After an extremely wet spring, researchers across Australia warn severe disease pressure will require proactive management to minimise damage to this year’s grain crops.
Having a good integrated disease management strategy will be critical in reducing grain yield losses. This includes avoiding susceptible varieties, removing the green bridge, paddock rotations, reliable agronomy practices (early grazing, sowing time, inter-row sowing, nutrition), rotation of fungicide groups and strategic fungicide applications.
Researchers also recommend that growers download the free StripeRustWM app for support with rust management decisions and the fungicide resistance management guide for information about fungicide best practice.
Speaking at this year’s GRDC Research Updates, Agriculture Victoria senior plant pathology researcher Dr Grant Hollaway said the use of up-front fungicides, such as flutriafol on fertiliser, would be important for managing the heightened risk of diseases carried over on stubble and volunteer plants.
Dr Hollaway said the frequency and volume of rain from August to November 2022 put cereal crops under “unprecedented disease pressure” and caused serious losses where diseases such as rust and Septoria tritici blotch (STB) were not effectively managed.
“Cereal rust appeared in Victorian wheat crops earlier than usual, and conditions favourable for disease development resulted in a damaging outbreak of stripe rust across Victoria,” Dr Hollaway said.
“Industry reports during the season confirmed that strategies of avoiding susceptible cultivars, using up-front fungicides, and timely foliar fungicide applications all contributed towards reduced stripe rust pressure in paddocks.
“Development of resistance to fungicides is increasing in cereal pathogens but can be slowed through the adoption of integrated control strategies and prudent use of fungicides.”
Yield losses from STB – the second-most-damaging wheat disease in 2022 – in highly susceptible varieties were highest in the high-rainfall zone. However, even in the medium-rainfall zone amounted to 35 to 43 per cent in Wimmera field trials, compared with less than 10 per cent in 2021. Fungicide sprays at growth stages 31 and 39 in susceptible cultivars increased yield by about 35 per cent.
Be on alert
The scenario in South Australia was similar, with most of the state receiving exceptional rainfall that lingered into late spring and early summer, causing rampant foliar disease and affecting grain fill and harvest.
South Australian Research and Development Institute (SARDI) cereal pathologist Dr Tara Garrard said wheat crops suffered from stripe rust, STB and powdery mildew, experiencing yield losses, reduction in grain quality and/or grain defects.
The first cases of stripe rust were reported in late July and disease pressure remained high in susceptible varieties.
Dr Garrard said growers should expect another rust epidemic this year, and be prepared for it to start earlier in the season with an integrated disease management plan that includes:
- green bridge control;
- variety selection – avoid susceptible varieties or growing wheat-on-wheat;
- up-front fungicide applications – in-furrow, seed dressings, coated fertiliser;
- proactive foliar fungicide applications; and
- use of support tools such as the StripeRustWM app.
A series of field trials by SARDI, in conjunction with Agriculture Victoria and GRDC, was undertaken during 2021 and 2022 in low and medium-rainfall zones to measure yield losses from STB in six wheat varieties.
They found STB control was uneconomical in 2021, but disease control in 2022 reduced yield loss by up to 22 per cent in susceptible varieties at one site and no significant yield losses were recorded at the other.
Dr Garrard said numerous reports of powdery mildew were received last year from the Lower Eyre Peninsula, Yorke Peninsula, South-East and even the Mallee, where it was rare.
Growers also reported difficulty in controlling it, likely due to the presence of strobilurin (Group 11) fungicide resistance and reduced sensitivity to triazole (Group 3) fungicides.
SARDI’s cereal pathology team identified seven causes of grain damage and lower quality from samples with severe grain shrivelling, fungal staining, black point and white and pink grains.
“A multitude of factors as well as fungal pathogens are likely to have contributed to these symptoms, with many samples displaying more than one impaired trait,” Dr Garrard said.
“Growers are advised to assess their seed quality going into the 2023 growing season and select a cleaner seed source if necessary.”
Wheat powdery mildew
The widespread occurrence of powdery mildew in wheat last year highlighted the importance of varietal resistance and rotating varieties in integrated disease management strategies.
Trengove Consulting principal Sam Trengove said the disease’s spread into new areas was driven by a combination of interacting factors. They included growers choosing to grow susceptible to very susceptible (SVS) varieties over long periods and early crop establishment in many regions, as well as inoculum carryover from previous years and conditions favouring the growth of large canopies and disease development.
Research into the extent of fungicide resistance in wheat powdery mildew across four regions started last year, with small-plot trials at Port Neill, Bute and Malinong, South Australia, and Katamatite, Victoria.
Under investigation was pre-emergent and post-emergent fungicide efficacy, and interactions with fungicide timing and varietal resistance.
Mr Trengove said the benefit of varietal resistance in limiting wheat powdery mildew build-up was clear in untreated plots, where Grenade CL Plus (moderately susceptible) had less disease than Chief CL Plus (SVS) and Scepter (SVS) treated with a two-spray fungicide strategy.
