Wheat disease update – rust and Septoria

Take home messages

• Disease pressure going into the 2023 season will be extreme, with carry-over of inoculum from 2022 crops on both stubble (for example, Septoria) and cereal volunteers (for example, rusts).
• Proactive disease management that combines variety selection (avoiding susceptible varieties), paddock selection (use good rotations that reduce cereal intensity) and appropriate fungicide use, provides proven sustainable and economic control of diseases.
• Up-front fungicides, such as flutriafol on fertiliser, will be important to manage the heightened risk posed by cereal rusts.
• The extremely wet conditions (both amount of rain and number of rain days) during August to November placed cereal crops under unprecedented disease pressure resulting in high losses where diseases were not effectively managed.
• Septoria tritici blotch, an important disease in HRZ, caused yield loss (35–43%) in highly susceptible varieties in field trials in the MRZ (Wimmera) during 2022. Fungicides were required to protect grain yield in the MRZ, increasing yield by ~35% with two sprays (Z31 and Z39) in susceptible cultivars.
• 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.

2022 in review

Disease pressure on cereal crops during 2022 was extreme. The combination of both the amount (Figure 1) and frequency (Table 1) of rainfall from August to November provided ideal conditions for disease development. Not only was 2022 the wettest August to November in the last 100 years (it exceeded the previous wettest in 1975 by 86 mm), it also had the highest frequency of rain days (rainfall ≥0.1mm), with rain on three out of five days (Table 1). During such a high-pressure year, the benefits of appropriate variety selection and proactive management strategies were demonstrated.

Figure 1. Annual total rainfall (mm) for August to November over 100 years (1923 to 2022) compared to the long-term average (156mm) at Longerenong, Victoria.

Table 1: Number of rain days (≥0.1mm) each month during 2022 compared with the 100-year average (1923 to 2022) at Longerenong, Victoria.

Cereal rust update

As expected during 2022, due to the substantial opportunity for rust to survive summer on volunteer cereals (the ‘green bridge’) in northern Australia, rust appeared in Victorian wheat crops earlier than usual. This early rust on-set, along with the favourable conditions for disease development, resulted in a damaging outbreak of stripe rust across Victoria. 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. Where control was inadequate, large yield losses due to stripe rust in wheat occurred. There were limited reports of wheat leaf rust and no reports of wheat stem rust in Victoria. There were multiple reports of barley leaf rust later in the season.

Cereal rust management for 2023

The rust risk going into the 2023 season will be extreme. The opportunities for rust to survive summer on volunteer cereals (the ‘green bridge’) and infect new crops will be immense due to widespread summer cereal volunteers across eastern Australia. Rust outbreaks are more severe following seasons with widespread volunteers, as rust can only survive from one season to the next on living plant material (it doesn’t survive on seed, stubble, or soil).

It is therefore essential that growers take the following steps to reduce their risk:

• remove the green bridge (volunteer cereals) by mid-March
• use a current cereal disease guide to check resistance ratings of their varieties and, where possible, avoid susceptible varieties
• develop a fungicide management plan, with an emphasis on up-front options, such as flutriafol on fertiliser, to provide early rust suppression
• download the StripeRustWM App; free for iPads and tablets for support with wheat stripe rust management.

When rust risk is high, the benefits from widespread use of up-front fungicide treatments (such as flutriafol on fertiliser) should not be underestimated in providing regional control. Such a practice on all at-risk cereal varieties on an industry wide scale can greatly reduce the rust risk across a district.

Septoria in wheat

Septoria tritici blotch (STB) was the second most damaging disease of wheat after stripe rust in 2022. The unprecedented spring rains (Figure 1 and Table 1) favoured STB, with the disease able to progress to the top of plants, causing yield losses in many crops where control was inadequate.

