Introduction
In Western Australia, stem rust typically causes losses of 10–50% in wheat, but this can increase to up to 90% when it occurs in early spring and is not controlled. Stem rust attacks wheat and triticale but barley can also be an important host during summer months. Stem rust produces large, reddish-brown oval to elongated spore masses on both sides of the leaf, on leaf sheaths, stems and outsides of heads. The pustules have tattered edges. The symptoms of stem rust and how the disease develops are discussed in detail on the Diagnosing stem rust of wheat MyCrop page.
Stem rust requires living plants on which to grow and reproduce. To infect crops during the season it must survive summer by infecting volunteer cereals or grass hosts, known as the ‘green bridge’.
Other rusts that infect wheat in WA are stripe rust and leaf rust but these do not occur on stems. More information on these can be found at Managing stripe rust and leaf rust in wheat in Western Australia.
Risk factors
While variety resistance will influence individual crop risk, the overall risk of serious rust outbreaks is influenced by several factors which can be considered each season. These include;
- the amount of stem rust present in the previous season – more stem rust in a given year means there is more chance of carry over into the next season. The PestFax map can show you where stem rust finds were reported last season.
- summer and autumn rains – summer rains permit the development of volunteer hosts (green bridge) and autumn rains permit the early build-up of rust on these volunteers. It primarily infects wheat and triticale but can infect barley and some grasses, including barley grasses and common wheat grass. For more information see the seasonal climate information page.
- spring rains and humid warm conditions – if all the other factors occur and early rust breaks out in crops, then a stem rust epidemic is more likely if the spring is suitably wet. Stem rust prefers temperatures of 15-30°C and humid conditions.
- keep up to date with stem rust finds across the wheatbelt by subscribing to PestFax.
- wind can spread spores large distances so any disease outbreaks in a season (even distant ones) pose a risk.
Yield losses
Yield loss will depend on the disease resistance of the variety and on how early the disease starts in the crop (Table 1). Quality reductions such as increased screenings and lower hectolitre weights can add to high yield loss impacts.
| Resistance rating | Definition | Potential yield loss (%) |
|---|---|---|
| Very susceptible (VS) | Early high disease build-up; can promote epidemic development | 50-90 |
| Susceptible (S) | High disease build-up | 25-50 |
| Moderately susceptible (MS) | Develops disease less quickly and so reduces loss risk | 10-35 |
| Moderately resistant to moderately susceptible (MRMS) | Some partial resistance; losses depend on disease pressure | 5-25 |
| Moderately resistant (MR) | High partial resistance; generally few losses | 5-15 |
| Resistant (R) | Highly effective resistance; no or slight losses | 0-5 |
| Highly resistant (HR) | Complete resistance | 0 |
Fungicide control may be required for varieties rated VS-MS. Varieties with a MRMS disease rating may benefit from fungicide control if early disease occurs or high spore loads are present (for example, next to another severely rusted paddock). High crop yield potentials also increase the rate of return from fungicides. Disease ratings for current wheat varieties are available from the Wheat variety guide for Western Australia.
Pre-season management
Remove summer volunteers that created the green bridge
Self-sown summer and autumn volunteers (predominantly wheat and barley), which help to produce the green bridge, should be killed with herbicides or heavy grazing as soon as they develop and at least four weeks before crop sowing. This will reduce local carryover of rust spores and reduce the risk of early infection. For more information refer to Control of green bridge for pest and disease management.
Sow resistant varieties
Sowing varieties with resistance to wheat stem rust is encouraged in Western Australia, particularly in years where rust risk is high and in rust prone environments. Varieties that are partially or fully resistant to rust are effective in reducing disease build-up and preventing losses. In rust-prone environments or high-risk seasons it is very important to avoid susceptible and very susceptible varieties. Use the Wheat variety guide for Western Australia to identify varieties with some resistance to stem rust.
Spring management
Paddock monitoring
Warming spring conditions favour stem rust so the disease becomes apparent after flag leaf emergence. When stem rust risk exists (such as in green bridge areas or after reports of stem rust in your region):
- monitor susceptible crops at 7-14 day intervals from flag leaf emergence to early dough grain development.
- inspect different parts of the crop by carefully examining plants (especially lower stems) for signs of stem rust infection.
- if stem rust is found, walk through the paddock in a 'W' pattern and collect 100 random stems from the crop (that is, 10 stems from 10 locations) and determine the number of stems with any stem rust.
- the MyCrop app can assist you to diagnose crop diseases
- if you need further help with diagnosis, a chargeable diagnostic service is available through AGWEST Plant Laboratories. Send 6-10 infected stems or leaves to: AGWEST Plant Laboratories, Department of Agriculture and Food, Western Australia, Locked Bag 4 Bentley Delivery Centre WA 6983. Post in a paper envelope (no plastic) with date, location, name, and contact details. Broadacre diagnostic submission forms are available from your local DAFWA office or via the AGWEST Plant Laboratories page.
