Barley stem rust on the Darling Downs in 2018

Take home messages

• All isolates identified as Scabrum rust – will not infect wheat crops
• The stem rust pathogen is a biotroph and needs a living host for survival – not stubble-borne
• Most barley varieties are vulnerable to stem rust infection and will host wheat, rye and scabrum rusts
• Susceptible varieties contribute to inoculum pressure and increase the risk of breakdown of resistance in other varieties
• Need to manage green bridge to limit pathogen survival

Background

In late 2018, stem rust was reported in QLD barley crops from Brigalow, Chinchilla, Dalby, Brookstead and Jandowae. These reports came as a surprise to many due to the dry conditions experienced throughout much of eastern Australia. Generally, most crops were planted in June - not particularly late for the Darling Downs. Most crops did not receive fungicide applications early in the season as a result of the dry conditions. Exceptionally good rainfall in early October resulted in the appearance of late tillers, increasing the vulnerability to stem rust and delaying maturity and harvest.

Stem rust of barley

The rust diseases are regarded as some of the most important diseases of cereals worldwide. Their importance can be attributed to their adaptability to a range of environments, their capacity to mutate rapidly and attack previously resistant cultivars, their rate of disease development and the ability of spores to remain viable when dispersed over long distances. The rust pathogens are highly specialised and have relatively narrow host ranges. They are obligate parasites and require a living host for survival.

Stem rust is primarily a disease of wheat, barley and triticale, but also infects some grass species. It is caused by the pathogenic fungus Puccinia graminis, comprising species that are specialised to different hosts. For example, P. graminis f. sp. tritici is specific to wheat and barley and cannot infect oats, whereas P. graminis f. sp. avenae is specific to oats and cannot infect wheat and barley.

Barley can be infected by up to three different forms of the stem rust pathogen, wheat stem rust (P. graminis f. sp. tritici), rye stem rust (P. graminis f. sp. secalis) and ’scabrum‘ rust. The latter is regarded as a hybrid between the wheat and rye stem rust pathogens and is most commonly found on Elymus scaber (formerly Agropyron scabrum), a native grass.

The stem rust pathogen is favoured by humid conditions and warmer temperatures (15-35^oC). Following infection, disease symptoms are generally visible within 5-8 days and sporulation occurs in 7-14 days. Severe infection hinders plant growth and can lead to lodging, resulting in yield loss and shrivelled grain. Stem rust can destroy seemingly healthy crops in a short period of time with 100% yield loss reported in susceptible varieties.

Stem rust is characterised by reddish-brown, elongated pustules. They can occur on stems, glumes and both sides of the leaves and pustules have a characteristic torn margin. At the end of the season, dark brown to black spores, known as teliospores are produced mainly on leaf sheaths and stems.

In recent years, Australia has seen a decline in stem rust levels and sample numbers for pathotype analysis. In some years, no infection was recorded in any commercial wheat or barley crop in Australia. The most recent stem rust outbreak in Australia was on triticale in 1982. Breeding for resistance to stem rust in wheat has been a priority in breeding programs and combined with green bridge control and pathogen surveys, played an important role in controlling the disease. In contrast, active resistance breeding for stem rust in barley has been a low priority since infection generally occurs late in the season with most barley crops maturing early, particularly in the northern region.

Heavy rust infection was observed on the native grass species, Elymus at most sites visited during a survey on the Darling Downs in January 2018. Rust samples collected are being pathotyped by the Plant Breeding Institute, University of Sydney.

Australian barley (Hordeum vulgare) varieties are categorised into 9 resistance categories through the National Variety Trial (NVT) disease screening process. The resistance ratings are assigned by the pathologists working group collating data from national disease nurseries conducted annually. These categories range from resistant (R) to very susceptible (VS) and are communicated to industry via the NVT website, field days and industry presentations.

Most barley varieties are vulnerable to stem rust. Fewer than 10% of varieties screened for stem rust in QLD in 2018 received a resistance rating higher than susceptible (S). Data to be available on the NVT website shortly.

Discussion

The reason(s) for stem rust outbreak in QLD in 2018 is not fully understood. However, the outbreak could be related to delayed sowing and conditions favourable for disease development late in the season. Active resistance breeding for stem rust in barley and research on the pathogen has been a low priority and resulted in limited knowledge about the disease in Australia.

All samples analysed to date have been identified as ’scabrum’ or hybrid stem rust. The pathogen is not able to infect wheat. Therefore, barley crops infected with ’scabrum’ rust pose no threat to wheat production. All rust pathogens are biotrophs, requiring a living host for survival. Hence they cannot survive in stubble. Volunteer plants, including native grasses such as Elymus, however allow these pathogens to survive over summer and controlling the green-bridge plays an important role in limiting pathogen survival.

Stem rust requires higher temperatures and tends to appear later in the crop season. Application of fungicide at early growth stages would not be effective for stem rust control. For chemical application to be successful, fungicide should be applied at the first sign of disease. Ensure thorough coverage. Application after successful establishment of the pathogen may reduce efficacy. Consider follow-up application if conditions favour disease development. Contact your adviser for suitable fungicides.

Rust samples are crucial to effectively identify the rust pathogen affecting barley. Rust samples can be sent to Lislé Snyman at 604 Yangan Rd, Warwick QLD 4370. Send up to a maximum of 20 infected stems/leaves with as much rust as possible in paper envelopes, do not use any plastic wrapping or plastic lined packages. Samples will be sent on to the Plant Breeding Institute, Cobbitty for pathotyping.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

Contact details

Lislé Snyman
DAF QLD
Hermitage Research Facility, 604 Yangan Rd, Warwick, Qld
Ph: 07 4542 6761
Email: lisle.snyman@daf.qld.gov.au

GRDC code: DAQ00189