Genetic diversity, population differentiation and intrachromosomal recombination inferring outcrossing in natural populations of Sclerotinia sclerotiorum

The main focus of this research was on the genetic diversity, population differentiation and evolutionary potential of the plant pathogenic fungus, Sclerotinia sclerotiorum. Populations isolated by distance evolve separately and might display great genetic and phenotypic differentiation. Two S. sclerotiorum populations obtained from canola fields in China and USA were genotyped using eight microsatellite markers and phenotyped for the production of oxalic acid, sclerotia and mycelium, sensitivity to fungicides benomyl, fluazinam and iprodione, and virulence. Significant population differentiation was observed for the genotypic and phenotypic traits except for virulence and fluazinam sensitivity. Half of the Chinese population was resistant to benomyl. Cross resistance between benomyl and iprodione was detected. Virulence was highly influenced by the environment, as evidenced by low heritability. Results suggest that even for a cosmopolitan pathogen like S. sclerotiorum, strict regulations on global pathogen movements should be applied.;The objective of the second project was to quantify genetic and phenotypic diversity of S. sclerotiorum from soil on a fine geographic scale. A high level of genetic and phenotypic diversity was observed among 40 isolates from 1 m2 soil in an alfalfa field. Five genetic clusters were found and recombination was detected within a subpopulation.;In view of the high genetic diversity in soil, it was hypothesized that relatively lower genetic diversity is present in the infected alfalfa plants due to selection by host. The genetic diversity of soil populations sampled at the beginning of the season was compared with the stem populations sampled near the end of the growing season using microsatellite markers. Both stem and soil populations were equally diverse and not differentiated suggesting host selection is minimal or less important in the alfalfa-S. sclerotiorum patho-system.;Recombination among presumably unlinked markers has been previously detected in S. sclerotiorum populations. Tests of pair-wise Linkage Disequilibrium (LD) between physically linked markers in this study revealed that, LD decayed with increasing distance between markers on two supercontigs for most of the populations. Results provide new evidence of frequent outcrossing in natural populations of S. sclerotiorum.