By: Kate Glastonbury, New South Wales Grains Biosecurity Officer and Marianne Coquilleau, Plant Health Australia
Breaking the green bridge is a critical step in reducing pest and disease risk before sowing begins. By managing volunteer growth and stubble, and staying informed about soil health and pathogen load, growers can make better decisions and maximise the return on every sowing opportunity.
Green bridge control helps protect crops by removing the plants that allow pests and diseases to survive, multiply and carry over between cropping seasons. If left unmanaged, volunteer plants and weeds not only increase disease and insect pressure but also compete for soil moisture and nutrients needed to establish crops.
Stubble management plays an equally important role in balancing productivity and biosecurity. While retained stubble delivers significant soil health benefits, including improved moisture retention and reduced erosion, it can also harbour pests and stubble-borne diseases if not carefully managed.
What is a green bridge?
A green bridge refers to any plant material that grows between cropping seasons which acts as a habitat for pests and diseases during the fallow period. These include volunteer crops (plants that germinate unintentionally) and weeds that allow pathogens and insects to survive and multiply until the next crop is sown. Green bridges can occur within cropping paddocks, along fence lines, roadsides and beyond paddock boundaries.
Why is it important to control?
Controlling the green bridge removes the green host plants that allow pests and diseases to carry over from one season to the next. Diseases such as stem, stripe and leaf rust and a range of viruses can persist, while insect pests such as aphids, slugs and snails are provided with a food source and shelter during the fallow period.
The green bridge also removes valuable soil moisture and nutrients that would otherwise be available to the next crop. Research has identified that yield losses between 0.5–1.0 tonne per hectare occur in cereals where the green bridge remains uncontrolled ahead of sowing. Researchers in Western Australia (WA) and New South Wales (NSW) have also identified that removing weeds early preserves between 50 – 75 millimetres of soil moisture[1].
How can I control it?
Effective green bridge control requires all volunteer plants to be completely dead at least four weeks before sowing. Control methods include herbicide application, ensuring products are used according to label directions and only within property boundaries, or tillage, noting that cultivation may also reduce stored soil moisture.
Green bridge management is most effective when approached as a community effort. Even if the green bridge is not present within your own paddocks, pests and diseases can still build up nearby and pose a risk to your crop. Coordinating control efforts with neighbours can significantly reduce regional pest and disease pressure.
What about stubble?
Unlike the green bridge, crop stubble left standing in the field after harvest can provide valuable benefits. Retaining crop stubble improves soil moisture retention, increases build-up of organic carbon, reduces top-soil erosion, and is particularly useful in dry regions or during dry seasons.
However, stubble retention requires careful management to ensure these benefits are realised. Decisions around how much stubble to retain, which crop or variety to plant next, and potential impacts on sowing and harvest efficiency all need to be considered. Understanding the pest and pathogen load carried in stubble and crop crowns is also critical for effective planning.
What are the risks of stubble?
Retained stubble can increase shelter for pests like snails, slugs and earwigs. It can also be a host of stubble-borne diseases.
In cereals, crown rot survives in crop residues and stubble and can be redistributed across paddocks during harvest, particularly when followed by a low-harvested break crop such as chickpeas[2]. While traditionally considered a northern issue, crown rot is present as far south as northern Victoria, where field trials are currently underway. (GRDC Project Code: CUR2302-002RTX).
Stubble-borne foliar diseases also remain a concern. In wheat, Septoria tritici blotch (STB) continues to pose a risk, while in barley, scald (Rhynchosporium secalis) increased in prevalence across Western Australia in 2025, including in varieties previously considered resistant. Net form and spot form net blotch (NFNB, SFNB) can survive on infected barley seed and stubble, as well as on volunteer plants within green bridges Researchers detected NFNB samples resistant to all three major fungicide groups in 2025[3].
Sowing canola back into canola stubble also increases blackleg infection[4].
Good management helps with biosecurity preparedness
Active green bridge control combined with proactive stubble management helps reduce the risk of both endemic and exotic pests and diseases. Managing retained stubble can reduce shelter for slugs and snails, while breaking the green bridge disrupts disease survival between seasons, strengthening on-farm biosecurity preparedness.
How to test the disease load in the paddock
Accurate, paddock-specific information on soil and stubble-borne pathogens can be obtained through the PREDICTA® B DNA-based soil testing service, available through the South Australian Research and Development Institute (SARDI). Accredited agronomists can collect and submit soil samples for testing. The lab processes soil samples weekly during the critical period from February to June and less regularly the rest of the year. A range of pathogens are tested for, including nematodes, crown rot, Rhizoctonia and more, and the results provide information on the soil’s pathogen composition and load ahead of crop selection and sowing. For more information, visit the Department of Primary Industries and Regions of South Australia (PIRSA) website[5].
Stubble testing is also available through an NSW Department of Primary Industries project invested in by GRDC, providing additional insights into disease carryover risks. For more information on the project, contact Sabita@riverineplains.org.au.
How to manage heavy stubble and reduce stubble-borne diseases
Mulching, baling or grazing reduces heavy stubble loads after harvest which also helps reduce spore load by bringing the stubble into greater contact with the soil. Burning can also be used as a last resort since it reduces soil carbon, water and nutrient capacity years after occurrence[6].
Crop rotation also plays a key role in reducing inoculum levels over time while delivering broader soil health benefits. The presence of SFNB in barley stubble can decline by 60 to 80 per cent over the two years following the harvest[7] when host plants are effectively managed. Further, selecting resistant varieties not only limits yield loss and disease impact, but also results in stubble with lower infection potential.
Soil health data across your property can help make informed decisions around green bridge control, stubble management, crop rotation and variety selection, helping ensure the best possible start to the sowing season.
If you or your agronomist notice anything unusual, such as a new pest or disease on varieties thought to be resistant, contact your state agriculture department or call the Exotic Plant Pest Hotline on 1800 084 881.
Visit the websites below for information and resources
For further information and support, contact your local GBO.
The Grains Farm Biosecurity Program is an initiative to improve the management of, and preparedness for, biosecurity risks in the grains
industry at both farm and industry levels.
Launched in 2007, it is managed by Plant Health Australia (PHA) and funded by growers through Grain Producers Australia (GPA), in partnership with the governments of New South Wales, Queensland, South Australia, Victoria and Western Australia.
References
[2] Simpfendorfer S (2025). Fusarium crown rot incidence and inoculum dynamics within cereal stubble learnings from northern farming systems GRDC Update Papers, July 2025.
[3] Triple fungicide resistance detected in Yorke Peninsula barley net form net blotch. GRDC.
[4] Blackleg management guide – autumn 2025. GRDC.
[5] PIRSA, Predicta B (broadacre).
[6] Agriculture Victoria, Managing stubble.
[7] Bradley J, Thomas G, Hills A, Chambers K, Hoque Z, Donovan D, Donovan C, Kidd J & Zhang B (2025). Spot form net blotch management in low rainfall areas – five years of research. GRDC Update Papers, February 2025.