LEAF AND STEM RUST OF WHEAT IN THE UNITED STATES

Authors: Kolmer, James; Jin, Yue; Long, David

Submitted to: Australian Journal of Agricultural Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2007
Publication Date: 7/1/2007
Citation: Kolmer, J.A., Jin, Y., Long, D.L. 2007. Leaf and Stem Rust of Wheat in the United States. Australian Journal of Agricultural Research. 58:631-638.

Interpretive Summary: In the U.S., wheat is attacked by two diseases called leaf rust and stem rust. Rust diseases are caused by rust fungi, which attack wheat plants by means of microscopic spores that are carried for hundreds of miles in the prevailing winds. Both rusts are also genetically variable, with many different types of races that differ in their ability to attack wheat plants that carry leaf rust or stem rust resistance genes. Collections of both rusts are made annually throughout the U.S. by the USDA-ARS Cereal Disease Laboratory in St. Paul MN. Forty to 50 different races of leaf rust are identified annually in the U.S. This high degree of variability in leaf rust has allowed the leaf rust fungus (Puccinia triticina) to adapt to new resistant wheat cultivars in a short period of time. Many wheat cultivars lose their resistance to leaf rust within a few years. Yield losses in wheat due to leaf rust can range from less than 1% to more than 30%. In recent years, only three to five races of the wheat stem rust fungus (Puccinia graminis) have been found in the U.S. Most wheat cultivars in the U.S. are highly resistant to wheat stem rust. New wheat stem rust races in East Africa are virulent to many U.S. wheat cultivars and may threaten the U.S. wheat crop. The results from virulence studies of wheat leaf and wheat stem rust can be used by wheat breeders and plant pathologists to determine which rust resistance genes can be added to wheat breeding programs in order to develop wheat cultivars with good resistance to the many different races of leaf and stem rust.

Technical Abstract: Leaf rust, caused by Puccinia triticina, is a common and widespread disease of wheat in the U.S. On an annual basis, more than 40 races of the leaf rust fungus are detected. There are at least five major groups of genetically distinct P. triticina isolates in the U.S. based on allelic variation at microsatellite loci. Distinct regional race populations of P. triticina are found in the U.S. due to the widespread use of race specific leaf rust resistance (Lr) genes in different market classes of wheat. In the Southeastern States where soft red winter wheats are grown, races with virulence to Lr9, Lr11, and Lr18, are the predominant races. Cultivars with these genes have been commonly grown in this region. In the Southern Great Plains region where hard red winter wheats are grown, races with virulence to genes Lr9, Lr17, Lr24, and Lr26, are common. In the Northern Great Plains region where hard red spring wheats are grown, races with virulence to Lr2a, Lr16, and Lr26, are most common. Due to the wide dispersal of P. triticina, some races are found in all regions of the U.S. Highly effective durable resistance to leaf rust has been difficult to achieve due to the high degree of virulence variation in the P. triticina population and the rapid selection of races with virulence to effective Lr genes in wheat cultivars. Hard red spring wheat cultivars with genes Lr16, Lr23, and Lr34 have been highly resistant for at least the last 10 years in Minnesota and the Dakotas. The interaction between Lr16, Lr23, and Lr34, most likely accounts for the resistance in selected hard red spring wheats. Stem rust, caused by P. graminis f. sp. tritici, has not been a common disease of wheat in the U.S. since the last major epidemics in the 1950s. The low levels of stem rust infections in the U.S. can be attributed to the increasing use of highly resistant winter and spring wheat cultivars that has greatly reduced the overall level of stem rust urediniospores. Eradication of the alternate host, Berberis vulgaris, has reduced the number of races and slowed the emergence of new races. Resistance genes Sr2, Sr6, Sr17, Sr24, Sr31, Sr36, and SrTmp, are common in the winter wheats. Genes Sr6, Sr9b, Sr11, and Sr17, are common in the spring wheats. Many spring wheat cultivars also most likely have adult plant stem rust resistance derived from the cultivar Thatcher. Many of the winter and spring wheats are susceptible to the new stem rust race from East Africa, however cultivars with resistance to this race can be found in each of the major wheat classes.