Take home messages

• Proactive disease management, which combines variety selection, paddock selection and appropriate fungicide use, provides proven sustainable and economic disease control.
• Septoria tritici blotch reduced grain yield in highly susceptible wheat varieties by 21% in the MRZ (Wimmera) Vic, during 2024. However, yield losses can be completely avoided if sowing time can be adjusted.
• Net blotches were common in barley and caused grain yield loss of up to 13% in susceptible varieties in 2024.
• Fungicide resistance in NFNB was found associated with the varietal distribution within a region, therefore improve diversity in variety selection within a region.

Background

Implementation of proactive strategies for the control of cereal diseases can prevent avoidable losses when seasonal conditions are suitable for disease. This paper provides an update on the latest research regarding cereal diseases for Victorian growers.

2024 in review

Disease pressure on cereal crops during 2024 was moderate to high, with high levels of inoculum carryover from 2023. Despite well-below-average rainfall for majority of the 2024 season, grain yield losses were found of 10–20% where susceptible varieties were grown with high yield potential on stored moisture. Stripe rust and septoria tritici blotch (STB) in wheat and net and spot forms of net blotch in barley were the common and most impactful diseases in Victoria during 2024. Avoiding highly susceptible varieties was an effective strategy, even during drier seasons, to limit disease risk, reduce fungicide use and minimise grain yield and quality losses. Resistance to fungicides was widespread during 2024 and was particularly more evident with net form net blotch in barley, highlighting the importance of adopting integrated management strategies to slow further resistance development.

Avoiding susceptible varieties for sustainable disease control

The most important component of an integrated disease management (IDM) strategy is the avoidance of highly susceptible (sucker) varieties. We often assume that for genetic control we need to grow resistant varieties (that is, those rated, R, RMR or MR), when varieties with a rating of MS (and in some cases MSS) or better will provide adequate protection from loss, especially when such varieties are grown on a large scale. Where additional IDM strategies are included (for example, crop rotation and green bridge control), complete protection from disease can be achieved.

With increasing susceptibility in varieties, the risk of yield loss increases exponentially, while avoiding highly susceptible varieties can result in large reductions in disease risk (Figure 1). Highly susceptible varieties produce large amounts of inoculum, which has implications not just for that season but also for the next. Use of susceptible varieties may also increase number of fungicide applications required for disease control leading to the development of fungicide resistance.

Figure 1. Varietal resistance rating and grain yield loss due to wheat stripe rust (mean of six sites across Vic, SA and NSW) in 2005.

Yellow leaf spot (YLS) in wheat is an example of how replacing highly susceptible varieties (for example, Yitpi, LRPB Scout, LRPB Phantom) with partially susceptible varieties (for example, Scepter, Rockstar, LRPB Trojan, Vixen), and including other established IDM strategies, can provide widespread disease control. Completely resistant varieties weren’t required.

Conversely, wheat powdery mildew is an example of how partially resistant varieties (for example, Yitpi, LRPB Scout, Axe) were replaced with highly susceptible varieties (for example, Scepter, Rockstar, LRPB Trojan, Vixen, Corack, Wallup) on a large scale, resulting in powdery mildew becoming an important disease.

Rust update

Rust, in particular wheat stripe rust, was common in south-eastern Australia in 2024 due to the high levels of rust present in the preceding seasons and its carry over on volunteer wheat growing over summer (the green-bridge). The common use of up-front treatment (for example, fungicide on fertiliser) provided good early suppression of disease, however high disease occurred when integrated control was not used.

At the time of writing this paper, rainfall during the preceding November and December had been below average and will therefore reduce the opportunity for rust to carry over on volunteer cereals going into the 2025 season. However, even with these reduced risks, good rust management is still important and includes:

• removing the green bridge (volunteer cereals) by mid-March
• using a current cereal disease guide to check resistance ratings of varieties and, where possible, avoiding susceptible varieties
• having a fungicide management plan, with an emphasis on up-front control options
• using the free StripeRustWM App, for iPads and tablets.

Wheat stripe rust

Pathotype analysis during 2024 identified five pathotypes of wheat stripe rust in eastern Australia: 239 E237 A- 17+ 33+, 238 E191 A+ 17+33+, 198 E16 A+ J+ T+ 17+, 238 E191 A- J+ T+ 17+ and 198 Ma+. Multiple pathotypes were frequently identified within single samples that were submitted to the University of Sydney for analysis. Among all pathotypes, the 239 pathotype (first detected in 2017) was the dominant pathotype across various regions, including Victoria, demonstrating its wide distribution on varieties commonly grown across eastern Australia. Pathotype 238 E191 A+ 17+33+ (2021) was the second most dominant pathotype across eastern Australia. The prevalence of 198 pathotype (2018) has declined overall, with it now only detected in northern Australia but not in the southern region. Pathotype 198 Ma+ (2023) is a new variant of the 198 pathotype with similar virulence, except for virulence on Maritime barley and an uncharacterised seedling resistance gene. It was only detected in NSW during 2024 and, like the 198 pathotype, remains virulent on various wheat varieties including DS Bennett, Emu Rock and LRPB Trojan. Due to the diversity in pathotypes present in eastern Australia, the resistance ratings in current disease guides reflect a ‘worse case’ against any of these five pathotypes.

