Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/14/2007
Publication Date: 10/25/2007
Citation: Ferguson, L.M., Carson, M.L. 2007. Temporal variation of Setosphaeria turcica sampled from 1974 to 1994 and the origin of races 1,2, and 23n in North America. Phytopathology. 97:1501-1511. Interpretive Summary: Northern corn leaf blight is a serious disease of corn throughout much of the world and is caused by the fungus, Setosphaeria turcica. We studied a collection of isolates of this fungus collected from the US from 1974-1994, a period of time in which three new races of this fungus were reported. The isolates were characterized by race, mating type, and DNA markers. Analysis of these genetic markers indicate the importance of both clonal and sexual reproduction in this fungus. Although race 1, which was first reported in the US in 1979, was predominately of a single mating type, our data suggest that it arose multiple times in different genetic lineages rather than as a mutant within a single clonal lineage. Races 23 and 23N were found in collections from Virginia in 1957, indicating that these races had been present long before they were initially reported in 1976 and 1989, respectively. These data mean that corn pathologists and breeders in the US should be aware that this pathogen is sufficiently variable that new races can arise from multiple lineages in diverse locations provided the new race is more fit. Emphasis in corn breeding programs should be placed on using broad-based partial resistance to northern leaf blight rather than race-specific resistance such as that conferred by Ht genes.
Technical Abstract: Setosphaeria turcica causes northern corn leaf blight, an economically important disease of maize throughout the world. Survey collections of S. turcica isolates from 1974 to 1994 provided a unique opportunity to examine temporal diversity in the eastern US. Two hundred forty two isolates of S. turcica from maize were studied using random amplified polymorphic DNA (RAPD) markers, mating type and virulence to Ht resistance genes, to examine changes over time. One hundred forty nine RAPD haplotypes were identified. Nearly 20% of these haplotypes recurred in more than one year. Race 0 isolates declined in frequency from 83% in 1974 to near 50% in the 1990’s in response to the widespread deployment of Ht1 in commercial maize hybrids. Races 23 and 23N were present in the collection at low levels throughout the study period and were also found among isolates from Virginia in 1957. The frequency of MAT1-2 isolates increased sharply after 1979 and was associated with the emergence of race 1 during the same period. RAPD markers were used to investigate the genetic diversity among a subset of isolates collected in the US from 1976 to 1982, the period in which this dramatic shift in race frequency occurred. Multilocus haplotypes were not exclusively associated with known races of S. turcica. Based on shared haplotypes and cluster analysis, race 1 isolates share greater similarity with race 0 than with 23 or 23N isolates, indicating race 1 probably evolved from multiple lineages of race 0. Sorghum spp.-infecting isolates share greater similarity with one another than with maize-infecting isolates and represent a distinct sub-group.