|Roberts, John - FORMER ARS EMPLOYEE|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 2, 1999
Publication Date: N/A
Interpretive Summary: Stem rust has been the most important disease of wheat, barley, and oat through most of agricultural history. Currently, stem rust is managed by planting rust resistant varieties, but resistance is not a permanent solution. There are many races of the fungus. Each race can overcome some resistant wheat, barley, or oat varieties. Therefore, we test rust collections from fields throughout the U.S. every year to see which races are most common in each area of the U.S. As we discover new races, we test them to see which varieties of the crops they can attack and to identify new sources of resistance that will be effective against the new races. The 1996 stem rust survey showed that stem rust did not build up early enough to cause significant yield losses in wheat, barley, or oat in any of the major production areas. No new stem rust races were discovered in 1996, which means that resistance that has protected wheat varieties well in recent years is still effective. Experimental wheat lines in our tests proved to be resistant to all races found in 1996. The stem rust race most virulent on barley varieties declined in prevalence in 1996. In past years, this virulent race survived mainly on a few highly susceptible winter wheat varieties in the central and southern Great Plains. Each spring it spread to the barley crop in the northern Great Plains. Recent changes in winter wheat varieties grown in the central Great Plains have eliminated this base from which the virulent race could spread to barley. Only one race of oat stem rust makes up more than 90% of the collections from oat. New sources of resistance to it are available. Preventing wheat stem rust epidemics averts potential losses of $100 million or more, which occurred frequently in the U.S. in the years before 1955.
Technical Abstract: Stem rust caused negligible yield losses in 1996 in the United States. Wheat stem rust was first found during the second week of April in a field of soft red winter wheat southwest of Houston, Texas. Race Pgt-TPMK continues to predominate with 66% of 273 isolates from 100 collections. TPMK represented 76 and 63% of the isolates from wheat in fields and nurseries, respectively. Race QFCS was identified at a frequency of 12% and 29% from farm fields and nurseries, respectively, and 26% overall. Eight other races consisted of 3% or less of the isolates. From barley, race QCCJ, virulent to the Rpg-1 gene for resistance to stem rust, was identified in only 12% of 77 isolates of 27 collections, while TPMK consisted of 64% of the isolates. No virulence was found to wheat lines with genes Sr9b, 13, 22, 24, 25, 26, 27, 29, 30, 31, 32, 37, Gt, or Wld-1. Oat stem rust was first found in late April in southern Louisiana and central Texas. Race NA-27, virulent to Pg-1, -2, -3, -4, and -8, was agai the predominant race in the United States, comprising 91% of 93 isolates from 36 collections. NA-5 and NA-16 were the other two races identified, comprising 4% each.