The world’s wheat supplies are under threat from fast-mutating new strains of stripe rust, also known as yellow rust. The new strains attack hitherto resistant varieties and are spreading to new areas. Unless they are stopped, a stripe rust pandemic could destroy millions of hectares of wheat, with low-income countries suffering the most.
In 2009-10, an epidemic of stripe rust swept across West Asia. Syria lost 25-30% of its wheat harvest in one year as a combined result of drought and stripe rust.
Shortly after the first signs of serious infection were spotted, national partners and ICARDA researchers began tracking rust pathogens and analyzing their virulence in different areas, and on different wheat varieties, in order to understand how the pathogens move and behave.
The epidemic was caused by a new strain that overcame the resistance provided by the widely used stripe rust resistance gene Yr27. Two varieties (Cham 8 and Cham 10) that were grown on 70% of Syria’s bread wheat area, were particularly susceptible.
By comparing differences and similarities in resistance between wheat varieties in Syria, neighboring countries and other continents, researchers identified varieties and breeding materials resistant to the new pathogen strain.
Diversity and genetic resistance
This work represents a major advance in our understanding of the evolution of new strains and the genetics of resistance. The best insurance against rust is to develop not one but a range of resistant varieties suitable for various environments.
The risk of a large-scale epidemic is greatly reduced when farmers across a region grow a range of rust-resistant varieties, not just one genetically uniform variety that might suddenly succumb to a new strain. ICARDA is running field trials to assess the resistance of different wheat varieties to different races of rust in different environments.
Researchers have identified stripe rust resistance genes that act in similar ways under different conditions. This information is being used to develop ‘slow rusting‘ varieties that, while not resistant, will suffer only minimal losses in yield and grain quality.
Such varieties are the best defense against stripe rust, particularly in areas where fungicides are unavailable or too expensive.