One of the classic host-pathogen relationships in plant pathology is between the common barberry, Berberis vulgaris, and Puccinia graminis, the cause of stem rust, a destructive disease of small grains. As the alternate host of P. graminis, the barberry is the key in the sexual stage of the pathogen's life cycle. The combination of extensive small grains production and widespread cultivation of the common barberry in the north central United States resulted in major stem rust epidemics by the late 1800s and early 1900s. In 1918, the Barberry Eradication Program was initiated by the U. S. Department of Agriculture in cooperation with important small grain producing states. Before the termination of the program in the late 1970s, more than 500 million barberry bushes were destroyed. Since the end of the program, however, scientists have voiced concerns about the potential for barberry to reemerge as a source of inoculum and of new genetic forms. The objectives of this research were to examine the history of barberry eradication in Minnesota, to determine whether barberry has reemerged on sites in Minnesota and what effect this reemergence may have on future stem rust epidemics. An evaluation of archival records revealed that the origin of the Barberry Eradication Program was in reaction to successive stem rust epidemics and concerns over production shortages during World War I. The program that developed after 1918 was an unprecedented collaborative effort among federal-state agencies, land-grant colleges, and agricultural industry. Initial survey work was focused on the removal of barberry bushes in cities, towns, and rural planted sites; priorities shifted and procedures changed with the discovery of large numbers of escaped bushes, particularly in southeastern Minnesota. More than one million barberry bushes were destroyed in Minnesota between 1918-1990. A field survey of 72 of the approximately 1200 active sites in Minnesota was conducted. Active sites were defined as those sites that remained to be inspected for barberry regrowth at the end of the eradication program in 1980. Barberry had reemerged on 32 of the 72 sites. More than 90% of the barberry bushes were found in counties with less than 400 ha of wheat per county, mostly in southeastern Minnesota, but one bush was found in a major wheat-producing county in northwestern Minnesota. Reemergence of barberry may play a role in future epidemics of stem rust, particularly with regard to sexual reproduction in the pathogen population. Aecial isolates of P. graminis collected from common barberry in Minnesota between 1912 and 2002 were obtained and used to evaluate changes in forma speciales over time. Uredinial isolates collected in Minnesota during the same time period were compared to the aecial isolates to evaluate changes in race structure of P. graminis f.sp. tritici. Forma speciales in aecial populations changed over the 20th century and coincided with changes in barberry populations. P. graminis f. sp. tritici has become the predominant forma speciales identified in collections since 1990, resembling the pathogen population structure before eradication. Removal of barberry from areas around wheat fields contributed to a reduction in race diversity in uredinial populations; however, diversity in aecial populations was unchanged in relation to barberry removal. With the lessons learned historically during barberry eradication, the knowledge that barberry has reemerged on many sites and the recent changes in the P. graminis population, there is reason to be concerned over the possibility of increasing stem rust epidemics in Minnesota.