Stem rust outbreak in Queensland barley

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Photo of stem rust in a barley crop
Stem rust in a barley crop near Dalby, Queensland, in November 2018. PHOTO: QDAF

Very dry conditions throughout much of eastern Australia in 2018 resulted in low overall disease incidence in cereal crops.

At the time of writing, rust reports in cereals were limited to: wheat leaf rust in New South Wales, Victoria, Tasmania, South Australia and Western Australia; barley leaf rust in SA and WA; and oat crown rust in Queensland, NSW, Victoria and WA. Wheat stripe rust was reported in Victoria’s high-rainfall zone and NSW’s central west and south-west slopes, while single samples were also recorded in Tasmania’s Midlands region and SA’s Roseworthy.

In view of the generally low 2018 disease incidence, it was surprising to see severe stem rust develop in several barley crops in south-east Queensland from October.

Reports of stem rust-infected crops were received from Brigalow, Chinchilla, Dalby and Jandowae, Queensland.

Australia’s stem rust pathogen

In Australia, stem rust occurs on all cereals, and it also infects some native grass species.

It is caused by the pathogenic fungus Puccinia graminis, which includes forms that are specialised to different hosts, known as ‘formae speciales’ (f. sp.). For example, P. graminis f. sp. avenae occurs on oats and cannot infect wheat. P. graminis f. sp. tritici occurs on wheat and cannot infect oats.

This has important implications in agriculture since an oat crop heavily infected with a specialised form of stem rust poses no threat to wheat crops and vice versa.

Barley stem rust

Barley may be infected with up to three different ‘special forms’ of the stem rust pathogen: wheat stem rust (Puccinia graminis f. sp. tritici) and rye stem rust (P. graminis f. sp. secalis), as well as a form that arises from hybridisation of these two pathogens, commonly known as scabrum rust.

The scabrum rust is so called because it is mainly found on a common native grass, Elymus scaber (formerly Agropyron scabrum). It is believed the hybridisation between wheat and rye stem rust pathogens that led to scabrum rust development, occurred on this native grass.

In Queensland crops, all samples investigated to date have proven to be scabrum stem rust. Identifying scabrum rust in stem rusted barley crops is important because this hybrid rust does not infect wheat, so poses no threat to adjacent wheat crops.

The reason for the stem rust outbreak in Queensland barley crops is not fully understood, but it could be related to sowing date.

Cultivated barley is generally vulnerable to stem rust. Because barley usually matures early in the winter cropping cycle, stem rust severity is typically low or absent, so active resistance breeding has been a low priority.

As a result of the low and erratic rainfall in the 2018 season, many barley crops were late-sown. Combined with good rainfall in October, some crops were vulnerable to late stem rust infection. Stubble regrowth in barley over summer can also result in severe stem rust infection, leading to ‘green bridge carry-over’ of the disease.

Rust samples are crucial for identifying the stem rust pathogen affecting a barley crop or plant. For this to be undertaken satisfactorily, a sample containing as much rust as possible – up to 20 infected stems or leaves – should be sent to the Plant Breeding Institute, Cobbitty, NSW. A large sample is required because there is no single cereal variety that is susceptible to all three special forms of P. graminis that infect barley.

Stem rust samples from barley are inoculated on a range of cereals to allow for preliminary identification in the laboratory. A large sample increases the likelihood of obtaining some infection and spore multiplication for determining the pathotype causing infection.

More information

Professor Robert Park
02 9351 8806
robert.park@sydney.edu.au

Dr Will Cuddy
02 9351 8871
will.cuddy@dpi.nsw.gov.au

Rusted plant samples can be mailed in paper envelopes only – do not use plastic wrapping or plastic-lined packages. Rust samples can be sent to:

University of Sydney
Australian Rust Survey
Reply Paid 88076
Narellan NSW 2567