days. The surveillance team walk approximately 385 kilometres each fortnight.

DAF senior scientist Joe Vitelli leads a team of three researchers looking into the biology and control of the plant, developing a three-pronged approach to defeating it.

This includes using host crops. A false host crop such as soybeans exudes a stimulant to trigger the red witchweed to germinate but does not allow the weed to attach or grow, which results in it dying (also known as suicide germination). A true host crop (such as corn or sorghum) is planted and then stringently monitored to detect any red witchweed germinations, which are then quickly destroyed. This technique is used to entice germination and reduce the seedbank within the soil.

Fumigation also plays an important role in the eradication program. Ethylene, a hydrocarbon gas, is injected by using a specifically designed tractor-mounted delivery system with tynes and rollers to spread the fumigant. This treatment also works to destroy the seedbank through suicide germination.

The third prong is to use herbicides to kill potential host crops such as tropical grasses within the soybean false host crops. These can be either pre or post-emergent herbicides and applied through various means to eliminate the risk of the weed attaching to the grass and setting seed.

Application of four of these various treatments annually in a 48-month trial reduced the red witchweed seedbank levels to between zero and three per cent.

“Like most weeds, moisture is key to germination and the onset of the wet season in Mackay triggers germination,” Mr Bowditch says.

“Therefore, using weather forecasts in the form of FWFA has increased the effectiveness of the program, kept the surveillance team safe in the hot weather, and assisted in predicting germination and treatment schedules.

“The program is unique in that it has been complemented by investment in simultaneous research, quarantining and delimitation activities to battle the weed. This allowed researchers to investigate several control options and understand the weed’s biology, while decommissioning infected paddocks and launching an intensive surveillance regime.”

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