Managing cereal diseases in high rainfall environments

Take home messages

• At the Hagley HYC site in 2022, fungicide management strategies for stripe rust and Septoria control combined with variety choice was shown to increase yield from 4.70t/ha to 12.37t/ha.
• The biggest agronomic lever for hyper yielding wheat and closing the yield gap over the last three years has been the correct disease management strategy which had elevated importance in the very wet conditions of 2022.
• For foliar sprays, target the following three key timings for fungicide intervention: first node–second node GS31–32, flag leaf emergence GS37–39, with an optional third application at head emergence GS59 when wet conditions prevail up to head emergence.
• 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.
• Spring sown barley gave no significant (LSD 0.33 t/ha) response in yield to the use of fungicides (Table 1). This result is in line with those in 2021 and 2020. This again demonstrates that intensive fungicide management (multiple applications and expensive chemistry) may not be needed to grow barley in spring sown systems.

Protecting yield potential in the HRZ

Many regions experienced just how important it is to protect yield potential in 2022, with many growers describing the stripe rust epidemic in 2022 as the worst in 20, if not 50 years. Disease management over the last three years has been shown to be one of, if not, the most important factors in securing high yielding crops in the Hyper Yielding Crops (HYC) project trials. It has also been demonstrated to be one of the most important factors in securing higher yields and closing the yield gap in seasons of higher yield potential (above average rainfall) in the Low and Medium Rainfall Zones (L-MRZ).

At the Tasmanian HYC site in Hagley, fungicide management strategies in the presence of Septoria tritici blotch (STB) and stripe rust combined with variety choice was shown to take yields from an average of 9.24t/ha without applied fungicides to 10.98t/ha with a full fungicide package (Systiva® (fluxapyroxad) seed treatment plus three foliar applications) (Figure 1). Varieties that showed better disease resistance also showed less yield response to fungicide application such as AGTW0005 (11.45t/ha–11.59t/ha) and AGFWH004818 (11.51t/ha–11.61t/ha). These two lines were the only varieties tested in this trial to show no significant (LSD 1.08 t/ha) difference in yield between fully treated with fungicide and untreated, indicating good inherent disease resistance (Figure 1). AGTW0005, which has also given similar results at the mainland sites, is expected to be released in 2024. This French red grained feed wheat has now been tested for three years in HYC trials and has excellent yield potential, favourable phenology, good disease resistance and standing power.

Alternatively, varieties that have had their disease resistance gradually eroded showed bigger yield differences between treated with fungicide and untreated, such as Accroc^A which has been tested at the Tasmanian HYC site since 2016 (7.44t/ha–9.85t/ha) and LongReach Beaufort^A (4.70t/ha–8.22t/ha). Susceptible varieties such as LongReach Beaufort^A (rated as MSS for STB) experienced such high STB pressure that, even under a full fungicide management strategy, STB was not properly controlled.

Figure 1. Effect of fungicide and cultivar on final grain yield (t/ha), harvested on 30 January 2023.

When assessed on 18 November 2022, over 70% of the crop canopy in LongReach Beaufort^A was recorded to have STB infection (Figure 2) under full treatment, compared to 96% untreated. Accroc^A also showed high disease infection at this stage in the season when left untreated but had shown much greater response to fungicide in controlling STB, however there was no significant (LSD 2.91 %INF) change in the control of stripe rust between the two management strategies (Figure 2). Varieties that gave little response to fungicide were also those that showed very low levels of disease during the spring.

Figure 2. Effect of fungicide and cultivar on stripe rust and Septoria tritici blotch severity (% plot).

