The development of understanding of the wheat rust pathogens, their hosts, and the interactions between these two organisms, is a remarkable chapter in the history of the plant sciences. The work began in earnest in the latter part of the 1880s in Australia with a series of Rust-in-Wheat conferences, which followed a stem rust epidemic in 1889 that was estimated to have caused some £2–3 million in losses.

Advances in understanding of how cereals and rusts interact have led to considerable success in developing and deploying rust resistant cereal cultivars, which remains the most economically sustainable way to control these diseases. Rust resistant cultivars have been very successful in reducing losses, particularly in wheat crops in rust-prone regions. In northern NSW and Queensland, for example, rust resistant wheat cultivars have provided industry-wide protection since the 1960s. In 2009, genetic resistance to rust was estimated to save the Australian wheat industry more than $1 billion annually.

Despite advances in resistance breeding, the rust pathogens continue to impact on global wheat production, principally because of their ability to generate new strains (races, pathotypes) that overcome the resistance genes incorporated in new cultivars by breeders.

Key points

• While fungicide usage to control rusts in cereal crops has increased substantially over the past 30 years, genetic resistance remains the mainstay;
• Long-term monitoring of rusts in Australia and New Zealand has clearly established how easily these pathogens move between the two countries;
• Ongoing changes in rust pathogens highlight the need for relevant and coordinated surveillance across the Australasian epidemiological zone.

The Australian Cereal Rust Control Programme (ACRCP)

The ACRCP has provided a service to all Australian cereal breeding groups since 1974. The programme has received long-term support from the University of Sydney as a hosting organisation and from the grains industry via the Australian Grains Research and Development Corporation (GRDC). The ACRCP has benefited from close collaborations with breeding groups and cereal pathologists. The service activities of the programme are underpinned by targeted research and the training of postgraduate students. It encompasses all cereal rust diseases in Australia, and has three main activities:

1. Pathogenicity surveys of the cereal rust pathogens. These surveys involve greenhouse characterisation of rust isolates in samples collected from all cereal growing regions. They provide advance warning by identifying new pathotypes before they reach levels likely to cause significant economic damage. Efforts to identify new sources of resistance, to screen breeders lines, and to incorporate effective resistance into advanced lines nominated by cereal breeders (activity 3) rely on the rust isolates identified and characterised by the surveys and maintained in liquid nitrogen.

2. Identification of new rust resistance sources. The efficient and effective exploitation of rust resistance relies on ongoing searches to identify new resistance sources and genetic studies aimed at determining the number of genes involved, their genetic relationships, and field assessments of the level of protection afforded by such new resistances. Any new potentially useful resistance genes are directed into the germplasm enhancement programme.

3. Germplasm screening and enhancement. All Australian cereal breeders are encouraged to submit material for screening for rust response in artificially inoculated greenhouse and field tests. The germplasm enhancement activities involve the incorporation of effective sources of resistance into advanced lines nominated by breeders.

Australasian rust surveillance

Annual pathogenicity studies on wheat rusts in particular have been conducted by the University of Sydney since 1921. These surveys included New Zealand for many years, but ceased after 2000 due to changes in Australian quarantine regulations. Data gathered prior to 2000 clearly demonstrated that Australia and New Zealand comprise a single epidemiological unit, with regular exchange of wheat rust inoculum. While this occurs primarily in a west to east direction, from Australia to New Zealand, there are many examples of rust movement in the opposite direction.

Pathotype surveys and rust control

The free and rapid movement of cereal rust pathogens throughout Australia and New Zealand provides strong justification for a coordinated Australasian approach to rust resistance breeding.

To have maximum impact in disease control, surveys of pathogenic variability in rust pathogens must be closely integrated with the development and management of new cereal cultivars.

Where this has been practiced, surveys have provided both information and pathogen isolates that have underpinned rust control efforts, from gene discovery to post-release management of resistance resources. Information generated by pathotype surveys has been used to devise breeding strategies, inform selection of the most relevant isolates for use in screening and breeding, define the distribution of virulence and virulence combinations, allow predictions of the effectiveness/ineffectiveness of resistance genes, and issue advance warning to growers by identifying new pathotypes that overcome the resistance of cultivars before they reach levels likely to cause significant economic damage.

Sustained genetic control of wheat rusts

Fungicidal control of cereal rust diseases has increased in Australia in recent years as a result of increased yields, improved application technology, and cheaper generic chemicals. While an important tool in combating these diseases, fungicides do not provide complete protection, are less effective in reducing the overall size of rust populations, and do not contribute to reducing the over-summering of rust inoculum on self-sown cereals.

Additionally, the economics of broad-spectrum fungicides are changing, as insensitivity in some target pathogen develops. In Australia, this has included insensitivity to some triazole fungicides (tebuconazole, flutriafol) in the barley powdery mildew pathogen B. graminis f. sp. hordei, detected in WA in 2009. More recently, insensitivity to several azole fungicides including triadimefon, triadimenol, tebuconazloe and propiconazole was detected in Mycosphaerella graminicola in NSW. While such insensitivity has been documented only once in a rust pathogen (soyabean rust), there is agreement that this could occur more commonly in the future.

The control of rust diseases by resistance is only sustainable if progress achieved through resistance breeding is managed responsibly. There are many examples from the past sixty years of resistance gene breakdowns occurring because a cultivar with a single resistance gene was deployed in a region with moderate to high rust inoculum levels. It is obviously undesirable to release cultivars with single gene resistance in these situations, and more complex resistance is needed. A policy of releasing and growing rust resistant cultivars only, reduces the threat to individual growers, and just as important, it ensures that the resistances in current and future cultivars are safeguarded by reducing rust populations and minimising the chance that new mutant pathotypes will develop.

Source: Robert F. Park and William S. Cuddy, University of Sydney/FAR