Take home messages

• Choose wheat varieties with higher levels of genetic resistance to stripe rust to reduce reliance on foliar fungicide applications
• Understand when adult plant resistance (APR) becomes active in individual wheat varieties and overlap timing of foliar fungicides if necessary to protect leaves prior to APR expression
• Know the nitrogen status of each paddock because a high soil nitrogen bank will delay the onset of APR and should therefore be managed as if one resistance level lower
• Delay the development of fungicide resistance by rotating fungicide use from different mode of action groups and use mixtures if available
• However, the critical first step remains correct disease identification. NSW DPIRD pathologists are here to assist!

Introduction

Stripe rust infection, caused by the fungus Puccinia striiformis f. sp. tritici, remains an on-going issue to wheat production across the northern region with multiple pathotypes complicating management decisions. Other cereal leaf diseases such as septoria tritici blotch and yellow leaf spot have also been present in cereal crops over the past few seasons but generally stripe rust is the primary target for integrated disease management.

At least six hours of leaf wetness is needed for a stripe rust spore to germinate and infect the leaf blade. Once established, further disease progression is purely dependent on temperature. The optimum temperature range for stripe rust development is 12–20 °C. At these temperatures it will take 10–14 days for a fresh batch of spores to emerge from infected leaves. This is called the latent period, during which time stripe rust infection within leaves is not visible. Temperatures above or below this optimum range DO NOT kill the pathogen. Rather the fungus slows and can become dormant outside these temperatures but importantly will continue to develop once temperatures return to the optimal range. Hence, the more time in a 24-hour period between these optimum temperatures, the shorter the latent period. Conversely, as temperatures normally warm in spring the stripe rust fungus stops developing during the day once above 22 °C but continues again overnight as temperatures drop. In these circumstances, the latent period extends to a 20+ day cycle time.

Season 2021 was wet, 2022 was excessively wet with 2023 being appreciably drier. The 2024 season was variable across the region being abnormally dry in south-eastern NSW but around average elsewhere. Another factor is the temperature during spring and how long crops are exposed to temperatures between 12 °C and 20 °C where stripe rust cycles the quickest. Consequently, the frequent rainfall and extended mild temperatures well into spring across much of the northern grain region in 2021 and 2022, favoured infection and multiple lifecycles of stripe rust. These conditions created a moderate-high pressure season for stripe rust across this region in 2021, very high in 2022, low in 2023 and low-moderate in 2024 (Figure 1). However, how did growers react to stripe rust management in these differing seasons? What is current fungicide usage in central and northern wheat crops?

Figure 1. Mean daily temperature for spring (Sep-Nov) in 2021 to 2024, and the resulting stripe rust pressure across seasons.

Annual random crop disease surveys

NSW DPIRD, with co-investment from GRDC, has been conducting annual crop disease surveys across NSW since 2018 including some crops in southern Qld. In the northern survey, which covers from Forbes in central NSW up into southern Qld, collaborating agronomists kindly collect the random samples from grower paddocks including background information on variety grown, paddock history and fungicide(s) applied. This provides an unbiased annual picture of wheat varieties being grown commercially across this region and fungicide usage. The number of crops surveyed with associated fungicide data varied between seasons with 185 in 2021, 165 in 2022, 154 in 2023 and 110 in 2024 (Figure 2).

In-crop fungicide applications

In 2021, nearly 31% of randomly surveyed crops did not receive an in-crop foliar fungicide application (Figure 2). In 2022, a season more favourable for stripe rust infection, only 3.6% of crops did not receive an in-crop foliar fungicide application. In the following seasons, which were both less conducive to stripe rust, 24% of crops in 2023 and 15.5% in 2024 were not sprayed with fungicide. Multiple in-crop fungicide applications were highest in 2022 at 49.7% of crops and lowest in 2023 at 3.9% of crops (Figure 2). Interestingly, the number of wheat crops with multiple fungicide applications was similar in 2021 (17.3%) and 2024 (18.2%) even though 2024 was a less conducive season to stripe rust development. Encouragingly, these results highlight growers adapted their management strategies and applied foliar fungicides according to seasonal conditions and disease risk.

Figure 2. Fungicide applications to wheat crops collected in random crop disease surveys from central NSW to southern Queensland across 4 seasons.

