Stripe rust of wheat
Given favourable conditions stripe rust can cause large losses in susceptible wheat varieties. However, growers have shown that by planning to manage this disease they can effectively minimise its effects.
Stripe rust in Victoria
Stripe rust is the most important disease of wheat in Victoria, with it occurring in most seasons and able to damage crops in all regions.
Since the first introduction of wheat stripe rust into Australia in 1979 there have been a further three introductions (2002, 2017 and 2018) of significance. Three of these incursions may have originated from Europe (1979, 2017 and 2018) while North America (2002) was implicated as the source of the other.
Over time these introductions undergo mutations which result in increased virulence (ability to attack more resistant varieties) and in some cases will displace older pathotypes. In some seasons a single stripe rust pathotype will predominate while in other seasons there will be a range of pathotypes present. For example, during 2020, the University of Sydney detected approximately seven different strains of wheat stripe rust in eastern Australia.
The resistance ratings provided in Victorian Cereal Disease Guide (AG 1160) and the National Variety Trials (NVT) website represent the dominant and most virulent of the pathotypes present in Victoria.
What to look for
Stripe rust is easiest to identify in the morning.
Examine the leaves, especially the older leaves low in the canopy, and look for yellow stripes of pustules. Pustules are raised above the leaf surface and can be easily wiped off onto a white cloth or tissue leaving a yellow stain.
Also, watch for hot spots in the crop. Hot spots are often 1–10 metres in diameter, and are generally well developed just before the disease becomes widespread in the crop.
Economic impact of stripe rust
When susceptible (S) and very susceptible (VS) varieties are grown, stripe rust is likely to cause annual average losses of up to 50 per cent with higher losses possible.
Disease cycle
Stripe rust is caused by Puccinia striiformis f.sp. tritici. The fungus is dispersed as wind-blown spores which produce new infections. This cycle is repeated many times during the cropping season causing epidemics to develop.
Conditions suitable for epidemic development occur from April to December in Victoria, and stripe rust can be expected in susceptible crops by September in most years.
The fungus has a wide temperature range, but its optimum is 6-12°C with a minimum of 3 hours of leaf-wetness (for example, dew) for new infections to occur. Once an infection is established the fungus can survive short periods of temperatures higher than 40°C.
Following infection of the leaf the fungus grows without symptoms within the leaf for 14 to 28 days (or longer depending on temperature) until the spores appear on the leaf surface as pustules. The time between initial infection and symptom development is known as the latent period, with this time reducing as temperatures warm during the spring. The spores are then spread by wind, allowing the cycle to repeat multiple times during the season.
Stripe rust survives from one season to the next predominantly on volunteer self-sown cereals but can also survive to a lesser extent on other cereal and grass weeds that grow over summer and is known as the ‘green bridge’. Consequently stripe rust epidemics are usually more severe in seasons following wet summers that have supported the ‘green bridge’.
Stripe rust management
Stripe rust can cause significant loss to grain yield and quality, given appropriate environmental conditions and susceptible varieties. However, farmers have shown that by planning to manage this disease they can effectively minimise its impact.
The most appropriate stripe rust management strategy will vary from one farm to another, from region to region, and from season to season.
There has been much discussion regarding the merits of various approaches to stripe rust management, such as variety seed choice or fertiliser fungicide treatment up-front, versus reliance on fungicide sprays alone.
Whichever strategy is used, provided it is implemented in a timely fashion, it will be effective. It is important that growers choose a strategy that is appropriate for their situation and follow it during the growing season.
The following management strategies are recommended to minimise the impact of stripe rust:
- Remove volunteer wheat plants (the 'green bridge') that will support stripe rust inoculum in the 6 weeks prior to sowing.
- Avoid growing Susceptible (S) and Very Susceptible (VS) varieties by selecting more resistant varieties.
- Use a seed or fertiliser treatment to suppress early infection.
- Monitor crops during the growing season and apply a foliar fungicide early in the epidemic, if required.
The 'green bridge'
Stripe rust can only survive from one season to the next on living plants (mostly wheat, and to a lesser extent barley, triticale, barley grass, brome grass and phalaris).
This is called the 'green bridge'.
Experience has shown that rust epidemics are worse following wet summers that have supported the widespread growth of volunteers.
Stripe rust does not survive on seed, stubble or soil. Therefore, the more susceptible volunteer wheat plants that grow during summer and autumn the greater the risk of a stripe rust epidemic.
