Mycosphaerella graminicola on wheat, genetic variation and histopathology.

The research described in this thesis was focused on a comprehensive understanding of the generation and extent of genetic variation, and its effects on host cultivars in the wheat- Mycosphaerella graminicola pathosystem. Inoculation experiments were conducted in the seedling stage and adult plant stages under field conditions over several years. These experiments encompassed nearly 100 isolates of the pathogen tested on some 50 wheat cultivars. Parametric and non-parametric statistical analyses were employed on large data and very small data sets, and indicated that in all experiments cultivar x isolate interactions were significant. Pathogen isolates originating from bread wheat and durum wheat appeared to be adapted to their hosts, respectively. Molecular analysis of the interternally transcribed spacer regions of isolates from both forms, indicated that these were taxonomically closely related. The inoculation experiments indicated that specificity is an important characteristic of the pathosystem. After having determined the wide genetic variation in the pathosystem, experiments were conducted to elucidate the mating system of the pathogen. The results indicated a bipolar heterothallic mating system and RAPD analyses of progenies showed regular Mendelian inheritance. Furthermore it was shown that the pathogen is able to complete several generative cycles within a season. Hence, genetic recombination appears to be the driving force behind the determined vast genetic variation. Biochemical and histological experiments were conducted involving compatible and incompatible interactions with the host plant in order to elucidate the resistance mechanism of wheat to the pathogen