Tackling cereal disease challenges head on in the HRZ

Take home messages

• At FAR Australia’s Victoria Crop Technology Centre near Bannockburn, stripe rust caused the greatest yield loss in wheat, with susceptible cultivars such as RockStar giving yield responses of over 3t/ha to a three-spray fungicide strategy.
• Drier than average conditions during stem elongation and the emergence of the top three leaves significantly reduced Septoria tritici blotch (STB) development in the upper canopy.
• In 2023, fungicide management strategies for stripe rust and STB control, combined with varietal resistance, lifted grain yield from 2.4t/ha to 6.76t/ha.
• For the first time at the FAR Australia Research Centre, net form of net blotch (NFNB) was poorly controlled in RGT Planet, even with four fungicide units, indicating that this pathogen is increasingly resistant to our fungicide arsenal, and we can see its effects in the field.
• An integrated disease management (IDM) strategy based on good variety resistance, cultural, and fungicide control remain the key ingredients to closing the yield gap in southern Victorian wheat and barley crops.
• In susceptible wheat varieties, target the following three key timings for fungicide intervention: first node – second node GS31–32, flag leaf emergence GS37–39, and with an optional third application at head emergence GS59 when wet conditions prevail up to head emergence.
• For stripe rust, with susceptible varieties such as RockStar, Genie and RGT_Cesario, apply flutriafol in furrow to kick start the disease management programme.
• Avoid gaps in fungicide application greater than 3–4 weeks when disease pressure is high.
• Avoid repeated use of the same fungicide active ingredients, and in the case of the newer Group 11 QoI (strobilurins) and Group 7 SDHIs, where possible restrict strategies to just one application per season in order to slow down and help prevent the selection of resistant strains.

Which disease caused the greatest yield loss in wheat in the southern HRZ region

In the 2023 season in southern Victoria, stripe rust caused by the pathogen Puccinia striiformis f. sp. tritici was the principal disease that caused yield loss in wheat. Despite well below average rainfall in the period from July to October, this disease was very aggressive in many cultivars and new lines evaluated. In contrast, the wet weather stubble-borne disease Septoria tritici blotch (STB) failed to develop substantially on the upper leaves as a result of drier conditions during the emergence of the top three leaves (often referred to as the money leaves) (Figure 1).

Selecting high yielding varieties with good resistance to these two diseases should be at the core of your disease management strategy for 2024. A number of new lines with high yield and good disease resistance were identified in GEN (Germplasm Evaluation Network) trials conducted across the HRZ by FAR Australia in 2023. These included AGFWH010222, AGTW005 (which was exceptionally resistant in 2022), RW 71608 and FAR WW2. Of those commercially available varieties, Anapurna, Longford (AGF 4818) and BigRedA have held up well in terms of stripe rust resistance compared to RGT_Cesario, however stripe rust was more evident in BigRed in 2023 than 2022. Varieties formerly moderately resistant to both stripe rust and STB (such as Accroc) are now susceptible to both diseases as well as leaf rust in the HRZ region of southeast Australia. With Accroc and RGT_Cesario, it will be essential to start a disease management programme with flutriafol in furrow to obtain early control of stripe rust in 2024, particularly bearing in mind the presence of a green bridge of volunteers, which in a number of regions was unexpected. The same will be the case with the milling wheats RockStar, Willaura, and Genie (IGW6754) which all gave significant responses (over 2t/ha) to a three-spray fungicide programme (Figure 2).

The milling wheats gave the greatest dollar return for fungicide spend in our 2023 research, with Willaura, Genie and RockStar giving returns of between $6.60–$10.10 for each dollar invested in a fungicide product and application (Table 1). The reward of controlling disease in these cultivars came from both yield gain and grain quality improvement. Using the same risk reward ratios, it was shown that the yield increases achieved with fungicides were not effective with RGT Waugh, RW 71608, AGF010222, AGTW005 and FAR WW2, all of which gave less financial return than dollars spent. The cost benefit ratio of using fungicides shown in Table 1 expresses the benefit of fungicide application in terms of $ return for each $ spent on the variety.

Figure 1. Influence of cultivar and fungicide management on stripe rust plot infection (p = <0.001, Lsd = 8.5), assessed on 27 September. Bars with the same letters are not significantly different (p=0.05).

Figure 2. Influence of cultivar and fungicide application on grain yield (t/ha). Bars with the same letters are not significantly different (p=0.05).

Table 1: Influence of fungicide application on income return expressed as ratio of $ return/ha for each $ spent.