Further investigation into fungicide management strategies were conducted on our Victorian site, with four varieties selected – RGT Cesario^A (MR for STB), Anapurna (MR/MS), Accroc^A (MR/MS) and SQP Revenue^A (MSS), ranging from most STB resistant to most susceptible, respectively. The fungicide management strategies were as follows:

• Nil – untreated control
• A single flag leaf fungicide applied at GS39 – based on SDHI/DMI mixture
• A two-spray approach at GS33 (3rd node) based on SDHI/DMI mixture, and Opus® 500mL/ha at GS59 (head emergence)
• A four-unit approach combining at sowing Systiva with three foliar sprays – GS31 Prosaro® 300mL/ha, GS39 based on SDHI/DMI mixture and GS59 (Opus)

Fungicide applications looked to represent a range of IDM approaches to develop profitable and sustainable disease management strategies for high rainfall zone wheat. The four-unit treatment looked to demonstrate a complete but targeted approach of disease management (primarily STB and rusts) by protecting the leaves that contribute most to yield, also known as the ‘money leaves’. In this treatment the Systiva at sowing and SDHI/DMI mixture on the flag leaf offer robust protection from STB, while the Prosaro and Opus offer protection against rusts and powdery mildew at stem elongation and head emergence respectively. The two-spray management looks at the idea of a ‘straddle approach’ where fungicides are applied between the traditional target growth stages. While a compromise on accurately hitting the ‘money leaves’, this management does provide an option for reducing fungicides if disease pressure, seasonal conditions and other IDM strategies allow. The single fungicide treatment centres on the scenario of lower disease pressure where good genetic resistance to disease is present allowing timing 1 (T1) at GS31-32 to be missed. In this case a single application of a robust fungicide on the flag leaf (the most important leaf in contributing to yield) could be the most appropriate.

The principal disease was STB caused by the pathogen Zymoseptoria tritici, with lower levels of stripe rust (pathogen Puccinia striiformis f.sp. tritici). Stripe rust was most apparent in Accroc^A and RGT_Cesario^A, with untreated Anapurna also recording high levels. STB was so severe in 2022 that not even the four-unit approach to disease management could be described as giving full control in the more STB susceptible cultivars. Except for the most resistant variety RGT_Cesario^A, none of the cultivars had sufficient genetic resistance to be farmed more profitably with no fungicides. RGT_Cesario^A, which in 2021 gave very little response to fungicides, suffered more STB and stripe rust pressure in 2022, but overall was the cultivar with the lowest disease levels (Figure 3). There were no statistically significant differences (LSD 15.88 t/ha) in STB levels amongst one, two and four units of fungicide when applied to RGT_Cesario^A, suggesting that it still maintains strong genetic resistance to STB relative to other cultivars tested, although in 2022, it was observed to be susceptible to stripe rust (Figure 3). All varieties performed as expected in response to STB, with an average of 46.0% of flag leaf area infected with STB in the susceptible SQP RevenueA under full fungicide control compared to just 0.8% in the more resistant RGT_Cesario^A. Although SQP RevenueA showed very low stripe rust infection, it is difficult to draw any conclusions from these results as to its susceptibility to this disease, as so much leaf area had already been occupied by STB.

Figure 3. Effect of fungicide management and cultivar on disease severity on the flag leaf when assessed on 15 December 2022, GS75.

Stripe rust control – active ingredient performance

FAR Australia in collaboration with Trengove Consulting have been looking at the influence of wheat powdery mildew (WPM) on wheat at Katamatite in a GRDC NGN project. The work, whilst not revealing insights into the effect of WPM on yield, did provide an excellent opportunity to examine the influence of stripe rust on the yield of the variety Scepter in the northern Victorian region. The work illustrated that, whilst it’s imperative to have the correct environmental conditions and N inputs to generate yield potential, in 2022, realising yield potential through disease management was even more important. Figure 4 shows the effect of different two-spray fungicide programs applied to Scepter in a scenario dominated by stripe rust (and Septoria tritici blotch (STB) – data not shown) infection. The first fungicide was applied at third node (GS33) as stripe rust was first infecting the crop canopy. The second application at head emergence (GS57) combined a head and flag leaf spray in what is referred to as a ’straddle spray program’, since the two applications are applied either side of, and miss, the flag leaf emergence at GS39. The disease pressure was so severe that differences in active ingredient efficacy were not only observed but associated with differences in both disease control and yield response.