The susceptibility of the volunteer wheat plants over summer influences the quantity of inoculum generated by the green bridge. If most varieties in a district are resistant there will be considerably less inoculum than if the majority of plants are susceptible or very susceptible.
It is critical that all volunteer wheat plants are removed either by spraying, cultivation or heavy grazing by the end of March. Care should be taken to destroy plants around sheds and silos, where stripe rust can survive.
Variety selection
Selecting wheat varieties for rust resistance is an extremely important part of rust management.
Select varieties with the highest levels of rust resistance possible, keeping in mind other agronomic and disease traits of the variety. The actual disease response that occurs in the field will depend on many factors including:
- the amount of inoculum carry over
- the timing of the rust outbreak in the crop
- the pathotypes (races/strains) of stripe rust occurring in a region.
Stripe rust resistance ratings are determined by multi-site field screening conducted nationally through the GRDC’s NVT disease screening program. The resistance ratings to stripe rust are based on data collected from around Australia. The disease resistance ratings are updated annually and published in the Victorian cereal disease guide and on the NVT website.
However, since stripe rust pathotypes (strains) are known to change over time, it is critical that only the current disease ratings in the Victorian Cereal Disease Guide (AG 1160) are used.
Varieties rated as Susceptible (S) or Very Susceptible (VS) to stripe rust should be avoided. In such varieties stripe rust is more difficult to manage, especially if the season is favourable for stripe rust. S and VS varieties have the potential to rapidly lose all leaf area to stripe rust.
The build-up of rust on these varieties can lead to infection of other crops in the district and increase the chance of resistance break-down occurring due to the large amount of spores they produce.
If S or VS varieties are grown it is critical that seed or fertiliser is treated with a fungicide before sowing. Crops of S and VS varieties should be monitored regularly for the first sign of rust, and a fungicide applied when necessary.
Varieties rated as Moderately Resistant to Moderately Susceptible (MRMS) or Moderately Susceptible (MS) generally have adult plant resistance. These varieties are unlikely to lose all their flag leaf to disease but may need a fungicide spray if rust is detected early (before flag emergence).
Varieties rated Moderately Resistant (MR) show only limited rust symptoms on their flag leaves under ideal rust conditions. Varieties rated as R, are those with resistance which persists for the duration of the plant's life. Even varieties rated as MR and Resistant (R) should be monitored with a view to fungicide application as mutations in the rust can occur.
Seed and fertiliser fungicide treatments
Seed and fertiliser fungicide treatments play an important role in stripe rust management.
In the Wimmera, Western, Central and North-East districts all varieties with a stripe rust rating of MS or lower should be treated with either a seed or fertiliser treatment to suppress early stripe rust.
In the Mallee, growers should use a seed or fertiliser treatment that suppresses early infection in crops when there is a high carry over of inoculum on the "green bridge" and susceptible varieties are grown.
These treatments are most effective when adopted across a region as they greatly reduce the inoculum levels in a district. The length of protection varies depending on the product selected.
For a comprehensive list of products, see the SARDI Cereal Seed Treatments guide.
Note that fertiliser treatments do not control bunts and smuts, so a seed treatment still needs to be applied to the seed.
Also, where crops are sown early for anticipated grazing benefits, issues such as withholding period will need to be considered.
Fungicide sprays
Effective fungicides for controlling stripe rust are available but should be regarded as a support, and not a substitute, for growing resistant varieties.
The requirement for fungicide sprays will depend on:
- the carry-over of rust inoculum on the 'green bridge'
- the timing of the epidemic (in relation to crop growth stage)
- the level of resistance in the variety.
For example, in 2011 where stripe rust was detected early (such as tillering to flag), a fungicide spray was required in many varieties to protect green leaf area until the onset of adult plant resistance, which starts around ear emergence.
Varieties without effective adult plant resistance (APR) may have required sprays beyond ear emergence. Sprays are generally more effective when applied early in an epidemic.
It is likely that the onset of a rust epidemic will be different in different years. The timing of the first occurrence in the crop may be different, and the area where it first occurs may also be different. It is, therefore, important that the decision to apply fungicides is made during the season, using available information, and is not based on previous experiences alone.
During the season crops should be monitored regularly (at least every 2 weeks) for the presence of stripe rust. The earlier that rust occurs within a crop the greater the potential loss but the easier it is to control.
If stripe rust is present before ear emergence, then crops must be sprayed before the level of infection reaches 1 per cent leaf area affected (this is when approximately 35 leaves per 100 have stripe rust). It is better to spray sooner than later.