“However, Scepter was the highest-yielding variety regardless,” he said.
Multiple diseases were present and broad-spectrum fungicides providing control of stripe rust were the highest-yielding treatments.
Testing of powdery mildew samples after spraying with products containing Group 11 fungicides showed an increase in the frequency of a specific mutation at three sites where initial resistance levels were low. The frequency of the gateway mutation conferring reduced sensitivity to Group 3 fungicides was more than 99 per cent at all sites, reflecting a history of relying on triazole fungicides.
Mr Trengove said the Group 13 fungicide Legend®, which contains the active ingredient quinoxyfen and can be used at rates of 200 to 300 millilitres per hectare in wheat under emergency permit PER93197, provided good control at Bute, but would have to be applied with an appropriate mix partner to also control stripe rust.
Resistance on the rise
Fungicide resistance in wheat powdery mildew is becoming more widespread across southern cereal growing regions.
Resistance to strobilurin (Group 11) and DMI/triazole (Group 3) fungicides has been confirmed and is common in New South Wales, SA, Victoria and Tasmania. Isolates carrying resistance to both groups also have been found at high frequencies in some locations.
Curtin University Associate Professor Fran Lopez-Ruiz said the more than 10 million hectares sown to wheat each year provided a large breeding ground for powdery mildew, which can cause yield losses of up to 25 per cent in cool, humid conditions.
Glasshouse experiments on LongReach Trojan wheat plants sprayed with fungicide and then spores of triazole-resistant and non-resistant powdery mildew found using different DMI fungicides did not control all diseases in all situations.
“It was demonstrated that propiconazole has a significantly lower activity against wheat powdery mildew DMI-resistant isolates,” Dr Lopez-Ruiz said.
“This has been further supported by the numerous field reports of lower wheat powdery mildew control rates by DMI fungicides in recent seasons.”
The Australian Fungicide Resistance Extension Network recommends five actions for avoiding and managing fungicide resistance. They are:
- avoid susceptible crop varieties;
- rotate crops – use time and distance to reduce disease carryover;
- use non-chemical control methods to reduce disease pressure;
- spray only if necessary and apply strategically; and
- rotate and mix fungicides/mode of action groups.
But Dr Lopez-Ruiz said the presence of multiple resistance at high levels and the lower effectiveness of Group 7 succinate dehydrogenase inhibitors (SDHIs) on powdery mildew made it difficult to rotate chemical groups.
It is important to sow clean seed during 2023 to ensure desired plant establishment and reduce disease carryover.
He suggested growers focus on using varieties with less susceptibility to powdery mildew, rotating crops to reduce disease carryover, and adopting non-chemical methods of managing disease outbreaks.
These practices would not only slow the spread of resistance, but also reduce the risk of the disease developing resistance to other modes of action.
“This is the logical approach, at least until disease levels drop and specific mildewcide products become available to growers,” he said.
Test retained seed
Pulse crops also copped a hammering from fungal diseases last year and Agriculture Victoria researcher Dr Joshua Fanning warned that high disease loads would be carried over in three ways: on stubble, in seed and in the soil.
Dr Fanning said proactive management of pulse diseases in 2022 proved essential for reducing the severity of disease and maintaining profitability, especially when crops established early and consistent rainfall extended the season.
The first step in this year’s disease management strategy should be testing any retained seed for diseases such as Sclerotinia white mould, Botrytis grey mould and Ascochyta blight before sowing.
“It is important to sow clean seed during 2023 to ensure desired plant establishment and reduce disease carryover,” he said.
“Plan a disease management strategy early that incorporates varietal resistance, paddock rotations, reliable agronomy practices (sowing time, inter-row sowing, nutrition), rotation of fungicide actives and strategic fungicide applications.”
Dr Fanning warned that growers without a solid strategy could expect grain yield losses of more than 90 per cent if conditions were again conducive to disease. Later sowing can help with disease management by delaying canopy closure and reducing how many disease cycles occur in a season.
This was demonstrated at Horsham last year when Genesis™ 090 chickpeas sown in May were affected more severely by Ascochyta blight than plots sown in July.
“It is important, though, to offset this with the potential yield reductions from late sowing,” he said. “It is a balance and should be based around plant establishment, keeping in mind that many areas have good soil moisture.”
No fungicide resistance in pulse crops of relevance has been detected in Australia, but many crops rely on the application of a single active multiple times during the season.
Dr Fanning said this made resistance likely if growers did not take steps to extend the longevity of the limited range of fungicides available.
More information: Dr Hari Dadu, firstname.lastname@example.org; Dr Josh Fanning email@example.com; Dr Tara Garrard, firstname.lastname@example.org; Dr Fran Lopez-Ruiz, Fran.Lopezruiz@curtin.edu.au; Sam Trengove, email@example.com