Historically, losses due to STB were most common in susceptible cultivars in the high rainfall zone. However, AgVic trials demonstrated losses of ~35–43% in the Wimmera (medium rainfall zone) during 2022 compared with <10% during 2021, clearly demonstrating the role of rainfall in the damage caused by STB. Figure 2 shows the relative progress of STB on a susceptible wheat variety grown in the Victorian low, medium and high rainfall zones (LRZ, MRZ and HRZ) during 2022.

Yield losses can be greatly reduced when resistant varieties are grown in preference to susceptible varieties. Field experiments conducted by AgVic at Longerenong (MRZ) during 2022 demonstrated yield losses of around 35–43% in susceptible (S) and susceptible to very susceptible (SVS) varieties, representing losses of ~1.7t/ha in yield (Table 2). The losses in less susceptible varieties (LRPB Lancer (MS) or Hammer CL Plus (MSS)) were 21% or less, demonstrating the benefit in avoiding highly susceptible cultivars.

Fungicides provided variable control of STB in a field experiment conducted at Longerenong (MRZ) (Table 3). All fungicide treatments reduced STB severity compared to the untreated control, but the early applications of seed only or a single spray at Z31 were not as effective as any of the three treatments that included a fungicide application at Z39. Within this trial, the best economic control of STB was achieved by two foliar fungicide applications at Z31 and Z39 (with or without a seed treatment) increasing grain yield by 37–39%.

Figure 2. Septoria tritici blotch severity (% leaf area affected) across time in wheat (cv. Razor CL Plus, susceptible to very susceptible to STB) at three different rainfall zones in Victoria, during 2022.

Table 2: Septoria tritici blotch severity (% leaf area affected) and yield loss of six wheat varieties treated with and without disease at Longerenong, Victoria, 2022.

AWithin column means with one letter in common are not significantly different (0.05). ** = statistically significant at 5% Lsd. BDate of assessment made andZadoks growth stages Z37 flag leaf emergence; Z51 ear emergence; Z75 milk developmentaccording to Zadoks et al. (1974). First two assessments were average of single plot assessments, while the third assessment was average of the top three leaves of ten tillers per plot. C Max. disease = Maximum disease treatment; Min. disease = Minimum disease treatment. D Yield loss % for each variety was presented as percentage yield decrease vs the minimum disease treatment.

To minimise losses from STB, avoid highly susceptible varieties (especially those rated S and SVS). If conditions are suitable for STB, then fungicide strategies should include applications at both growth stages Z31 and Z39. Also, avoid sowing wheat into paddocks with one- or two-year-old wheat stubble, noting that early STB infection will also come from wind borne spores from adjacent paddocks.

Table 3: Septoria tritici blotch severity (%) and yield loss in the wheat variety Scepter (S) in response to different fungicide treatments at Longerenong, Victoria, 2022.

AWithin a column, means with one letter in common are not significantly different at 0.05. First two assessments were average of single plot assessments while the third assessment was average of the top three leaves of ten tillers per plot. BMax. disease = Maximum disease treatment. CYield gain % for each treatment was presented as percentage yield increase vs the untreated control. #ai = active ingredient

Fungicide resistance in cereals

Resistance to fungicides is becoming an increasing threat to Australian cereal crops. Recent research by the rust research team at the University of Sydney have convincing evidence from growth room studies of insensitivity (resistance) to several DMI fungicides in the leaf rust pathogens of wheat and barley. These findings come from the screening of more than 800 cereal rust isolates of wheat (stem rust, leaf rust, stripe rust), barley (leaf rust) and oat (crown rust, stem rust) for sensitivity to the DMI fungicide tebuconazole in recent years. In both cases, the insensitivity was to not only tebuconazole, but also triadimefon, propiconazole and prothioconazole. At this stage field failures have not been observed and further field validation of these findings are required.

In barley leaf rust, the insensitivity was found in the most common isolates in Australia. The insensitive isolates were able to grow and sporulate despite rates of tebuconazole (not registered for control of leaf rust in barley) of more than 10 times the recommended high field rate of 290mL/ha.

