GENETIC-STRUCTURE OF SETOSPHAERIA-TURCICA POPULATIONS IN TROPICAL ANDTEMPERATE CLIMATES

Northern leaf blight, caused by Setosphaeria turcica, is a serious dis ease of maize in temperate and tropical environments. To examine the p athogen's population structure, we analyzed 264 isolates from four dif ferent continents with 70 random amplified polymorphic DNA markers and determined their mating types. Tropical populations (from Kenya, Mexi co, and southern China) had an extremely high genotypic diversity, no or only weak gametic phase disequilibrium. and an even distribution of the two mating types, indicating frequent sexual recombination. Tempe rate populations (from Europe and northern China) had a much lower gen otypic diversity, strong gametic phase disequilibrium, and an uneven d istribution of mating types, indicating that sexual recombination has been rare. Populations in different continents were genetically isolat ed. They shared no haplotypes and carried several ''private'' alleles. The number of migrants between continents and between regions (betwee n northern and southern China, western and central Kenya, and Europe w est and east of the Alps) was estimated to be less than one per genera tion. Multivariate statistics suggested a greater relatedness of popul ations from the same continents than from different continents. Within agroecological zones, migration must be extensive. The potential with in populations of S. turcica for adaptation should be regarded as very high, especially in tropical climates.