Rust in wheat appears as brown to orange dusty pustules on leaves. Stripe rust and leaf rust can be distinguished by the colour and shape of pustules and the location of the infection.
Stripe rust pustules are yellow-orange. Individual pustules are small and circular but develop into yellow stripes on upper leaf surfaces, leaf sheaths and awns and inside glumes. It can be seen from a distance in patches known as 'hotspots'. For further information refer to the Diagnosing stripe rust of wheat MyCrop page.
Leaf rust pustules are orange-brown in colour, circular to oval in shape and chiefly found scattered on the upper surface of leaves (Figure 3). Colour depends on the freshness of the pustule. Because most spores are produced overnight, pustules are best observed in the morning. For further information refer to the Diagnosing leaf rust of wheat MyCrop page.
Stem rust pustules are a darker red-orange and are often found with ragged edges and going through to the other side of the leaf and present on the stem or leaf sheaths. For further information refer to the Diagnosing stem rust of wheat MyCrop page.
Very susceptible (VS) to susceptible (S) varieties increase the risk of rust infection in two ways. During the growing season, rust can develop and multiply rapidly on these varieties. During the summer and autumn non-cropping period, varieties in this category represent good volunteer wheat hosts that greatly enhance the survival opportunities for rust to carryover in the non-cropping period. Rust is more difficult to control with fungicides in varieties with low resistance ratings (very susceptible to moderately susceptible). When selecting varieties, consider responses to rust diseases and avoid varieties that are highly susceptible. Check disease resistance ratings on the Wheat disease ratings page. Please note that variety ratings have changed for some varieties due to the presence of new leaf rust pathotypes. For further information refer to Implications of known wheat leaf rust pathotypes present in WA.
Green bridge proximity
Summer rains permit the development of volunteer cereal hosts and autumn rains permit the early build-up of rust on these volunteers known as the 'green bridge'. This happens readily after wet summers. Cropping areas that receive summer rain resulting in self-sown green bridge cereals are at risk of early infection with stripe or leaf rusts. Wheat regrowth is the primary risk for carryover of both wheat leaf rust and wheat stripe rust. The amount of rust present in the previous season also determines the risk of leaf and stripe rusts. The more rust in a given year means there is more chance of carryover into the next season.
While resistance will influence individual crop risk, the overall risk of serious rust outbreaks is influenced by summer and winter weather factors (rainfall and temperature) which can be considered in your region each season. Both stripe and leaf rusts require moisture (rain or heavy dew) or high humidity for spores to germinate and infect leaves. Usually 4-6 hours of leaf wetness are required at optimum temperatures (warm days and dewy nights). Each rust has an optimum temperature for infection and growth (Table 1). Rust outlooks are provided as part of the plant pathology group's seasonal Crop diseases: forecasts and management page.
A stripe or leaf rust epidemic is more likely if the winter and/or spring is suitably wet. Seasonal outlooks are available on the Seasonal Climate Information page. Leaf rust has a warmer mean daily temperature optimum than stripe rust (Table 1). The mild winters in Western Australia result in leaf rust being relatively active in winter and into spring, particularly in the northern agricultural areas. Early sown crops, on which infections establish prior to the cooler winter months, are more at risk from early leaf rust which can develop rapidly in spring. The lower temperature optimum for stripe rust results in the disease being relatively more active in winter than later in the season. Warm spring conditions, particularly in the northern agricultural areas, can increase the time taken between infection and resultant new spores being produced (the latent period). Rust spores easily spread on wind.
|Disease||Latent period (days)||Optimal daily temperature (°C)|
|Stripe rust||10-14||12-20 (dormant >23)|
How to monitor crops
The aim of crop monitoring is to detect infection at the earliest stage feasible. Inspect the most susceptible and earliest sown crops carefully over a wide area of the paddock. Examine leaves at the top and bottom of the canopy for scattered light infections. In green bridge areas also look for infrequent heavily-infected hot spots. Crops prone to infection at young stages (rated very susceptible to moderately resistant) should be inspected at seven to 10 day intervals from early stem elongation (growth stage Z31) or from early flag leaf emergence (growth stage Z37) if seeding fungicide treatments registered to control rust diseases have been used.
A chargeable service is available to assist with disease diagnosis, send ~10 infected leaves to:
DAFWA Diagnostic Laboratory Services (DDLS)
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 office or from the DAFWA Diagnostic Laboratory Services (DDLS) page.
Rust pathotype testing
Leaf rust and stripe rust occur as different strains because they can readily mutate and strains can easily move around the country and the world on the wind or people's clothing. Possible new strains need to be continuously monitored in order to understand the implications for existing varieties and to assist wheat breeders in developing new resistant varieties.
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. Post leaf samples in paper envelopes to: University of Sydney, Australian Rust Survey, Reply Paid 88076, Narellan NSW 2567. Further instructions on submitting samples and printable dispatch forms are available from the University of Sydney website.
Because there are different strains of leaf, stem and stripe rusts, care must be taken when travelling interstate or receiving interstate or 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.
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.