Yorke Peninsula barley growers should review their disease management practices after locally collected net form net blotch (NFNB) fungus samples showed combined resistance to Groups 3, 7 and 11 fungicides.

Key points

• Laboratory testing has confirmed that NFNB samples collected from the Yorke Peninsula, South Australia, and from a site in Streatham Victoria, contained genetic mutations associated with resistance or reduced sensitivity to all three currently available fungicide groups used to control net form net blotch (NFNB): Group 3 (DMI), Group 7 (SDHI) and Group 11 (QoI).
• Growers must use agronomic disease management tactics to control NFNB and limit the build-up of these fungicide-resistant mutations.
• Fungicide-resistance mutations can spread rapidly and are likely to already be present in other areas, making practices that prevent their spread imperative.

How and where the resistance was detected

In 2024, South Australian Research and Development Institute (SARDI) research scientist Dr Tara Garrard collected diseased barley samples from paddocks in the Warooka, Corny Point and Paskeville districts across South Australia’s Yorke Peninsula.

As part of a Grains Research and Development Corporation investment, the Centre for Crop and Disease Management (CCDM) analysed the samples affected by net form net blotch (NFNB) for fungicide-resistance mutations. They confirmed the presence of resistance to the Group 11 (QoI) fungicide azoxystrobin.

They also detected mutations associated with resistance to Group 3 and Group 7 fungicides in the same samples. These ‘triple mutants’ have been connected to at least one case of fungicide failure in the paddock.

The results suggest that Group 3 (DMI), Group 7 (SDHI) and Group 11 (QoI) fungicides might all be compromised for NFNB control in South Australia. Growers in SA must employ agronomic disease management tactics to avoid encouraging further proliferation of these resistant strains of the fungus that causes NFNB.

A sample from Streatham in Victoria also contained the triple mutations, but disease control is not currently compromised at that site. Growers in Victoria need to remain aware of the potential for fungicide resistance and reduced sensitivity developing in their region.

Growers should report any lack of disease control to their agronomist, state government pathologist or directly to the CCDM.

Failures affect different locations, cultivars and treatments

The resistant Warooka Point samples were collected from a paddock of Zena barley where treatments of epoxiconazole (Group 3, DMI), plus mixtures of cyproconazole + azoxystrobin and propiconazole + azoxystrobin (DMI + Group 11 QoI) had a limited effect on NFNB.

The resistant Corny Point samples were collected from two separate paddocks: one where Zena CL had been sprayed with a cyproconazole + azoxystrobin (DMI + QoI) mixture; and one where RGT Planet seed had been treated with fluxapyroxad (Group 7, SDHI) before the crop was treated with a cyproconazole + azoxystrobin (DMI and QoI) mixture. NFNB was not controlled in either paddock.

Triple mutants were also identified from a National Variety Trials site in Paskeville, growing Zena CL. The effect on disease control at this site is unknown, but growers in the area should be aware of the potential for fungicide resistance developing.

Emerging Group 11 resistance reveals risk factors

Group 11 fungicides, also known as quinone outside inhibitors (QoI), are single-site actives that include the fungicides azoxystrobin and pyraclostrobin.

The QoI site mutation F129L, detected in the samples, has previously been associated with reduced sensitivity overseas to QoI fungicides. It was first detected in NFNB samples collected from barley crops on the Yorke Peninsula in 2022. However, this latest detection is the first time F129L has been linked to a paddock fungicide failure in the area.

Associate Professor Fran Lopez-Ruiz, Project Lead for the Australian Fungicide Resistance Extension Network (AFREN) at CCDM, says “the scientific evidence suggests the observed fungicide failure is due to the triple resistance combination of mutations. Lack of control by successive single and mixed fungicide actives may encourage selection of the triple-resistant population, increasing the disease prevalence.”

NFNB in barley samples from Warooka and Corny Point in SA, and Streatham in Vic, carry the F129L mutation in combination with mutations associated with reduced sensitivity and resistance to DMI fungicides and SDHI fungicides. The samples from Paskeville carried F129L with a different combination of DMI and SDHI resistance mutations. However, the potential for fungicide resistance developing in this area is the same.

Fungicide advice for SA barley growers

The compromised status of all three fungicide groups for NFNB means the chemical control options available to South Australian growers are now severely limited in paddocks where the triple mutants are allowed to proliferate.

