Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/4/1999
Publication Date: N/A
Interpretive Summary: The spread of plant disease is an important consideration in international trade of agricultural products as well as in the design of strategies to control diseases of major food crops. Stem rust, which is potentially the most devastating of all diseases of wheat, first appeared in South America in 1811, more than 150 years after wheat was introduced into Brazil. Our research was conducted to determine whether the fungus causing wheat stem rust in South America is genetically similar to the stem rust fungus that evolved in Central Asia and spread to Europe with the development of cereal-based agriculture, or whether the stem rust in South America developed independently from a similar rust fungus infecting South American species of barberry related to the barberry species that serves as the alternate host for the European stem rust fungus. We compared virulence types, enzyme forms, and specific DNA segments in collections of the European and South American rust fungi. The results show that wheat stem rust in South America had its origin in Europe. This is important because it establishes that wheat stem rust from South America does not pose a special threat to wheat production in North America, where stem rust from Europe has been present since 1660. Also, it shows that new sources of resistance to the South American stem rust are most likely to be found in wild wheats in the Near East where wheat and stem rust originated. This information will be used by other scientists, particularly those involved in developing new disease resistant varieties or studying the spread of agronomically important plant pathogens.
Technical Abstract: We tested whether the South American population of Puccinia graminis f.sp. tritici may have originated from Europe by comparing the genetic similarity of collections of P. graminis f.sp. tritici from South America and Europe. RAPD, isozyme, and virulence markers were used to compare 20 isolates, diverse for virulence, from South America and 19 similarly diverse isolates sfrom Europe. The average similarity coefficients between the South American and European isolates was 0.67 for isozymes, 0.65 for virulence, and 0.70 for RAPD markers. These values are comparable to the average similarity coefficient when comparing South American among themselves of 0.64 for isozymes, 0.63 for virulence, and 0.72 for RAPDs. Relationship patterns between all 39 isolates based on RAPDs was more similar to the pattern based on isozyme markers (r = 0.52) than that based on virulence (r = 0.32). The correlation was low (r = 0.16) between the similarity matrix based on isozyme markers and that based on virulence markers. There were no isozyme alleles that were unique to the South American collection, but there were alleles unique to the European collection. For isozyme and RAPD makers, both the South American and European collections consisted of two main groups; each South American group was more closely related to a European group than to the other South American group. These results indicate that the current South American population could have arisen from European introductions. The data are most consistent with at least two separate introductions of P. graminis f.sp. tritici into South America.