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Australia researchers confirm new dual fungicide resistance in barley disease

Australian researchers have confirmed a new form of dual fungicide resistance in the common barley disease, net form net blotch (NFNB).

They have identified a new genotype in NFNB that is – for the first time known – showing dual resistance to both a succinate dehydrogenase inhibitor (SDHI) fungicide as well as some DeMethylation Inhibitor (DMI) fungicides.

The discovery was outlined this week at the 2019 Crop Protection Forum at Moama in New South Wales and the national fungicide resistance workshop in Melbourne (Victoria).

Working together to test barley samples collected near Minlaton on South Australia’s Yorke Peninsula, researchers from the South Australian Research and Development Institute (SARDI), the research division of Primary Industries and Regions SA, and the Centre for Crop and Disease Management (CCDM) found fungal strains showing reduced fungicide sensitivity to SDHI fungicide fluxapyroxad.

The samples were identified by SARDI plant pathologists Tara Garrard and Hugh Wallwork and confirmed as resistant via tests conducted by CCDM’s fungicide resistance team led by Fran Lopez-Ruiz.

“The samples, collected from two paddocks a short distance apart, clearly showed high NFNB disease levels when the SARDI team was first alerted to the problem by agronomist Sam Holmes,” said Dr Wallwork.

Additional testing by the CCDM team then confirmed the higher levels of resistance correlated with a genetic mutation in the Sdh complex which is the NFNB target of SDHI fungicides.

Following these initial tests more detailed studies were undertaken, reinforcing the discovery of resistance.

“While the type of SDHI resistance and the mutations found in the Yorke Peninsula isolates have been recorded previously in Europe, this is the first confirmation of this genotype in Australia,” said Dr Lopez-Ruiz.

Dr Wallwork said the samples were from the barley variety Spartacus CL, grown in a barley-on-barley rotation with fluxapyroxad used in both years. In terms of fungicide application, soil moisture in the area was considered to be adequate to allow for good uptake and further testing on residual seed clearly showed that the efficacy was greatly reduced.

Additional samples collected at the site also showed resistance to the Group 3 DMI fungicide tebuconazole, and although the resistance mechanism is still being investigated, Dr Lopez-Ruiz said it was known to be different to the DMI resistance mechanisms already reported in barley net blotches in Western Australia.

Grains Research and Development Corporation (GRDC) diseases manager, Friday Obanor, said the discovery of these new genotypes reflected the importance of research and development in the Australian grains industry and investment in research conducted by both the CCDM (co-supported by Curtin University and the GRDC) and SARDI.

“Australia has some of the best crop disease experts in the world working here and their efforts are critical for identifying and quantifying new risks to agriculture in a rapid and highly co-ordinated way,” said Dr Obanor.

“The faster we can alert growers, the quicker we can get management strategies into the paddock.”

The Yorke Peninsula results are a reminder for growers to consider using different modes of action when applying fungicides, as continuous use of the same fungicides will increase selection pressure further, increasing the risk of even more rises in resistance levels.

This also applies to the closely related spot form net blotch disease which has already developed high levels of resistance to some DMIs in WA.

Dr Lopez-Ruiz said the results also reinforced the need for growers and advisers to implement a fungicide resistance management strategy that encompasses an integrated approach, involving:

Avoid growing barley on barley – look for suitable alternative break crops
Choose fungicide mixtures with different modes of action (if available)
Do not use Group 7 fungicides for net blotch control in the areas where resistance to this group of fungicides has been reported
Group 7 fungicides (seed dressing and foliar) should not be used more than once per season in any crop rotation – alternate them instead with other fungicides with different modes of action
Do not apply the same Group 3 fungicides twice in a row – look at alternate sprays
Avoid using tebuconazole, propiconazole and epoxiconazole as a stand-alone product in barley for any disease as a way of avoiding indirect fungicide resistance selection
Use fungicides as early as possible after symptoms develop if conditions are highly conducive for disease development
Do not spray below label rates to avoid fungicide underperformance.

Growers on Yorke Peninsula, south of Port Wakefield and Kadina, should avoid using any SDHI fungicides for the control of NFNB on barley during 2020. This will help to contain the further development of resistance and hopefully allow for the return of SDHI use in the region at a later date.

Growers and agronomists in SA are urged to send NFNB stubble samples to SARDI or CCDM for their analysis to determine whether resistance to SDHI and DMI fungicides is also present in other regions of SA.