Older Group 3 demethylation inhibitor (DMI) fungicides (triazoles) dominate

When a foliar fungicide was applied, there was a heavy reliance on two older triazole (DMI) active ingredients, tebuconazole (e.g. Folicur^®) and propiconazole (e.g. Tilt^®), to manage stripe rust in each season (Table 1). Each season either of these two fungicide actives were present in between 73% to 87% of in-crop fungicide applications across the region with tebuconazole being more commonly used (Table 1). These two older actives are normally used as single ingredients within fungicide applications across the region but there has been an increasing trend for their use in mixture with especially azoxystrobin (Group 11), epoxiconazole (Group 3) or prothioconazole (Group 3) since the 2023 season (data not shown).

Table 1. Proportion of foliar fungicide applications which contain tebuconazole or propiconazole across seasons.

The fungicide usage pattern suggests this region is potentially slowly progressing towards fungicide resistance issues with the reliance on two older Group 3 DMI actives, but at a reduced rate than if multiple applications of the same product were applied within an individual season. The increasing use of these older triazoles in mixtures with fungicides from another mode of action (MoA) group (e.g. Group 11, strobilurins) and even newer Group 3 DMIs (e.g. epoxiconazole and prothioconazole) will also assist in reducing the rate of fungicide resistance development within fungal pathogen populations. Note that annual University of Sydney testing within the Cereal Rust Laboratory has not identified any reduced sensitivity of current stripe rust pathotypes to fungicides within Australia. Even though rusts are generally considered a lower risk pathogen for the development of fungicide resistance, fungicide application for stripe rust within susceptible wheat crops still has selection pressure on other higher risk fungal pathogens if also present at the time of application such as Blumeria graminis f. sp. tritici (wheat powdery mildew) or Zymoseptoria tritici (Septoria tritici blotch). For the status of fungicide resistance in cereal pathogens across NSW and integrated disease management strategies to reduce risk refer to recent collaborative research between NSW DPIRD and CCDM (Baxter et al., 2025).

The data reflects the general practice of growers north of Forbes to avoid using multiple fungicides on their wheat crops. This is related to the heavy clay soils that dominate the region. These soils potentially pose trafficability issues for spraying using ground rigs in wetter seasons, which are also more conducive to leaf diseases. In such seasons, logistical difficulties can also occur when chickpea crops represent a high proportion of the rotation across this region and require fungicide management of Ascochyta blight.

What varietal stripe rust resistance levels are being grown?

In 2021, 61% of wheat crops surveyed across this region were rated resistant to moderately resistant (RMR) to stripe rust, while 17 per cent were moderately resistant to moderately susceptible (MRMS), and only nine per cent were moderately susceptible to susceptible (MSS) (Figure 3).

Figure 3. Selected wheat varietal stripe rust resistance levels grown across seasons from central NSW to southern Qld.
RMR = resistant to moderately resistant, MRMS = moderately resistant to moderately susceptible, MSS = moderately susceptible to susceptible.

In 2022, the number of RMR varieties randomly surveyed dropped to 40%, with growers exploring increased areas of newer, higher-yielding varieties, which unfortunately have reduced levels of genetic resistance to stripe rust but were still predominantly MRMS. However, following issues associated with managing stripe rust in more susceptible varieties in a high infection season, the proportion of RMR varieties planted surged back to 61 per cent in 2023 (Figure 3). The proportion of RMR varieties dropped again in 2024 to only 39% but was replaced mainly by MR and MRMS varieties. Even though the proportion of RMR varieties fluctuates between seasons, growers are not replacing this area with wheat varieties with resistance levels any lower than MRMS. The proportion of newer varieties with an MR rating has progressively increased from 7% in 2021, to 12% in 2022, 16% in 2023 and 22% in 2024 (data not shown). This highlights the importance growers across this region place on wheat varieties having adequate levels of stripe rust resistance.

Fungicide application across varietal resistance levels

The average number of foliar fungicide applications applied to wheat varieties with different levels of stripe rust resistance was also evident in this random survey data across seasons. In 2021, on average growers, applied less than one (0.6) foliar fungicide spray to RMR varieties, 0.9 to MRMS varieties and 1.5 to MSS varieties (Table 2). In 2022, this increased to 1.4 fungicides in RMR varieties because the season was more conducive to stripe rust infection, especially at early seedling stages, and under drier conditions in 2023, it dropped back to 0.7 and was 0.9 in 2024.

