Very rusty, quite rusty, or rotten with rust?

Photo of barley lines being screened for responses to leaf rust and barley grass stripe rust

Each year, staff from the Australian Cereal Rust Control Program test thousands of breeding lines and varieties in field rust nurseries, such as these barley lines being screened for responses to leaf rust and barley grass stripe rust.

PHOTO: University of Sydney

In the previous GroundCoverTM rust column, we discussed the roles played by William Farrer and the Reverend Dr Nathan Cobb in the late 1800s and early 1900s in laying the foundation for Australia’s rust resistance breeding efforts. While most people would be aware of the contribution of Farrer, few know of the impact of Cobb, whose work on what we now often call ‘data capture’ is still used today by most of the world’s rust researchers.

Cobb realised the importance of having a reliable means of documenting the response of wheat lines to rust and developed a “scale of rustiness” that could be used to record the percentage area of leaves or stems occupied by rust pustules. He published such a scale in 1892, stating: “I have found this scale so convenient and so much more expressive and accurate than the indiscriminate use of terms like ‘very rusty’, ‘quite rusty’, ‘rotten with rust’”.

While similar indiscriminate terms are still in use (for example ‘rust sucker’), a modified Cobb scale has become a global standard in recording the rust response of cereal lines to most if not all of the rust diseases. It is often combined with an additional descriptor that is commonly used as a rating of a variety’s rust resistance: VS (Very Susceptible), S (Susceptible), MS (Moderately Susceptible) and so on.

Every year, GRDC-supported staff at the Plant Breeding Institute (PBI) test thousands of breeding lines, cultivars, experimental lines and research populations for rust response in the field, using the modified Cobb scale to record results. In 2016, we tested 75,000 lines of wheat, 1600 barleys and 4500 oats for breeders – along with thousands of lines being used in our research on rust genetics – in rust nurseries that were artificially infected with Australian rust races selected as being important based on our annual national rust race surveys. The rust races selected also enable screening of varieties with races that are not yet present in the region in which they are grown. The isolation of the PBI at Cobbitty, NSW, from commercial cereal-growing regions means that the chance of rust spreading from our artificially infected field nurseries is minimal.

For the most important lines, the field tests are integrated with seedling tests in the greenhouse to allow an accurate appraisal of all-stage resistance (also known as major gene resistance and seedling resistance) and adult plant resistance.

Undertaking this process annually is important to ensure all current wheat cultivars are included and also because the response of a given variety can change dramatically when a new rust race arises.

In the absence of such rigorous testing, varieties that are susceptible to rust would slip through the recommendation system, threatening not only individual crops but also crops in neighbouring regions, and place resistant cultivars at risk of mutation change that may result from a large pathogen population.

More information:

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

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