Eradication of red witchweed on track

September 15, 2021

Early detection, a comprehensive surveillance and eradication program, and the willingness to experiment with new technologies means the Red Witchweed Eradication Program (the 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 over a 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 and Livestock Australia and Canegrowers funded the initial response until 2025.

The initial plan was independently reviewed in April 2019 and although it was technically feasible to eradicate red witchweed, it was found that the proposed time frame was too short. The NMG approved the review panel’s recommendations for an extended treatment period, redesigned 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 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 (QDAF).

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

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

Due to the excellent surveillance, seedling detection and destruction, and treatment activities undertaken by the Program team, the first of the 100 hectare quarantined paddocks 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 100’s of hectares every 8 – 10 days. Approximately 385 km is walked by the surveillance team every fortnight.

Joe Vitelli, Senior Scientist at QDAF 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 soybean 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 tines and rollers to spread the fumigant. This treatment also works to destroy the seedbank through suicidal germination. In areas where the weed has been detected but it is difficult to use soybean or ethylene, the granular fumigant Dazomet is used. The granules are applied to the surface of the soil, requiring 10 – 25 ml of rainfall within a few days of application to move it into the soil.

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 (based on the requirements) and applied through various means to eliminate the risk of the weed attaching to the grass and setting seed.

Since depletion of the seed bank is vital to eradication, the research team conducted a 48-month trial to test the effectiveness of the treatments in reducing seed viability.  After four annual applications of either ethylene (suicide germination), sorghum (true host), dazomet or soybean (false host) the viability of the red witchweed seed bank was reduced to 0%, 0.3%, 2% and 3 % respectively.

“Like most weeds, moisture is key to germination and the onset of the wet season in Mackay triggers germination,” explains Mr Bowditch. “Therefore, using weather forecasts in the form of Forewarned Forearmed (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. These measures are instrumental in managing the weed effectively and keeping the Program on track for eradication. This comprehensive approach is the key to success and often employed in eradication programs delivered in Australia, positioning them for a successful outcome and preparing us to respond in the future if needed.

*FWFA is producing prototype multi-week to seasonal climate forecasts aimed at supporting on-farm decision-making. The forewarned is forearmed project is supported by funding from the Australian Government Department of Agriculture, Water and the Environment as part of its Rural R&D for Profit program. Project research partners include the Bureau of Meteorology, South Australian Research and Development Institute, University of Melbourne, University of Southern Queensland, Birchip Cropping Group, Agriculture Victoria, Monash University and Queensland Department of Agriculture and Fisheries. Rural R&D Corporation partners: Meat & Livestock Australia, Dairy Australia, Wine Australia, Sugar Research Australia, Grains Research & Development Corporation, Agrifutures Australia, Cotton Research & Development Corporation and Australian Pork.