Table 2. Average number of foliar fungicide applications to wheat varieties with different levels of stripe rust resistance grown across seasons from central NSW to southern Qld.

The average number of fungicide applications applied by growers in MSS varieties also rose during 2022 to 1.9. This was because these varieties needed more protection against disease in a high-pressure season. This data highlights that growers across this region are generally adjusting their in-crop fungicide management strategies to match seasonal conditions and associated stripe rust pressure. Their management strategy also reflects the reduced need for foliar fungicide applications in wheat varieties with increasing levels of genetic resistance.

Did fungicide applications reduce infection levels?

As part of these annual surveys random plant samples from each paddock are collected by the collaborating agronomists during grain fill and then rated visually for infection by NSW DPIRD at Tamworth. The three top leaves, which contribute most to yield in wheat crops, were also analysed using a quantitative polymerase chain reaction (qPCR) to determine the DNA levels of a range of fungal pathogens, including stripe rust. The study therefore allows regional mapping where stripe rust infections were detected and most severe in the top three leaves, highlighting the role of seasonal conditions in epidemic development.

However, individual crop data can also be linked to integrated disease management strategies used by growers (varietal resistance and fungicide application) to determine if this influenced stripe rust severity (pathogen DNA levels in leaves). No RMR or MRMS varieties were treated with three fungicides in any season, nor MSS varieties grown without fungicide (Table 3), indicating that growers made the right management decisions based on levels of genetic resistance.

Table 3. Effect of fungicide application and varietal resistance on level on stripe rust infection in wheat crops between seasons¹.

The qPCR data highlights the power of growing an RMR-rated variety for protection against stripe rust, with lower infection levels noted across seasons and numbers of foliar fungicide applications (Table 3).

In the wetter seasons of 2021 and 2022, the stripe rust pathogen load in MSS varieties was much higher than in MRMS varieties with fungicide applications reducing infection levels. If an RMR variety was grown, the stripe rust pathogen load was significantly lower again, and the need for any foliar fungicide application appeared marginal based on pathogens levels in the top three leaves. In 2022, an MSS variety required three foliar fungicide applications and an MRMS variety needed two, while RMR varieties required one or none to achieve a similarly low pathogen load in the top three leaves (Table 3).

In contrast, the much drier and warmer 2023 season during spring, in hindsight, questions the need to have applied any foliar fungicides to RMR and MRMS varieties with the conditions being so non-conducive to stripe rust development.

Using adult plant resistance in wheat varieties – the spray dilemma

Seasonal conditions not only affect the stripe rust pathogen, but they also affect crop development and expression of resistance genes in different wheat varieties. Most varieties rely on adult plant resistance (APR) genes for protection from stripe rust, which as the name implies, become active as the plant ages. Consequently, all varieties, unless rated resistant (R), are susceptible as seedlings and move towards increasing resistance as they develop, and APR genes become active.

The stripe rust fungus is what is termed a ‘biotroph’ which means it needs living wheat cells to survive. APR is related to a mechanism of programmed cell death which destroys the cells infected by the pathogen and those surrounding these infected plant cells. This eliminates the infection because the pathogen is essentially deprived of a living host. However, APR activates at different times related to the genetic resistance rating of a variety. Therefore, if your crop has experienced a significant infection when APR is expressed, the green leaf area will be stripped from the crop canopy to kill this infection, which is not ideal for producing higher grain yields. However, if APR is already expressed and conditions are still conducive to stripe rust infection then only mild flecking in the leaves will be visible if held up to the light. This is often best observed on the margin of stripe rust hotspots where disease pressure from the hotspot shows as leaf flecking on plants adjacent to this area when APR is active.

Applying a foliar fungicide at key growth stages two to three weeks before APR becomes active in seasons conducive to stripe rust development is the ultimate aim of an integrated approach to disease management. Keeping the top three leaves green during grain filling is key to producing high wheat yields. But remember, foliar fungicides only protect emerged leaves at the time of application, for roughly two to three weeks.