When stripe rust is first detected at ear emergence, only the most susceptible (S and VS) crops or longer season crops may need spraying. After a fungicide application, crops should continue to be monitored as fungicides only provide between 2 to 4 weeks protection.
There is often an apparent increase in stripe rust for a few days after spraying. This is caused by the development of symptoms of infections that occurred just before spraying. Control becomes apparent within a week of spraying, and the period of protection is normally about four weeks.
Early season protection versus foliar sprays
There has been much discussion as to the relative merits of either applying or not applying early season seed or fertiliser treatments (with follow up fungicide spray if required) versus just relying on applications of foliar fungicides.
Both approaches can effectively manage stripe rust, with similar costs to the grower, if used appropriately.
The disadvantage of early season protection is that expense is incurred before knowing if rust will be an issue, or the yield potential of the crop. The advantage of the early applied long season protection is that in the presence of rust, the likelihood that a fungicide will be required before flag leaf emergence is reduced. This minimises the need for timely fungicide applications during the season.
The disadvantage of the foliar spray option alone is that crops must be sprayed early in the rust epidemic, in a timely fashion, keeping in mind the difficulty of spraying during a period of continuous wet weather. The advantage of this approach is that expense is only incurred when, and if, stripe rust is an issue within the crop.
Both methods are effective if used appropriately. To determine which approach is the most suitable, growers need to consider rust carry over on the 'green bridge', variety selection, local conditions, and the ability to spray for stripe rust in a timely fashion.
Resistance to stripe rust in wheat
In general, there are two types of resistance to stripe rust deployed in Australian wheats:
- major gene resistance (all stage resistance)
- adult plant resistance.
These resistance sources may be used either alone or in combination.
All stage resistance (major gene resistance)
All stage resistance (also known as major gene resistance) is a race specific resistance that is very effective against some strains of rust but ineffective against others.
Typically, when these major genes are first deployed they are completely effective but through mutation of the rust these resistances are often short lived in wheat as they are overcome or 'broken down' by the pathogen. An example of this is the acquisition of virulence toward the Yr17 gene deployed in many varieties.
When a major resistance gene is 'broken down' the level of resistance in a variety will depend on the other genes present in that variety.
Adult plant resistance (APR)
APR is a resistance that is widely used in Australian wheats.
APR genes are often partial resistance genes that work by slowing down the rate of rust development. They do not stop the disease progress completely.
There are a number of APR genes used in Australian commercial wheats. The relative effectiveness of APR genes can be influenced by factors such as:
- temperature (they often working better at higher temperatures)
- crop nitrogen status (there may be a delayed onset in high nitrogen status crops)
- the wheat variety that they are deployed in
- the number of APR genes present (their effects are often additive)
- the pathotypes of stripe rust present.
Even though APR genes are widely used in Australia they are often not well understood. Some APR genes may be pathotype specific and therefore prone to being overcome by new pathotypes, while other APR genes are regarded as "durable" and, therefore, less likely to be overcome.
In general, APR becomes effective at around ear emergence and works best if rust levels are not excessive in the crop at this time. In varieties that have APR as their only source of resistance it may be important to protect the earlier growth stages of the crop with seed or fertiliser treatments and/or fungicide sprays.
In general, varieties rated as MS with effective APR will rarely lose all their flag leaf to disease, whereas varieties rated as S and VS are at risk of losing 100 per cent of their leaf area to disease.
Many cultivars with APR can be very susceptible as young plants. Growers using such varieties must plan to protect their crops from stripe rust before the onset of effective APR to minimise rust build up. The level of susceptibility of young crops will vary from one variety to another. This early susceptibility can result in build-up of rust in some years.
Further information
Books
- Wallwork, H (2000) Cereal Leaf and Stem Diseases
Rust identification
For rust identification, send plant samples in a paper envelope (do not use plastic wrapping) to:
Australian Cereal Rust Survey
Plant Breeding Institute
Private Bag 4011
Narellan NSW 2567
Contact
Dr Grant Hollaway
Cereal Pathologist – Horsham
03 5450 8301
Field Crops Pathology
Grains Innovation Park
110 Natimuk Rd
Horsham 3400
03 5450 8301
Or call the Customer Service Centre, 136 186
Acknowledgements
Grant Hollaway, Luise Sigel. Support by the Grains and Research Development Corporation is gratefully acknowledged.