GENETIC-VARIABILITY IN NUCLEAR-DNA IN-FIELD POPULATIONS OF STAGONOSPORA-NODORUM

Anonymous nuclear DNA markers were used to characterize the genetic structure of field populations of the wheat pathogen Stagonospora nodorum. High levels of genetic and genotypic diversity were found when DNA probes that hybridized to single restriction fragment length polymorphism (RFLP) loci and to a dispersed, repetitive DNA sequence were used. The populations surveyed contained a high number of different genotypes distributed on a small scale. In the majority of cases, separate lesions on the same leaf were caused by different genotypes of S. nodorum. Pairwise comparisons between individual RFLP loci showed that the majority of alleles at these loci were in gametic equilibrium, as is expected in a random-mating population. All of these observations support the hypothesis that ascospores were a significant source of primary inoculum. A probe that hybridizes to a repetitive sequence present on several chromosomes may be useful for differentiating all the genotypes present in a population. These genetic markers will be useful for further studies on the population biology and epidemiology of S. nodorum.