Septoria in wheat

Septoria tritici blotch (STB) has become the most widespread disease in wheat across Victoria, with yield and quality losses common in many parts due to increased area of susceptible varieties and conditions conducive to disease. AgVic trials sown during late April 2024 demonstrated grain yield losses of up to 21% in susceptible varieties in the medium rainfall zone (MRZ-Wimmera, Table 1). However, when sowing was delayed by approximately a month and was still within the optimal sowing window, the risk from disease significantly reduced (P <0.001) and no yield was lost due to sowing time or disease. Equally by avoiding susceptible varieties (as mentioned above), yield loss was minimised even when sown early. These trials clearly highlighted the benefit of integrating different management strategies to avoid disease impacts, which may even mitigate the need for fungicide intervention.

Table 1: Septoria tritici blotch severity and grain yield of wheat varieties with and without disease sown on 24 April (early) and 20 May (late) at Longerenong (MRZ), Victoria, 2024.

^A Dadu (2024) Cereal Disease Guide 2024.^B Within column means with one letter in common are not significantly different (0.05). ** = statistically significant at 1% and * = statistically significant at 5%; ns = not statistically significant when the disease and fungicide treatments were compared. ^C Date of assessment made and Zadoks growth stage. ^D Disease treatment = No disease control with 1kg STB infected wheat stubble; Fungicide treatment = No stubble, Seed (Fluquinconazole 167g/L @ 300mL/100kg seed) + Foliar applied fungicide at Z31 (Benzovindiflupyr 40g/L + Propiconazole 250g/L @ 500mL/ha) + Z39 (Epoxiconazole 500g/L @ 125mL/ha). ^E Yield loss per cent for each variety was presented as per cent yield decrease vs the fungicide treatment.

Net blotches in barley

Net form net blotch (NFNB) and spot form net blotch (SFNB) are common foliar diseases of barley in Victoria due to the adoption of susceptible varieties (for example, RGT Planet (SVS) and Spartacus CL (S)) and stubble retention. During 2024, AgVic trials in the Wimmera (Longerenong) demonstrated losses of up to 11% and 13% in the susceptible varieties RGT Planet and Spartacus CL, respectively (Table 2). Losses in RGT Planet were the result of a combined infection by both NFNB and SFNB, whereas in Spartacus CL, yield loss was primarily due to SFNB and not NFNB. The varieties with better resistance had less infection and no yield loss, again demonstrating the risk of growing very susceptible varieties even during dry seasons and the importance of using resistant varieties to manage net blotches.

Interestingly, NFNB infection levels within the trial were low on the susceptible variety Spartacus CL and high on the moderately susceptible Neo, indicating likely pathogenic diversity in the pathogen population. Such observations are not unusual when diversity is present within a pathogen across the region. These observations suggested the presence of a pathotype that is more virulent on RGT Planet and Neo than others. NFNB’s virulence is understood to be associated with the increase in area sown to RGT Planet in Victoria. Therefore, where RGT Planet and similar varieties are grown, additional control strategies are needed to manage NFNB.

Consistent with previous seasons, fungicides were effective in managing NFNB (Table 2), however, low levels of NFNB were still found on RGT Planet at milk development stage, suggesting only a partial control was achieved. Our annual surveillance activities in barley crops have revealed resistance to fungicides was widespread across the state during 2024 but was more frequent on farms growing RGT Planet and related varieties (Table 3). This suggests a link between the RGT Planet pathotype and fungicide resistance. In contrast, fungicides were found to protect varieties of different genetic background (for example, Spartacus CL) to RGT Planet from NFNB. While more investigations are required to validate such observations, it is recommended that growers employ diversity in variety selection within a region rather than large scale cropping to a single variety, such that it slows down the evolution of pathotypes of concern.

The reduced efficacy of the fungicides in Victoria, as confirmed by the Centre for Crop Disease Management (CCDM) in 2023, was due to fungicide resistance of Pyrenophora teres f. teres (Ptt) to both succinate dehydrogenase inhibitor (SDHI, Group 7) and demethylase inhibitor (DMI, Group 3) fungicides. Hence, these fungicides are no longer reliable options for NFNB control. BASF also detected a mutation affecting the efficacy of quinone outside inhibitor (QoI, Group 11) fungicides used for NFNB control in Yorke Peninsula, South Australia. While its distribution is unknown currently, this detection showcased the rapid adaptability of the pathogen to different groups of fungicides and the potential threat for NFNB management in barley crops.