Within wheat leaf rust, insensitivity to four DMI fungicides was identified in a single pathotype (93-

3,4,7,10,12 +Lr37). Fortunately, nationally this pathotype is only present at low levels, but it could grow and sporulate on leaves treated with rates of tebuconazole up to 25 times the recommended high field application of 290mL/ha. Further field validation of these findings are required.

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. Where possible select the most resistant crops suitable and/or avoid putting susceptible crops in high-risk paddocks
• Rotate crops. Avoid planting crops back into or adjacent to their own stubble
• Use non-chemical control methods to reduce disease pressure. Delaying sowing, early grazing are examples of strategies that can reduce disease pressure
• Spray only if necessary and apply strategically. Avoid prophylactic spraying and spray before disease gets out of control
• Rotate and mix fungicides/modes of action. Use fungicide mixtures formulated with more than one mode of action, do not use the same active ingredient more than once within a season and always adhere to label recommendations.

For more information on the management of fungicide resistance consult the ‘Fungicide Resistance Management Guide’ available from www.afren.com.au

Wheat head diseases in 2022

There was widespread disease in the heads of wheat crops during 2022 which resulted in shrivelled grain and/or reduced grain number. Work is on-going to define all the causes, but preliminary results indicate that the primary cause, in many cases, was stripe rust damage to the head, with other pathogens and saprophytes also involved.

Molecular testing of affected wheat heads in collaboration with SARDI found stripe rust in all samples tested, along with Septoria tritici. The pathogen causing yellow leaf spot was common, but not in all samples. The crown rot fungus, Fusarium pseudograminearum,was identified in wheat heads but generally at low levels. No other Fusarium spp. have been identified at this stage.

Symptoms of false or pseudo black chaff (a physiological condition associated with the stem rust resistance gene Sr2) were often confused with glume blotch. Subsequent testing identified Stagnospora nodorum (glume blotch pathogen) in a small number of samples, but this was generally at low levels and not a widespread issue.

The major causes of the wheat head issues during 2022 were most likely the exceptionally wet conditions during flowering and grain fill (Figure 1 and Table 1) and high levels of stripe rust. It is recommended that seed lots for 2023 sowing are tested for germination and vigour.

Conclusion

In the absence of proactive disease control, yield losses due to diseases can be greater than 20%. The risk from rust diseases in 2023 will be high due to widespread volunteer cereals carrying rust inoculum from last season. Given this heightened rust risk this season, the widespread use of up-front fungicides (for example, flutriafol on fertiliser) is highly recommended, especially on rust susceptible varieties, to help with early season control. Disease management plans should consider paddock and variety selection and, where the risk warrants it, the proactive and prudent use of fungicides that avoid overuse to protect their longevity.

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 author 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, DJP2103-005RTX, DJP2003-011RTX, DJP1905-002SAX, University of Sydney (9175448). Thanks to Agriculture Victoria’s Cereal Pathology Team: Jordan McDonald, Glenn Sluggett, Joshua Fanning, Melissa Cook, Luise Fanning, Chloe Findlay, Bhanu Kalia, Swapan Brar and Sandra Mayberry. Thanks also to the Birchip Cropping Group for field trials within the Victorian Mallee and to our research collaborators Andrew Milgate (NSW DPI), Julian Taylor (University of Adelaide) and Tara Garrard (SARDI).

References

Zadoks JC, Chang TT, Konzak CF (1974) A decimal code for the growth stages of cereals. Weed Research 14, 415-421.

Cereal disease guide

Australian cereal rust survey

Field crop diseases Victoria 

Contact details

Grant Hollaway
Agriculture Victoria
Private Bag 260, Horsham VIC 3401
03 5450 8301
grant.hollaway@agriculture.vic.gov.au
@Grant_Hollaway

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

Mark McLean
Agriculture Victoria
Private Bag 260, Horsham VIC 3401
03 5450 8301
mark.s.mclean@agriculture.vic.gov.au
@msmclean777