The pathogen that causes the world's most common wheat disease is a moving target, but scientists are now better equipped to keep track of it, thanks to some genetic sleuthing by Agricultural Research Service (ARS) scientists.

Up to 60 resistance genes have been known to combat Puccinia triticina, the fungus that causes wheat leaf rust. But the pathogen is so genetically diverse and quick to adapt that most wheat resistance genes prove ineffective within a few years.

Leaf rust is the world's most widely distributed wheat disease. In Kansas alone wheat producers lost 14% of their crop – some 50 million bushels – to a leaf rust epidemic in 2007. Emerging strains of P. triticina are an increasing threat to soft red winter wheat in the southeastern U.S. and to hard red winter wheat and hard red spring wheat in the Great Plains, says James Kolmer, plant pathologist, ARS Cereal Disease Laboratory in St. Paul, MN.

Kolmer says the strains of P. triticina infecting wheat in North America fall into five genetically distinct groups, with two widely distributed groups accounting for 90% of the total population. The five groups also differ in their ability to overcome a number of resistance genes, an indication that different groups of P. triticina develop virulence traits at different rates.

The work, to be published in the journal Phytopathology, will help researchers identify the origins of emerging strains of the disease, unravel clues about migration patterns, monitor shifts in virulence and figure out why some resistance genes are more effective and long-lasting than others.