Along with the triple fungicide resistances detected on the Yorke Peninsula:

• Reduced sensitivity or resistance to Group 3 fungicides epoxiconazole, propiconazole and prothioconazole has been confirmed in several parts of SA since 2019.
• Resistance to Group 7 fluxapyroxad was confirmed on the Yorke Peninsula in the 2019 growing season. Reduced sensitivity to fluxapyroxad has also been reported in the Eyre Peninsula and Kybybolite regions of SA.
• Dual resistance or reduced sensitivity to Group 3 fungicide propiconazole and Group 7 fungicide fluxapyroxad has also been detected on the Yorke Peninsula.

As a result, barley growers must manage all fungicide use with extreme care to avoid selecting further resistance in their paddocks. Growers should also be acutely aware that resistant fungus can spread to neighbouring paddocks with wind-borne spores, meaning resistance could easily spread to barley paddocks with no previous issues around fungicide effectiveness.

It is also essential to remember that applying a fungicide to control one disease, such as powdery mildew, can still select for resistance in other pathogens that are present in the crop.

Leverage agronomy to reduce disease

NFNB is caused by the fungus Pyrenophora teres f. teres, which prefers susceptible hosts, early sowing, mild temperatures (15-25°C) and extended periods of leaf wetness. It can survive on infected seed and stubble, as well as volunteer plants.

Adhere to the principles and priorities described in the Australian Fungicide Resistance Extension Network’s ‘The Fungicide Resistance Five’ (AFREN FR5) to help reduce disease risk before, between and during growing seasons.

It is critical that barley growers now:

• Choose less susceptible varieties
  Any level of genetic resistance to NFNB will help slow the rate of pathogen and disease development within a crop. Without effective fungicide control, a lower yielding variety may prove more profitable than high yielding cultivars with greater susceptibility to NFNB.
• Sow later if possible
  Later sowing avoids the warm and damp period of late autumn that favours damaging disease development in young plants. Later-sown crops also tend to establish a less dense canopy, which is less conducive to fungal disease development. However, growers also need to consider any potential yield penalty from later sowing.
• Encourage airflow through the canopy
  Reduce humidity in the canopy by taking steps to encourage air circulation, such as using wider row or plant spacings (balanced against yield potential) or grazing with livestock. Grazing early-sown crops can also remove infected leaves and help prevent runaway infections.
• Reduce inoculum between seasons
  Manage stubble-borne inoculum by grazing, rolling or cutting standing barley stubble, to reduce the spread of wind-borne spores at the start of the growing season. Take care to eliminate all volunteer barley and green bridge hosts to help prevent disease carry-over.
• Use crop rotations to break the disease cycle
  NFNB only infects barley, and inoculum loads will decline as barley stubble breaks down in the paddock. Planting pulses, oilseeds and alternative cereals in a succession of break crops will help prevent disease build-up in the paddock and across seasons.

New cultivars can improve performance

A new wave of barley varieties is currently being introduced to the industry, including several with yield potential that matches the established market leaders. Growers should review the GRDC National Variety Trials 2025 South Australian Crop Sowing Guide to identify available varieties that combine improved disease resistance with growth characteristics and yield potential to suit local conditions.

The choice of malting barley varieties is also increasing, with newer cultivars either receiving accreditation or advancing through the accreditation process in recent years. Grains Australia provides a Malting Varieties guide detailing varieties that are preferred, accredited or under evaluation (with their current status).

With the reduction of fungicide efficacy for NFNB control, improved genetic resistance is now an essential factor in disease control. The risk of other crop diseases developing similar multiple resistances to available fungicides must also be considered – making agronomic disease management essential for protecting future control options and yields.

Useful resources

• GRDC fact sheet: Barley net form net blotch Group 11 (QoI) resistance (Apr 2024)
• GRDC fact sheet (PDF link): Managing fungicide resistance - net form net blotch of barley (Jun 2022)
• GRDC podcasts - various podcasts with AFREN on fungicide resistance
• GRDC publication: Fungicide resistance management in Australian grain crops (Apr 2021)
• GRDC National Variety Trials: 2025 South Australian Crop Sowing Guide (Nov 2024)
• Grains Australia: Barley malting varieties

Contact details

Nola D’Souza  
AFREN Project Coordinator, Centre for Crop and Disease Management (CCDM), Curtin University
AFREN@curtin.edu.au 
0408 995 669