Figure 4. Influence of fungicide product applied in two-spray programs (GS33 and GS57) – FAR Australia Disease Management Centre, Katamatite, VIC 2022 cv Scepter. Columns denote stripe rust leaf area infection, dots denote grain yield, FL-1 denotes the leaf one position below the flag leaf. Notes: As per registered labels, a maximum of one application of tebuconazole can be applied per crop. Two applications were used in this trial to demonstrate relative efficacy only. For resistance management purposes, sequential use of the same mode of action is not advised. Two sequential applications were used in this trial to demonstrate relative efficacy only. Always read and follow product labels.

At Katamatite, two fungicide applications increased yield in Scepter from 1.75t/ha (untreated) to a maximum of 4.14t/ha. With the second fungicide being applied to established infection, it’s important to note that treatments with greater curative activity tended to be more effective. The application of a third fungicide in this research increased yield by a further 0.36t/ha (data not shown).

Whilst fungicides can only be considered an insurance (that is, we don’t know what the economic return will be when they are applied), it is clear that when the stem elongation period is wet as the principal upper canopy leaves emerge (flag, flag-1, flag-2), fungicide application is essential to protect yield potential. Infection was so severe in 2022 that fungicide timing and the strength of the active ingredients being used made a profound difference in productivity. Long ‘calendar gaps’ of over four weeks between fungicides (as was the case in our own work) resulted in many crops losing control of the epidemic as unprotected leaves became badly infected in the period between sprays and applications became more dependent on limited curative activity rather than protectant activity.

The wider issue the success of fungicide management raises is that pathogen resistance to fungicides is primarily driven by the number of applications of the same mode of action. This is why it is imperative for HYC research to incorporate the most resistant, high yielding and adapted germplasm available in order to reduce our dependence on fungicide agrichemicals.

Barley disease – a spring sowing solution

Results from 2022 again showcased the reduced disease pressure that comes with spring sown barley. When compared to results from the Tasmanian ‘Hyper Yielding Cereals’ project (2016–2020) and 2022 autumn sown trials on the mainland, RGT Planet^A’s biggest ‘Achilles heel’ is disease and the need for an extremely robust fungicide program. At the Victorian and South Australian HYC sites, yields were improved by 1.33t/ha and 1.84t/ha respectively between the untreated and the best yielding fungicide managed plots.

With the reduced growing season length and quicker development of the crop, no significant (LSD 0.33 t/ha) yield differences were found in 2022/23 in response to fungicide (Table 1). This is despite low levels of Net form of net blotch (NFNB) and scald being found in the trial. There was also no influence of fungicides on grain quality with the exception of retention, with untreated (92.6%) significantly (LSD 1.1%) lower than most other treatments (average 94.2%) (data not included).

These yield and grain quality results are consistent to those found in 2021 and 2020 and affirm the idea that spring sown barley may not need intensive and expensive fungicide input, especially when compared to autumn sown systems.

*Aviator Xpro applied no later than GS45 in accordance with product label.
** Experimental treatments omitted from table.

Table 1: Influence of fungicide management on grain yield (t/ha).

Acknowledgements

The research undertaken as part of these projects 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 and extension partners in the Hyper Yielding Crops project. These are CSIRO, the Department of Primary Industries and Regional Development (DPIRD) in WA, Brill Ag, Southern Farming Systems (SFS), Techcrop, the Centre for eResearch and Digital Innovation (CeRDI) at Federation University Australia, MacKillop Farm Management Group (MFMG), Riverine Plains Inc and Stirling to Coast Farmers. FAR Australia gratefully acknowledges the collaborating support of Trengove Consulting, SA and CCDM in Perth with the research into wheat powdery mildew.

Useful Resources

FAR Australia Resources and Publications (https://faraustralia.com.au/resource)

Contact details

Darcy Warren
Field Applied Research (FAR) Australia
Shed 2/63 Holder Rd, Bannockburn VIC 3331
03 5265 1290
0455 022 044
darcy.warren@faraustralia.com.au