Green bridge increases risk of early onset of fungal and virus leaf diseases, soilborne pathogens and pests

Volunteer cereal plants
Volunteer cereal plants forming a green bridge. Photo courtesy of: DPIRD.

Growers and consultants are urged to identify what fungal, viral or soil pathogens are lurking in their green bridge and manage their cropping programs accordingly this season. If you are unsure of what you are finding, use the PestFax Reporter app to send a report (preferably with good photos) to the PestFax team requesting identification.

DPIRD staff are conducting green bridge surveys in some areas but due to the widespread nature of the regrowth this year your assistance with reporting any disease, virus and insect finds with the PestFax Reporter app would be particularly welcome.

Significant rainfall experienced by some growers in the eastern, central and southern wheatbelt in early March has resulted in weeds and crop volunteers germinating in paddocks and along roadsides. Follow up rain is keeping it alive in many areas. This vegetation can serve as a ‘green bridge’ for diseases which need a living plant to survive, such as rusts, viruses and their vectors, and powdery mildew.

These weed and crop volunteers can also act as a between-season host for root diseases and invertebrate pests such as aphids (several species including Russian wheat aphid), caterpillars (such as diamondback moths) and mites such as bryobia mite and wheat curl mite (which is a vector of wheat streak mosaic virus).

If weeds and/or volunteers are present at the start of the new cropping season, particularly in or adjacent to cropping paddocks, there is a greater risk of spread of pests and diseases to newly emerging crops.

Autumn regrowth/green bridge risks include:

  • Foliar diseases
    • Cereal volunteers significantly increase the chances of outbreaks of foliar diseases such as rusts and powdery mildew during the cropping season.
  • Viruses
  • Soil-borne pests and diseases
    • Major soilborne fungal diseases and nematode pests that impact our broadacre cropping areas (plant parasitic nematodescrown rotrhizoctonia and take-all) are all favoured by the moist soil conditions that occur after summer/early autumn rainfall events. They can readily multiply on roots and crowns of cereal volunteers and weeds, then transfer directly to emerging crop seedlings if there is no ‘fallow’ period between death of sprayed autumn volunteers and sowing of this year’s crop. Rhizoctonia inoculum will decline during summer/autumn rains in the absence of any grass weed hosts, but will survive dry summers.

Disease management strategies

Growers need to eradicate weeds and crop volunteers, including those along fencelines, prior to the start of the cropping program to reduce potential pest and disease outbreaks. Ideally there needs to be a break (a fallow period) of at least four weeks prior to sowing. To achieve this, the weeds and volunteers should be sprayed with herbicides at least 4-6 weeks before sowing, to ensure weeds are completely dead.

Alternatives to herbicide application are to graze heavily or cultivate weeds and crop volunteers which will reduce their potential as a ‘green bridge’ or host of diseases and pests. Cultivation may actually increase crown rot as the fungus survives in stubble pieces for several years, so ensure that the disease is diagnosed prior to cultivating.

When managing the green bridge growers need to consider retaining at least 50% ground cover to reduce the risk of erosion during crop establishment. Especially on susceptible soils that are prone to erosion.

In addition to managing the green bridge, to reduce risk of early diseases, growers can utilise an integrated disease management approach including;

  • Sow clean seed.
  • Know the latest disease ratings of your varieties and plan accordingly. Use adult plant resistant varieties. For more variety details refer to DPIRD’s 2021 Western Australian Crop Sowing Guide page.
  • Consider applying in-furrow or seed dressing fungicides to reduce your risk of early infection of rusts and powdery mildew in susceptible varieties and rhizoctonia, crown rot, take-all, smuts and bunts. For more information refer to Seed dressings and in-furrow fungicides for cereals in Western Australia.
  • In case of early disease outbreak, budget for early foliar fungicide sprays where upfront fungicides are not used. For more information refer to Registered foliar fungicides for cereals in Western Australia.
  • Stubble management. Reduce exposure to stubble through rotation and careful paddock planning (to avoid sowing on or adjacent to infected stubble) or stubble management (such as grazing, windrowing, baling, incorporating or burning). Stubble retention can be very useful in seasons where aphids/virus are high risk.
  • Earlier sown crops may be more at risk of some foliar diseases such as powdery mildew, so consider later sowing of susceptible varieties/at risk paddocks or at least plan to monitor earliest sown paddocks closely for disease.

There are a range of decision support tools available that can aid growers in their early disease management decision making. A BlacklegCM for canola webinar is being held on Wednesday 31 March at 8.30am AWST (to attend this webinar register here). There are also spore release predictions for all WA districts, such as blackleg for canola and blackspot for field peas. Forecasts for these will begin weekly in April, refer to DPIRD's Crop diseases: forecasts and management page.

For more information see the department’s Control of green bridge for pest and disease management page and Grains Research and Development Corporation's (GRDC) Green Bridge fact sheet.

For further disease forecasts and information on managing specific diseases refer to DPIRD's Crop diseases: forecasts and management.

For more information contact South Perth research officers Benjamin Congdon (viruses) on +61 (0)8 9368 3499, Geoff Thomas (foliar diseases) on +61(0)8 9368 3262, Sarah Collins (nematodes) on +61(0)8 9368 3612 or Daniel Huberli (root diseases) +61 (0)8 9368 3836.


Article author: Cindy Webster (DPIRD Narrogin)

Article input: Andrea Hills (Esperance), Benjamin Congdon (DPIRD South Perth), Ciara Beard (Geraldton), Daniel Huberli (DPIRD South Perth), Janette Pratt (DPIRD Moora) and Svetlana Micic (DPIRD Albany).