Fungicide resistance in barley

Call to action/take home messages

• Reports of resistance to Systiva^® in spot form net blotch in South Australia have been refuted.
• The first detection of resistance (reduced sensitivity) to Group 3 fungicides has been confirmed for spot form net blotch in Western Australia.
• Continuous use of popular Group 3 fungicides without alternation with chemicals of different modes of action will accelerate the development of resistance in eastern Australia.
• Industry must heed and adopt best management fungicide practices to maintain the efficacy of fungicides registered for use on winter cereals.

The status and evolution of fungicide resistance in grain crops was covered in a paper by Lopez and Kay presented at the 2017 Goondiwindi GRDC Grains Research Update. The authors reported seven cases of fungicide resistance and reduced sensitivity to three fungicide Groups in pathogens of Australian grain crops.

Application of fungicides is now routine procedure in many cropping situations and the product of choice based on registration, efficacy and cost is usually a triazole. In seasons favourable for disease infection and spread, and particularly when genetic resistance ratings are low, repeat applications are often necessary for adequate control of a range of diseases.

Lopez and Kay (2017) warned that overuse of fungicides with the same mode of action accelerates the development of resistance in the target pathogens; therefore, responsible stewardship of fungicides demands rotation with chemicals with different modes of action. For example, with a disease like spot form of net blotch (Pyrenophora teres f. maculata), if a second application was required, an alternative chemical like a strobilurin should be used. This strategy reduces the increase of infection propagules of the fungus, that may have been selected from the wild population by the initial application of the triazole fungicide. While this is a sound management approach, fungicides with alternative modes of action and efficacy against the major foliar diseases are very limited.

It was therefore of particular interest when the new generation carboxamides (Group 7 chemicals) were registered in Australia. These showed very good efficacy against some crop diseases as foliar sprays and/or seed treatments. Industry now had another alternative to the dimethyl inhibitors (DMIs Group 3 chemicals) that have been the mainstay of disease control in cereal crops.

To keep abreast of developments in resistance to fungicides, pathogen populations are routinely monitored by the Centre for Crop and Disease Management (CCDM) based at Curtin University in Western Australia. Disease samples are collected Australia wide and submitted to CCDM for analysis to determine the levels of resistance present in a range of pathogens to the fungicides registered for control.

This paper reports recent information from the CCDM on spot form net blotch (SFNB) of barley to guide better management of this and other cereal diseases in the coming season.

Systiva

Fluxapyroxad was registered by BASF for commercial use in 2015 as Systiva^®. Through extensive trialling pre-release, Systiva demonstrated an affinity for control of foliar diseases in barley, as a seed treatment, providing protection against some diseases up to the middle of heading (Z55). From over 80 trials conducted for registration, BASF reported an average yield increase of 350kg/ha. The levels of disease control, particularly with SFNB and the resultant yield increase from an ’up front‘ application resulted in widespread use of the product in 2017.

Later in 2017, reports from South Australia indicated that Systiva was not performing to expectations in some trials. This sparked fears that the pathogen was demonstrating resistance to fluxapyroxad. Apparently, the levels of disease on some varieties particularly Spartacus were much higher than expected considering the efficacy of Systiva against SFNB (H. Wallwork pers.comm.). Samples were collected and sent to the CCDM for testing.

In an email received from CCDM on 22^nd January, Dr Fran Lopez-Ruiz reported, “We have not been able to confirm in the lab the lack of activity for fluxapyroxad reported from the field”. Fluxapyroxad was tested with several other SDHI chemicals and all showed similar levels of activity against all SFNB isolates used.

It is therefore concluded that resistance to fluxapyroxad has NOT been detected.

This then poses the question, ‘Why was fluxapyroxad not performing to expectations in these instances’?

Answers to this question may be:

1. Uneven application of seed treatment
2. High early disease pressure and
3. Rates of application less than recommended.

None of these options has been confirmed as an explanation for the higher than expected levels of disease and further observations and monitoring will be undertaken in 2018.

Resistance in spot form net blotch

In a media release on 12^th February 2018, the CCDM reported that samples of SFNB from the South Stirling region showed resistance to some Group 3 DMI fungicides (tebuconazole, epoxiconazole and propiconazole). Another SFNB sample from the Esperance region showed reduced sensitivity (a shift in sensitivity that does not cause field failure) towards the same Group 3 DMI fungicides.

This follows the detection of multi-DMI resistance in net form net blotch ((NFNB) Pyrenophora teres f. teres) as reported by Lopez and Kay (2017). In that publication, the authors questioned why resistance had not developed in SFNB as it has many similarities with NFNB. It has! It simply took a little more time to be detected.

This is an alert to industry in all regions to implement integrated disease management (IDM) strategies to minimize the opportunities for SFNB to develop resistance elsewhere.

Appropriate IDM strategies involve:

• Avoid sowing barley into barley stubble
• Sowing varieties resistant to SFNB where available
• Application of fungicides at the recommended rates and
• Alternating fungicides from different groups where repeat applications are required.

The DMI fungicides remain the backbone of foliar disease control in winter cereals. It is time to implement best management practices to protect their efficacy before it is too late.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support.

References

Wallwork, H (2017) Cereal Seed Treatments 2018

Lopez and Kay (2017) Fungicide resistance in grain crops – what’s happening and how should we respond?

GRDC Media Release (2018) Fungicide resistance found in barley spot form of net blotch.

Contact details

Greg Platz
Department of Agriculture and Fisheries
Hermitage Research Facility, 604 Yangan Road, Warwick QLD 4370
Ph: 07 4542 6733 / 0408 733 055
Email: Greg.Platz@daf.qld.gov.au

Lisle Snyman
Department of Agriculture and Fisheries
Hermitage Research Facility, 604 Yangan Road, Warwick QLD 4370
Ph: 07 4542 6761 / 0408 733 055
Email: Lisle.Snyman@daf.qld.gov.au

Fran Lopez-Ruiz
Curtin University, Centre for Crop and Disease Management
School of Molecular and Life Sciences, Perth WA 6845
Ph: 08 9266 3061
Email: fran.lopezruiz@curtin.edu.au

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