Ground Cover Supplement : GC Supplement - Cereal foliar fungal diseases
6 Disease resistance Wild genes help tame rust pathogen the grdc-funded australian cereal rust control Program is facing a very tough opponent – the ever-changing cereal rust pathogen – but so far it seems to be winning the game By Janet Paterson A SINGLE RUST-RESISTANCE gene inserted into the now famous 'green revolution' wheat varieties bred by Dr Norman Borlaug lasted for more than 40 years before a pocket of stem rust living in east Africa mutated to overcome it. The mutant, known as Ug99 because it was first found in Uganda and documented in 1999, quickly spread to Kenya and Ethiopia and has since spread thousands of kilometres, into South Africa and east into Iran. It is now only a stone's throw from Pakistan -- a major wheat-producing country with a population of 180 million people. Seven strains of the original Ug99 mutant have now been documented. University of Adelaide cytogeneticist Dr Ian Dundas says the potential consequences of the ever-evolving Ug99 rust pathogen are on par with the devastating effects of the potato blight disease that hit Ireland and Scotland in the 1840s. "Dr Borlaug's rust-resistance gene was so effective against stem rust that the disease was thought to have effectively disappeared and many took their eye off the pathogen and stopped looking for new resistance genes," Dr Dundas says. Due to its associated poor dough quality, Australian wheat breeders decided not to use the Dr Borlaug gene. Several other rust-resistance genes were used instead, which until recently have held Australia's wheat cultivars in good stead in the face of the ever- evolving stem rust pathogen. "It was the foresight of Professor Robert McIntosh -- previous director of rust research at the University of Sydney -- that put Australian wheat cultivars in a better position to cope with the rust pathogen," Dr Dundas says. "It was Bob who warned us not to let our guard down against rust in a plant breeding sense because he could see the potential for the pathogen to overcome genetic resistance to the disease in our wheat cultivars." AUSTRALIAN CULTIVARS AT RISK But with Ug99 and other stem rust strains spreading and developing new mutations, most of the Australian rust-resistance genes in commercial wheats are now vulnerable. "It is a sobering thought that if all seven Ug99 strains arrived in Australia tomorrow then the majority of our cultivars would have no effective resistance," Dr Dundas says. Given Australia's potential vulnerability to new rust strains, Dr Dundas says it is imperative that the search continues for new sources of genetic resistance against the costly pathogens. "Rust is seemingly so bent on its own survival that it jumps at every opportunity to generate new strains -- some of which will inevitably overcome the resistance bred into current Australian varieties." Reassuringly, Dr Dundas says there is no shortage of genetic resistance to the wheat rust pathogen. "There is only a shortage of resistance in modern wheat varieties," he explains. WILD RESISTANCE GENES Dr Dundas and his colleagues from the Australian Cereal Rust Control Program (ACRCP) have been scouring the genomes of wild wheat relatives from across the Middle East for resistance to stem, leaf and stripe rust since the early 1990s with the aim of transferring durable rust resistance into Australian and international wheat varieties. PHOTO: UNIVERSITY OF ADELAIDE Rust-resistance genes from wild wheat relatives (pictured) are being isolated and incorporated into modern wheat breeding lines to avert the growing worldwide threat posed by stem and stripe rust pathogens to global wheat production.
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