During March, CIMMYT, the International Maize and Wheat Improvement Centre in Mexico, announced a significant stem rust epidemic in Uganda. This was brought about by a mutational change in the stem rust fungus. The question asked of me was why is this event so important for CIMMYT, and what are the implications for Australia?
Many modern varieties produced by CIMMYT breeders and released by national programs throughout the developing world possess a segment of chromosome obtained from cereal rye. This segment carries genes for disease resistance and superior adaptation. On the negative side, there is a potential quality defect that has warned Australian breeders off using it.
The new rust in Uganda has the ability to overcome the resistance provided by Sr31 present in the rye segment. Because Sr31 is widely used throughout Northern Africa, the Middle East and Western Asia, to the Indian subcontinent and even in China, there is the danger that the new pathotype will spread.
Indeed, a similar event occurred during the late 1980s and early 1990s with the stripe rust fungus overcoming the gene Yr9 in the same chromosome segment.
Stem rust is much more damaging than stripe rust. There is no doubt that some wheats with Sr31 will have additional genes to protect them, but breeders and pathologists will need to identify them as a matter of urgency.
Australia is geographically isolated and is not at risk from airborne rust spores from North Africa and Asia. Even if such a pathotype were to appear here we would not be threatened because our rust resistances are not widely based on Sr31 (exceptions are Grebe, Mawson, Triller and Seri 82-AUS).
The event in Uganda is more a timely reminder that a breakdown of genetic resistance can occur at any time. Most likely this will be the result of a change in a current local pathotype.
The mutant pathotype will increase on varieties carrying only the resistance gene for which the mutation occurred.
The strategy of the National Cereal Rust Control Program, strongly supported by its Steering Committee representing wheat breeders and pathologists, and the National Rust Forum, a subcommittee of the Grains Council, is to minimise the level of rust by the universal use of resistant wheats.
Farmers are encouraged to use only rustresistant wheats and to choose two or three varieties with different genes for resistance.