Association Mapping of Leaf Rust Resistance in Wild Barley.

Barley was first introduced to the United States by English settlers during the 17^th century, and has since developed into a major US crop.  In 2008, just over four million acres of barley were planted in the United States, leading to an annual average of 273 million bushels per year from 1999 to 2008 with an annual value of $633 million as a raw product.  The brewing industry alone contributes over $189 billion dollars annually to the US economy, a number that would decrease significantly without a healthy brewing barley crop.  Epidemics of leaf rust occurred regularly in the United States barley crop, causing yield losses as high as 40%, until the development of resistant barley cultivars in the early 1960's.  In most early cultivars, leaf rust resistance was controlled by the dominant gene Rph7.  The first race of leaf rust with Rph7 virulence appeared in North America in 1990 and, unlike its predecessor, this new strain was capable of completely destroying a crop.  Three hundred and eighteen accessions of wild barley were analyzed for resistance to two different leaf rust isolates.   Three replications for each isolate were inoculated, incubated, and rated on a 0 to 4 scale.  TASSEL software was used to statistically associate leaf rust phenotypes of wild barley accessions with molecular markers, which have previously been assigned to a chromosomal position in the barley genome. This association mapping (AM) approach allowed for efficient mapping of leaf rust resistance genes to chromosomal positions and identification of closely linked molecular markers that may be used in marker-assisted selection.