- to monitor rust strains in Western Australia, growers and consultants are encouraged to send rust samples at no cost to the Australian Rust Survey, particularly from varieties showing unusually high levels of rust. Sampling instructions and dispatch forms are available from the University of Sydney page. Post samples in paper envelopes to University of Sydney, Australian Rust Survey, Reply Paid 88076, Narellan NSW 2567.
Foliar fungicides
Where wheat stem rust occurs in susceptible varieties, economic control can be achieved with fungicide applied at early disease onset. Economic responses result from improvements in yield and in grain quality (reduced screenings, increased hectolitre weight). A range of fungicide active ingredients are registered in Western Australia for stem rust control. For more information refer to Registered foliar fungicides for cereals in Western Australia.
Application of a registered foliar fungicide can effectively control stem rust. Early disease detection is important as experiments show that fungicide reduces subsequent rust severity on plant parts that were only slightly infected at the time of fungicide application, but is not effective on plant parts that were more heavily infected when treated. The degree and duration of control depends on the application rate. Control is more effective if the fungicide is applied early in the development of the disease (Table 2). The disease becomes more difficult to control as it progresses. Poor control is likely at rates below those recommended, particularly when disease levels are high. Under high disease levels, high rates should be used.
| Crop stage at which stem rust is detected | Crop stage at which fungicide is applied | Tillers1 detected with stem rust (%) | Crop yield potential (t/ha) | Yield response with one spray (t/ha) | Yield response with two sprays (t/ha) |
|---|---|---|---|---|---|
| Flag emergence | Booting | Trace (<1) | 3.0-3.5 | 0.9 | 1.5 |
| Early head emergence | Mid-head emergence | 5 | 2.5-3.0 | 0.2 | 0.2 |
| Early grain fill | Late milk-early dough | 90 | 1.5-2.0 | 0.5 | n/a |
1 Any stem rust detected on stem or leaves from each of 100 tillers.
Strategies for using fungicides to manage rust depend on when the disease starts and the level of risk acceptable to the grower. All strategies assume that rust is detected soon after the start of the epidemic. Monitoring susceptible crops is essential to achieve this.
Foliar spray strategies
Reducing early epidemic development, particularly in VS-MS varieties, is most important. Economic responses have been achieved in susceptible varieties from registered foliar fungicide sprays from pre-head emergence to grain filling. Responses in less susceptible varieties will be less with later sprays.
Crops infected with stem rust before flowering are considered at high risk as yield losses of 50% or more are possible. If infection starts or re-starts after flowering, losses of around 25% are possible.
When and what rate to spray
If any stem rust is detected spray susceptible or moderately susceptible crops as soon as possible with an appropriate fungicide at a high rate. For crops with intermediate resistance continue to monitor and spray if infection exceeds an average of 5% of random stems infected.
If stem rust is detected after head emergence, short term control can be achieved with standard application rates provided infection is not severe. However, high fungicide rates have been found to be more profitable in experiments.
If stem rust is detected after grains are at the dough stage, apply fungicide if the crop has at least four more weeks of grain filling to go before hard dough stage. It is important to consider withholding periods when choosing an appropriate fungicide.
Biosecurity measures
Because there are different strains of stem rust, care must be taken when travelling interstate/overseas or receiving interstate/overseas visitors, since spores carried on clothing could introduce new strains of rusts. Implement biosecurity measures to minimise rust becoming established or spreading on your farm. Rust spores are small, light and may survive for several days without a host. Rust spores can spread long distances by wind, on machinery/vehicles, on tools, clothing and footwear. Remember that if you walk through an infected crop, follow biosecurity protocols and thoroughly clean your boots, hands and trousers before entering another paddock or travelling, as rust spores can be unknowingly transferred via people locally and also from overseas. Also check biosecurity measures taken by your visitors and agronomists.
Be particularly vigilant when returning from eastern Australia or internationally, as rust pathotypes with different virulences exist outside WA. The Australian Cereal Rust Control Program conducts rust pathotype testing and produces reports each year detailing new rust incursions that are a threat to Australian cereal varieties.
If entering a paddock suspected to be infected with rust, biosecurity suggestions include:
- Wear protective overalls and rubber boots;
- After crop insprection clean any material off boots with a brush. Prepare footbath of bleach (10% household bleach, 90% water) and spray bottles of methylated spirits brew (95% metho, 5% water) for use to disinfect footwear, pants and hands,
- Decontaminate vehicles, tools and machinery,
- Walk instead of driving through crops,
- Ask visitors/agronomists to leave their vehicle at the gate and only travel on your property in your vehicle.
Research
Department of Agriculture and Food, Western Australia has published findings of wheat stem rust research and these can be accessed via the external links on the right hand side. This research was funded by Grains Reasearch and Development Corporation.