The rust resistance 'arms race'
In the last issue of Ground Cover, (Issue 123, July–August 2016), we discussed the use of LED lighting to accelerate the delivery of rust-resistant cereals to growers. In that article we noted that more than 40 per cent of the wheat cultivars grown in Australia carry rust-resistance genes provided to wheat breeders by the pre-breeding program at the University of Sydney’s Plant Breeding Institute (PBI).
Two important rust resistances characterised at PBI and delivered to Australian growers provide excellent examples of the impact of rust research in Australia under the national rust research effort – the Australian Cereal Rust Control Program.
The rust resistances Lr24/Sr24, and Lr37/Sr38/Yr17 were introduced into wheat from related grasses, the first by researchers in the US, and the second by researchers in France. They were then provided free to researchers at the PBI to assess their potential for use in Australia.
The Lr24/Sr24 resistance comprises two all-stage resistance genes that are always found together, one providing resistance against leaf rust (Lr24) and the other stem rust (Sr24). The resistance was found after crossing the grass species Thinopyrum ponticum with wheat. The stock carrying Lr24/Sr24 that was sent to PBI from the US was red-seeded, and before it could be used here, the close association between this red grain colour and the rust resistance had to be broken. After some time and effort, white-seeded rust-resistant lines were generated by researchers at the PBI and provided to Australian breeders.
The first Australian wheat cultivar to carry Lr24/Sr24 was Torres, released in Queensland in 1983. Since then, 56 cultivars with Lr24/Sr24 have been released in Australia and many have been sown across large areas, providing protection against leaf rust and stem rust. For example, in 1993 alone, the area sown to cultivars with Lr24/Sr24 was about 45 per cent in Queensland and 35 per cent in New South Wales. While leaf rust virulence to Lr24 was detected in eastern Australia in 2000, this gene is still effective in WA where it provides complete protection against leaf rust. The linked stem rust resistance gene Sr24 remains effective across Australia. It is a significant contributor to the genetic control of stem rust in wheat here – the overall value of which is estimated to be some $438 million a year. Current cultivars carrying Lr24/Sr24 include: Bremer, Cutlass, Elmore CL Plus, Gazelle, Impress CL Plus, Lancer, Magenta, Shield, Sunguard and Supreme.
These three all-stage resistance genes are always found together in wheat, and provide triple rust resistance to leaf rust (Lr37), stem rust (Sr38) and stripe rust (Yr17). Also known as the ‘VPM resistance’, the genes were transferred to common wheat from the wild relative Aegilops ventricosa. The name ‘VPM’ comes from the lines used to generate it: Ae. ventricosum, Triticum persicum and T. aestivum cv Marne.
The first cultivar released in Australia with the VPM triple rust resistance was Sunbri (1990). Since then 70 cultivars have been released, including more than 30 that are currently grown (for example, Envoy, Hatchet CL Plus, Impala, Mace, Scenario, Scepter, Scout, Sunlamb and Tenfour). Since the initial use of the VPM resistance, the three wheat rust pathogens have developed the ability to overcome each of the component resistance genes (Yr17 stripe rust virulence in eastern Australia in 1999; Sr38 stem rust virulence in WA in 2001; Lr37 leaf rust virulence in WA in 2002).
The Yr17 gene continues to provide resistance to stripe rust in WA because the stripe rust pathotype that can overcome this gene does not occur there. This is important given the presence of this resistance in the popular cultivar Mace, which in 2014-15 occupied 59 per cent of WA acreage. Based on previous estimates of savings of $45 million a year due to genetic resistance to stripe rust in WA, it could be said that Yr17 contributed some $22 million in protection in that year alone.
The battle goes on
Breeding for disease resistance is akin to an arms race, in which breeders must stay one step ahead of a constantly evolving pathogen. The discovery and delivery of new rust-resistance genes, and of tools to accelerate their incorporation into high-yielding and high-quality cultivars, are important in ensuring Australian wheat growers are spared the ravages of these important diseases. The pre-breeding pipeline continues to identify new resistance genes and deliver them to breeding programs. Over the past five years at PBI, 22 new rust-resistance genes (including 16 all-stage and six adult plant resistance genes) have been found and characterised in wheat. Many of these have already been delivered to breeding programs in adapted backgrounds, along with linked markers, and will likely find their way into new cultivars in the same way as Lr24/Sr24 and the VPM resistances have.
More information:Professor Robert Park,
02 9351 8806,
Dr Will Cuddy,
02 9351 8871,
GRDC Project Code US00067, US00053, CSP00164
Region Overseas, South