Identifying and managing rusts in cereals
In last week’s 2024 PestFacts WA newsletter we discussed stem and leaf rust being found in oat and wheat crops respectively at Wickepin, Highbury and Ravensthorpe. For more information refer to the 2024 PestFacts WA Issue 16 article Monitor oat and wheat crops for rusts.
It is important that growers check their crops, particularly susceptible varieties for rusts, now as moist, warm weather conditions encourage these diseases to develop.
When checking cereals, growers and consultants are also urged to investigate the canopy and full length of stems for the presence of other rusts that have not yet been reported this growing season such as stem rust and stripe rust in wheat, and leaf rust in barley
Wheat rust symptoms
Leaf rust pustules are small, circular to oval and can vary in colour from orange to light brown. They usually appear on the upper surface of leaves and on leaf sheaths in cases of heavy infection. Leaf rust requires moisture (rain or heavy dew) or high humidity for spores to germinate and infect leaves. Usually, 4 to 6 hours of leaf wetness are required at optimum temperatures 15 to 25°C (warm days and dewy nights) for this polycyclic disease to spread rapidly. The primary risks for carryover of rusts are volunteer cereals at edges of paddocks or roadsides. This means that it is worth inspecting volunteer wheat/oats/barley as well as sown crops.
Stem rust pustules are a darker red-orange and are often found with ragged edges. Stem rust is favoured by warm spring conditions (20 to 35°C).
Stripe rust pustules are yellow-orange in colour. The pustules are initially small and circular but develop into yellow stripes along the leaf veins on upper leaf surfaces and leaf sheaths. In severe cases the awns and inside glumes can also become infected. Stripe rust can be seen from a distance in hotspots/patches 1 to 10 metres in diameter.
Barley leaf rust symptoms
Leaf rust symptoms in barley are similar to those in wheat. That is, small, circular or oval pustules with light brown powdery spores on upper surface of leaves, and on leaf sheaths in cases of heavy infection. As the crop matures, pustules darken and produce black spores embedded in leaf tissue.
Oat rust symptoms
Oat stem rust and oat leaf rust (also known as crown rust) can impact the yield and quality of both hay and milling crops. Both rust types can affect the leaves, leaf sheaths and panicles of oats and can even infect the same plant.
Both oat leaf and stem rust species are oat specific and do not infect barley or wheat.
Oat leaf rust produces round- to oblong-shaped pustules, containing yellow-orange powdery spores. These pustules are most commonly found on the leaf surface. The leaf area surrounding the pustule can also turn pale green in colour.
Oat stem rust pustules are darker in colour containing darker red-brown powdery spores and are larger in size.
The rust pustules can turn black in colour as the disease ages, and heavily infected leaves can senesce and die.
Management
Variety selection is the best defence against rust diseases in-crop. Varieties rated as moderately resistant to moderately susceptible (MRMS) or better will show significantly less rusting than a susceptible (S) and very susceptible (VS) variety. These varieties are unlikely to require a fungicide application to maintain grain yield except in exceptional years with very high disease pressure. For variety disease ratings, refer to DPIRD’s 2024 WA Crop Sowing Guide.
If rust is detected in a susceptible variety (rated VS to S), fungicide is more likely to be needed to protect yield. In these cases, fungicide should be applied before the epidemic becomes severe, taking into consideration the stage of crop development. Spraying after crop flowering is normally not economically beneficial for leaf rust. However on the other hand, stem rust is more devastating than leaf rust and is favoured by warmer temperatures and may need prompt spraying in susceptible varieties, even into grain filling. As the season progresses and warmer, drier conditions occur, the likelihood of an economic favourable response to fungicide will diminish, but less so for stem rust. Spray decisions should consider the level of disease in the crop, varietal susceptibility, the time of the season and growth stage of the crop. Additionally, consider the chance of ongoing rainfall after spraying, as this is favourable for disease development. Use high rates of fungicide for longer duration of protection, for example, when season conditions favouring infection are likely to persist, or for more susceptible varieties. Ensure the cost of fungicide treatment is aligned with the crop's yield potential and the length of the growing season for optimal economic return.
For a list of registered fungicides to use as foliar sprays, visit DPIRD’s Registered foliar fungicides for cereals in Western Australia page. It is important to follow fungicide label recommendations for application rates and withholding periods for fungicides.
Rusts predominantly spread via wind from infected plants, but spores can also be transported through the movement of people, machinery and plants. Growers are urged to adopt hygiene (biosecurity) measures during their paddock checks to reduce the risk of spreading the disease.
Rust pathotype testing
Growers and agronomists are encouraged to send samples of all rusts for pathotype testing at any time of the year to the Australian Rust Survey. This is a free testing service that identifies new rust strains. This information will assist wheat breeders in developing new resistant varieties as new strains could become problematic in existing varieties. Infected leaf samples should be mailed in paper envelopes (do not use plastic wrapping or plastic lined packages) along with your details and collection information (location, variety etc.) directly to:
University of Sydney
Australian Rust Survey
Reply Paid 88076
Narellan NSW 2567.
Optional free reply-paid envelopes can be ordered from the University of Sydney. For further details, refer to the University of Sydney’s Australian Cereal Rust Survey page.
Further information
To read about earlier wheat and oat rust activity this season refer to the 2024 PestFacts WA Issue 16 article Monitor oat and wheat crops for rusts and Issue 8 article Leaf rust, powdery mildew, yellow spot/nodorum blotch and flag smut are appearing in wheat.
For more information on wheat rusts, refer to DPIRD’s:
- Diagnosing leaf rust of wheat page
- Managing stem rust of wheat page
- Managing stripe rust and leaf rust in Western Australia page.
For further information on barley rusts visit DPIRD’s Diagnosing barley leaf rust and Managing barley leaf diseases in WA pages.
For further information on oat rusts visit DPIRD’s Diagnosing stem rust in oats or Oats: leaf diseases pages.
For more information on cereal diseases contact Plant Pathologists Kylie Chambers, Northam on +61 8 9690 2151, Kithsiri Jayasena in Albany on +61 8 9892 8477, Ciara Beard in Geraldton on +61 8 9956 8504, Geoff Thomas in Perth on +61 428 947 287 or Andrea Hills in Esperance on +61 8 9083 1144.
Article authors: Cindy Webster (DPIRD Narrogin), Ciara Beard (DPIRD Geraldton), Kylie Chambers (DPIRD Northam), Geoff Thomas (DPIRD Perth), Kithsiri Jayasena (DPIRD Albany) and Andrea Hills (DPIRD Esperance).