Understanding the genetic resistance of individual wheat varieties grown is therefore crucial to the correct timing of foliar fungicide application to overlap protection of the crop until APR is active. In an MR variety such as LRPB Raider, APR begins at growth stages 30 to 32. For an MRMS variety like Sunmaster, APR activates at growth stage 37 to 39, while for MSS varieties like Coolah, this resistance doesn’t become active until around growth stage 61 to 75 (Figure 4).

A significant proportion of stripe rust enquiries in 2024 related to identifying whether the disease was still active in MRMS crops like Sunmaster or if leaf speckling was the crop expressing APR. Many growers who had generally correctly applied their first fungicide around GS30-32 saw leaf flecking symptoms sometimes with a few necrotic stripes with very limited pustule formation around GS39-49 and wanted to know what to do because they thought stripe rust was still active in their crops. However, stripe rust was not rampant; APR had taken out the infections, and a small amount had very limited pustule formation. It was a case of trusting the variety’s genetic resistance and refraining from applying another fungicide when APR was active and had it covered.

Figure 4. The spray dilemma with stripe rust management.
Note: MR = moderately resistant, MS = moderately susceptible.
Flag-3 means the plant has developed to the first node on the main stem and three leaves below the flag leaf. Flag-2 means the second node is visible on the main stem, and two leaves below the flag leaf have emerged. Flag represents when the flag leaf becomes visible on the main stem.
The varieties shown above are all protected under the Plant Breeders Rights Act 1994. Lancer and Raider are LongReach varieties.

Another management option, especially in more susceptible wheat varieties, is treating starter fertiliser with the fungicide flutriafol to protect the crop to growth stages 31 to 32. This approach significantly reduces early stripe rust pressure in more susceptible varieties and within a localised area if used widely by growers, as stripe rust spores are readily windborne.

The nitrogen effect

High soil nitrogen levels can further complicate stripe rust management decisions. When a paddock has elevated nitrogen levels, such as after a pulse crop or when large amounts of urea have been applied upfront, an MR variety will perform like an MRMS variety because high nitrogen levels delay the expression of APR. Accordingly, growers should adjust their fungicide management strategy in wheat crops with higher nitrogen status and treat them as if they had one category lower level of genetic resistance.

Correct identification

Importantly, the first step prior to considering the in-crop application of any fungicide is to ensure correct identification. The ability to understand when a fungicide application is not required, is just as important to know than when one is required. NSW DPIRD diagnostics testing highlights that around 10-15% of samples and enquiries annually are incorrectly assumed to be disease when they are related to non-biotic factors (e.g. nutrition, herbicide or frost damage) or simply physiological reactions to stress within cereal plants (e.g. leaf tip necrosis or melanism). NSW DPIRD pathologists can assist with correct diagnosis and management advice to ensure growers only apply fungicides when appropriate.

Conclusion

Random survey data highlights the value of starting with a solid base of varietal resistance. More than 60% of the wheat varieties grown north of Forbes are RMR or MR to stripe rust which reduces disease pressure in these crops and across the region. Many growers say that if a variety needs more than one foliar fungicide spray for stripe rust within a season, it is thrown out in the north because these varieties are too high maintenance. Growers are adapting their foliar fungicide management strategies to match seasonal disease pressure and level of varietal resistance which is great.

References and further resources

Australian Fungicide Resistance Extension Network

Crop sowing guides

NVT disease ratings

Managing stripe rust in a high-pressure season

Baxter B, Lopez-Ruiz F, Chang, S, Turo C, Mair W, Dodhia K, Zulak K, Poole N, Park R, Chhetri M, Williams M, Ovenden B, Milgate A & Simpfendorfer S (2025) NSW fungicide resistance update – status of cereal pathogens. GRDC Grains Research Update paper.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers and their advisers through their support of the GRDC. The author would also like to acknowledge the ongoing support for northern pathology capacity by NSW DPIRD. This research would also not have been possible without the support of growers and advisors through collection of annual random cereal survey samples for testing and provision of background crop management information including fungicide use.

Contact details

Steven Simpfendorfer
NSW DPIRD, 4 Marsden Park Rd, Tamworth, NSW 2340
Ph: 0439 581 672
Email: steven.simpfendorfer@dpi.nsw.gov.au
X: @s_simpfendorfer or @NSW_AGRONOMY

Date published
February 2025

^® Registered trademark
Varieties displaying this symbol are protected under the Plant Breeders Rights Act 1994.