Table 2: Net form net blotch (NFNB) and spot form net blotch (SFNB) severity (%) and grain yield of five barley varieties with and without disease at Longerenong, Victoria, 2024.

^A Dadu (2024) Cereal Disease Guide 2024. ^B Within a column, means with one letter in common are not significantly different at 0.05. ** = statistically significant at 1% and ns = not statistically significant when the disease and fungicide treatments are compared. ^C Date of assessment made and Zadoks growth stage. ^D Disease treatment = No disease control with 1kg NFNB infected barley stubble; Fungicide treatment = No stubble, seed (Fluxapyroxad 333g/L @ 150mL/100kg seed) + foliar applied fungicide at Z31 (Prothioconazole 150g/L +Bixafen 75g/L @ 500mL/ha) + Z39 (Prothioconazole 210g/L + Tebuconazole 210g/L @ 300mL/ha) + Z55 (Azoxystrobin 133g/L + Prothioconazole 100g/L @ 600mL/ha). ^E Yield loss per cent for each variety was presented as per cent yield decrease vs the fungicide treatment.

Table 3: Mean net form net blotch (NFNB) severity score (1–9 scale) of barley varieties treated with multiple fungicide applications at National Varietal Trial (NVT) sites, Hamilton and Kaniva, Victoria during 2024.

^A Net form net blotch (NFNB) scoring guide: 1 – Resistant (R), 2 – Resistant to moderately resistant (RMR), 3 – Moderately resistant (MR), 4 – Moderately resistant to moderately susceptible (MRMS), 5 – Moderately susceptible (MS), 6 – Moderately susceptible to susceptible (MSS), 7 – Susceptible (S), 8 – Susceptible to very susceptible (SVS), 9 – Very Susceptible (VS). Fungicide treatments at Hamilton NVT, in furrow (Flutriafol 500g/L @ 0.2L/ha) + seed (Fluxapyroxad 333g/L @ 150mL/100kg seed) + foliar at flag –2 (Prothioconazole 210g/L + Tebuconazole 210g/L @ 300mL/ha) + Z49 (Azoxystrobin 320g/L + Epoxiconazole 250g/L @ 250mL/ha). Fungicide treatments at Kaniva NVT, foliar at Z31 (Prothioconazole 210g/L + Tebuconazole 210g/L @ 300mL/ha) + Z39 (Propiconazole 250g/L @ 300mL/ha) + Z55 (Prothioconazole 210g/L + Tebuconazole 210g/L @ 300mL/ha).

Note: Fungicide applications at both the NVT sites were applied to control a range of foliar diseases that affect barley including scald, net blotches, powdery mildew and leaf rust. Therefore, apply above results with caution. Consult respective fungicide labels for appropriate usage.

Fungicide resistance management

There are five strategies that growers can adopt to slow the development of fungicide resistance and therefore, extend the longevity of the limited range of fungicides available.

• Avoid susceptible crop varieties.
• Rotate crops.
• Use non-chemical control methods to reduce disease pressure.
• Spray only if necessary and apply strategically.
• Rotate and mix fungicides/modes of action (MoA).

For more information on the management of fungicide resistance, consult the ‘Fungicide Resistance Management Guide’ available from the Australian Fungicide Resistance Extension Network (AFREN) (https://afren.com.au/resources/#management-guide).

Conclusion

In the absence of proactive disease control, yield losses can be greater than 20%, even during drier seasons in susceptible varieties. It is, therefore, important that plans are developed to effectively manage cereal diseases this season. Disease management plans should consider paddock and variety selection and, where the risk warrants it, the proactive and careful use of fungicides (avoiding their overuse to protect their longevity) should be adopted.

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. Funding for this work was provided by the Victorian Government (Agriculture Victoria) and the GRDC through the GRDC projects: DJPR2104-004RTX, DEE2403-003SAX; DEE2404-004RTX; DAQ2304-008RTX, DAW2112-002RTX and CUR2302-002RTX. Thanks to Agriculture Victoria’s Field Crops Pathology Team. Thanks to our research collaborators at Birchip Cropping Group (BCG) and Drs Lisle Snyman (QDAF), Fran Lopez Ruiz (CCDM), Rohan Kimber, Tara Garrard (SARDI), Mark Mclean (Project Platypus) and Julian Taylor (University of Adelaide).

Useful resources

Cereal disease guide (http://agriculture.vic.gov.au/cereal-disease-guide)

Australian cereal rust survey (https://www.sydney.edu.au/science/our-research/research-areas/life-and-environmental-sciences/cereal-rust-research/rust-reports.html)

Septoria tritici blotch in wheat (https://grdc.com.au/resources-and-publications/all-publications/factsheets/2022/septoria-tritici-blotch-in-wheat)

Australian Fungicide Resistance Extension Network (https://afren.com.au)

Contact details

Hari Dadu
Agriculture Victoria
Private Bag 260, Horsham VIC 3401
03 5450 8301
hari.dadu@agriculture.vic.gov.au
@Imharidadu