Notes: Gross income of untreated and fungicide treated crop based on the value of the grain yield and quality grade, with the treated crop values expressed after fungicide and application cost ($/ha) has been subtracted. Genie (IGW6754) and IGW6755A classed as milling quality, RW 71608 as red feed grain for purposes of this calculation. Fungicide costs based on a three-spray programme of Prosaro® 300mL/ha (Group 3 DMIs prothioconazole & tebuconazole), Aviator Xpro® 500mL/ha (Group 3 DMI prothioconazole & Group 7 SDHI bixafen) and Opus® 500mL/ha (Group 3 DMI epoxiconazole) with $15/ha spray application cost. Total cost of fungicide programme and application $106.50. Grain costed at FED1 – $307/t, SFW1 and AGP1 – $327/t, APW1 – $376/t.

Poor field control of net form of net blotch (NFNB) in barley

A new GRDC research initiative is already indicating the importance of cultivar resistance in barley crops grown in the HRZ. Although fungicide resistance testing at the Centre for Crop and Disease Management (CCDM) is still taking place on samples taken from the trials, net form of net blotch (NFNB) has been increasingly difficult to control in susceptible cultivars such as RGT Planet, with evidence of poor control now showing up in the field. Unlike the situation in wheat where fungicides are still very effective on diseases such as stripe rust, NFNB in barley is not being adequately controlled by fungicide programmes based on up to four units of fungicide. Whilst the research indicated a yield response (Table 2) to fungicide application, the grain yield responses were based on much poorer levels of disease control than those which have traditionally been experienced over the last four years. This would appear to indicate that the NFNB pathogen is becoming more resistant to our current fungicide groups (all of which were incorporated into these strategies). The research at the FAR Australia Victoria Crop Technology Centre near Bannockburn has indicated that, despite up to four units of fungicide, control of NFNB has been no better than 63% on the most important leaves in barley such as F-1 (Figure 3).

Table 2: Influence of fungicide management on grain yield (t/ha) – cv RGT Planet sown 30 April.

Notes: Systiva seed treatment (Group 7 SDHI fluxapyroxad), Opera (Group 3 DMI epoxiconazole & Group 11 QoI pyraclostrobin), Aviator Xpro (Group 3 DMI prothioconazole & Group 7 SDHI bixafen) and Opus (Group 3 DMI epoxiconazole)

Figure 3. Influence of fungicide management on NFNB severity on flag -1 at GS71 cv RGT PlanetA sown 30 April assessed 3 October.

Fungicide resistance and reduced sensitivity

The wider issue in these results is that pathogen resistance to fungicides is primarily driven by the number of applications of the same mode of action. The more fungicides we apply, the more selection pressure we apply to the pathogen population, and the more resistant to fungicides the population becomes. For this reason, it is imperative for HYC research to incorporate the most resistant, high yielding and adapted varieties available in order to reduce our dependence on fungicide agrichemicals.

The following are fungicide resistance management strategies which should be used within broader IDM:

• With wheat and barley crops, where two to three fungicide applications occur within a season, avoid repeat applications of the same product/active ingredient and, where possible, also avoid the same mode of action in the same crop. This is particularly important when using Group 11 QoI (strobilurins) and Group 7 SDHIs, which preferably would only be used once in a growing season.
• Avoid using the seed treatment fluxapyroxad (Systiva) year after year in barley without rotating with foliar fungicides of a different mode of action during the season.
• Avoid applying the same DMI (triazole) Group 3 fungicide twice in a row.
• Avoid the use of tebuconazole alone and flutriafol for STB pathogen control in regions where reduced sensitivity is problematic, as these Group 3 DMIs are more affected by reduced sensitivity strains than other DMIs.
• Group 3 DMIs such as epoxiconazole (Opus) or triazole mixtures such as prothioconazole and tebuconazole (Prosaro) when used alone are best reserved for less important spray timings, or in situations where disease pressure is low in higher yielding scenarios.
• With SDHI seed treatments such as fluxapyroxad (Systiva) or QoI fungicides used in-furrow such azoxystrobin (Uniform®), use a subsequent foliar fungicide with a different mode of action.

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. FAR Australia gratefully acknowledges the support of all of its research partners in the GRDC Net form of net blotch initiative. FAR Australia also gratefully acknowledges the collaborating support of the breeders in the germplasm evaluation network.

Useful resources

FAR Australia Resources and Publications 

The Australian Fungicide Resistance Extension Network 

Contact details

Nick Poole
Shed 2/63 Holder Rd, Bannockburn VIC 3331
03 5265 1290
0499 888 066
nick.poole@faraustralia.com.au

Darcy Warren
Shed 2/63 Holder Rd, Bannockburn VIC 3331
03 5265 1290
0455 022 044
darcy.warren@faraustralia.com.au