Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2004
Publication Date: 4/1/2004
Citation: Ferguson, L.M., Carson, M.L. 2004. Spatial diversity of Setosphaeria turcica sampled from the eastern United States. Phytopathology. 94:892-900. Interpretive Summary: Northern leaf blight, caused by the fungus Setosphaeria turcica, is an important disease of corn worldwide. In this study, over 250 isolates of the fungus were collected from across the midwestern, eastern and southern corn belts of the U.S. The isolates were analyzed for virulence on resistant corn varieties, genetic (DNA) markers, and mating type. Although there were no distinct subpopulations of the fungus in the U.S., isolates from the midwest were more closely related to one another than with isolates from the south or east. The population of this important fungal pathogen in the U.S. was found to be genetically quite diverse and likely the result of both clonal (asexual) and sexual reproduction. This study demonstrates why the U.S. should not rely on on corn hybrids with race-specific, single gene resistance to control this disease. The primary audience for this report is scientists in both the public and private (hybrid seed industry) sector.
Technical Abstract: Randomly Amplified Polymorphic DNA (RAPD) markers and mating type were used to examine regional population structure of Setosphaeria turcica in the Eastern United States. Of 251 maize-infecting isolates studied, 155 multilocus haplotypes were identified using 21 RAPD markers. Twelve isolates of the most common haplotype, which were identified from seven states, represented 5.2% of the sample. Although variation in genetic diversity was greatest within states rather than between either regions or states within regions, multidimensional scaling based on average taxonomic distances among state samples showed a close association of samples from IL, OH, IN, IA, MN, MI/WI, and NC. Isolates from GA/SC, VA/TN, PA/NY, and FL were distant from this core group that included Midwestern states and NC, and also distinct from one another. The high genotypic diversity, near equal mating type frequencies and gametic phase equilibrium in samples from several states is inconsistent with a strictly clonal population. The population genetic structure of S. turcica is likely the result of both asexual and sexual reproduction. It is not clear whether sexual recombination actually occurs in the eastern US or occurs elsewhere in tropical America and the recombinant genotypes routinely migrate to North America.