Strobilurin resistance in Septoria tritici blotch confirmed in South Australia

Wheat growers in South Australia are being urged to monitor the effectiveness of fungicides this season after the common wheat disease, Septoria tritici blotch (STB), was discovered to be resistant to Group 11 fungicides (strobilurins).

Researchers from the Centre for Crop and Disease Management (CCDM), a co-investment by the Grains Research and Development Corporation (GRDC) and Curtin University, made the discovery in Millicent in South Australia, as part of their annual monitoring program.

As a result, researchers are now encouraging growers with any concerns about the effectiveness of Group 11 fungicides this season to send crop samples to CCDM for resistance testing.

Dr Fran Lopez-Ruiz, leader of CCDM’s fungicide resistance research group, said the discovery was an unexpected chance circumstance, as the diseased leaf sample came from a trial aimed at another pathogen.

“This monitoring program involves applying different fungicides to multiple trial sites across Australia’s grain growing regions and testing diseased leaf samples for fungicide resistance,” Dr Lopez-Ruiz said.

“Thanks to this program, we have picked up the mutation before growers have noticed fungicide failure in the paddock, which gives us a time advantage to stay on top of it.

“This is fortunate, because we know from global reports that when selection pressure is high, strobilurin resistance can spread very quickly.”

Dr Lopez-Ruiz said multiple genetic tests had confirmed the presence of mutation G143A – a mutation associated with resistance to all Group 11 fungicides.

“However no other fungicide groups are affected by this mutation, so growers still have chemical alternatives they can use to control STB,” he said.

Dr Lopez-Ruiz is now advising growers to adopt a resistance management strategy that includes the use of different modes of action (MoA) and avoids, if possible, the application of formulations containing Group 11 fungicides in their rotations.

CCDM Director Professor Mark Gibberd said the timeliness of the discovery has highlighted the importance of disease monitoring across all grain growing regions.

“Growers depend heavily on fungicides for disease control and this discovery demonstrates the value of regular monitoring and early detection of developing resistance,” Professor Gibberd said.

“Together we can then work with growers to ensure they have the information to make the best decisions with fungicide management to ensure fungicides are here for the long run.”

Dr Andrew Milgate at NSW Department of Primary Industries (DPI) works closely with the STB pathogen, and said when choosing products to control STB growers should keep in mind not all demethylase inhibitor fungicides (DMI, Group 3) are completely effective anymore.

“Research by my team has revealed that STB has reduced sensitivity to older DMI chemistry such as tebuconazole and this is widespread in the STB population,” Dr Milgate said.

“However, products with newer active ingredients such as epoxiconazole, remain effective at field rates, as do registered SDHI (Group 7) products. This allows product selection that will avoid repeated use of strobilurins to target STB and help reduce the selection of the G143A mutation.”

The Australian Fungicide Resistance Extension Network (AFREN), a GRDC investment, suggests an integrated approach tailored to local growing conditions. AFREN has identified the following five key actions, ‘The Fungicide Resistance Five’, to help growers maintain control over fungicide resistance, regardless of their crop or growing region:

1. Avoid susceptible crop varieties

2. Rotate crops – use time and distance to reduce disease carry-over

3. Use non-chemical control methods to reduce disease pressure

4. Spray only if necessary and apply strategically

5. Rotate and mix fungicides/MoA groups.

Growers and agronomists who suspect reduced sensitivity or resistance to Group 11 fungicide should contact the CCDM’s Fungicide Resistance Group at frg@curtin.edu.au. Alternatively, contact a local regional plant pathologist or fungicide resistance expert to discuss the situation. A list of contacts is available on the AFREN website.