Courtesy: GRDC – Net form net blotch triple fungicide resistance detected in WA
Western Australian barley growers should prioritise varietal resistance and review their disease management strategies following detection of net form net blotch (NFNB) isolates with combined resistance to Groups 3, 7 and 11 fungicides.
Key points
- Laboratory testing has confirmed that NFNB samples collected from South Stirling, WA, contained 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).
- A clear difference between disease impact in paddocks of RGT Planet (SVS) and Laperouse (MRMS) highlights the value of varietal resistance in disease management.
- Growers must use agronomic disease management tactics to control NFNB and limit the build-up of these triple fungicide-resistant populations.
- Fungicide-resistant mutations can spread rapidly and are likely to already be present in other areas, making practices that prevent their spread imperative.
- Barley growers are advised to: choose less susceptible varieties and rotate them; sow later if possible; encourage airflow through the canopy; reduce inoculum between seasons; and use crop rotations to break the disease cycle.
Fungicide failure in South Stirling paddock
WA’s Department of Primary Industries and Regional Development (DPIRD) research scientist Kithsiri Jayasena collected the infected barley samples from two paddocks in the South Stirling region during 2024. This work was part of a Grains Research and Development Corporation (GRDC) surveillance investment. The grower was concerned by the lack of effective disease control in one of the paddocks.
The paddock was planted with RGT Planet and had severe disease levels despite several applications of fungicide:
- Seed dressing Tebuconazole (Group 3)
- With in-furrow fertiliser Flutriafol (Group 3)
- First foliar spray Prothioconazole + Tebuconazole + Azoxystrobin (Group 3 + 11)
- Second foliar spray Prothioconazole + Tebuconazole + Azoxystrobin (Group 3 + 11)
- Third foliar spray Prothioconazole + Bixafen (Group 3 + 7)
RGT Planet is the most NFNB-susceptible barley variety at adult stage. It is rated Moderately Susceptible (MS) to the predominant NFNB Beecher avirulent pathotype in WA, but Susceptible to Very Susceptible (SVS) at adult stage to the aggressive Oxford virulent pathotype, which is most prevalent in the Albany and Esperance port zones of WA.
The adjacent paddock was planted with Laperouse. It was treated with the same fungicide regime but without the need for the third foliar spray. Adult plants of this variety are rated Moderately Resistant to Moderately Susceptible (MRMS) to the Beecher avirulent pathotype and Moderately Susceptible to Susceptible (MSS) to the Oxford virulent pathotype. This crop had minimal NFNB disease.
Triple fungicide resistance detected
As part of a GRDC investment and working in partnership with DPIRD, the Centre for Crop and Disease Management (CCDM) analysed the samples affected by NFNB for fungicide-resistant mutations.
In total, 41 isolates were tested by the fungicide resistance team at CCDM. Of those, 39 isolates contained mutations associated with resistance or reduced sensitivity to Group 3 DMI, Group 7 SHDI and Group 11 QoI fungicides.
Forty isolates carried the F129L mutation, which is associated with reduced sensitivity to Group 11 QoI fungicides.
First reported in South Australia in 2022 and present in the recently reported South Australian and Victorian triple mutant isolates, the F129L mutation has not been previously detected in WA.
Along with the triple mutations detected at South Stirling:
- Resistance and reduced sensitivity to the Group 3 fungicides propiconazole, prothioconazole, and epoxiconazole has been confirmed in various regions of WA since 2013.
- Resistance and reduced sensitivity to the Group 7 fungicides fluxapyroxad and bixafen has been confirmed in various regions of WA since 2021.
- Dual mutations have been detected for resistance or reduced sensitivity to both Group 3 azoles and Group 7 fluxapyroxad and bixafen fungicides, and most recently to both Group 3 azoles and Group 11 azoxystrobin fungicides.
Variety choice will help
The compromised status of all three fungicide groups for NFNB means that WA growers may now have limited chemical control options for managing NFNB.
With triple resistant NFNB mutants now detected in Victoria, South Australia and Western Australia, it is highly likely that the resistance is widespread and is already present in many more paddocks.
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 combination of mutations present in these triple resistant NFNB populations. Lack of control by successive single and mixed fungicide actives may allow further selection of the resistant populations, increasing the disease’s prevalence.”
However, the marked difference in infection rates between the two paddocks at South Stirling highlights the importance of variety selection in disease control.
Despite both paddocks receiving similar fungicide treatments and showing widespread occurence of triple resistant mutations, only the paddock of SVS-rated RGT Planet was severely affected. MRMS-rated Laperouse has withstood high levels of fungicide-resistant NFNB infection.
What barley growers can do now
Associate Professor Lopez-Ruiz emphasises “barley growers must manage all fungicide use with extreme care to avoid selecting for further resistance in their paddocks”.
Resistant fungi can spread to neighbouring paddocks with wind-borne spores, meaning resistance could easily spread to barley paddocks with no previous fungicide effectiveness issues.
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.
Growers should report any loss of disease control to their agronomist, state government pathologist or directly to the CCDM fungicide resistance team.
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 AFREN’s 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 and rotate them
As the South Stirling samples show, there can be a clear advantage in choosing even MRMS-rated barley over an SVS cultivar. Without effective fungicide control, a lower yielding variety may prove more profitable than high-yielding varieties with greater susceptibility to a prevalent disease. Just as rotating fungicide groups reduces selection for fungicide resistance, rotating varieties reduces selection of pathogens that have adapted to overcome host plant resistance. - 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 (e.g. barley grass is a potential weed host) 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 might be better
New barley varieties are currently being introduced to the industry, including several with improved disease resistance and yield potential that compares well with the established market leaders. Growers should review the GRDC National Variety Trials 2025 Western Australian Crop Sowing Guides to identify available varieties that combine improved disease resistance with agronomic 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 Barley net form net blotch Group 11 (QoI) resistance fact sheet
- AFREN Fact Sheet: Managing fungicide resistance: Net form net blotch of barley
- AFREN FR5 podcast series
- AFREN Fungicide resistance management guide
- GRDC Western Australian Crop Sowing Guide
- Barley malting varieties – Grains Australia
More information
Nola D’Souza
AFREN Project Coordinator,
Centre for Crop and Disease Management, Curtin University
AFREN@curtin.edu.au
08 9266 3541
GRDC Project Code: CUR2302-002RTX,