The two major pathotypes (W & R) of the cereal eyespot fungus, Pseudocercosporella
herpotrichoides, show variation of major agronomic importance, specifically in sensitivity to
fungicides and in differences in host species specificity. This thesis describes the genetic
analysis of these characters using the parasexual cycle to generate recombinant progeny
between W and R pathotypes. Interspecific hybrid progeny from a parasexual cross between P.
herpotrichoides and P. anguioides were included in the analysis. Marker differences present in
the parental isolates including auxotrophic requirements, isozyme banding patterns, sensitivity
towards benzimidazole fungicides and spore length have been used as proof of recombination
in the parasexual progeny, and to characterise the pattern of genetic segregation in the crosses.
Analysis of fungicide sensitivity focussed on the ergosterol biosynthesis inhibitors (EBI), where
it is found that W-types are significantly more sensitive to the majority of the déméthylation
inhibiting (DMI) fungicides than are the R-tvpes. Determination of sensitivity of progeny to
EBI fungicides revealed the presence of 'major' and 'minor' gene(s) controlling the expression of
fungicide resistance. Major gene segregation resulting in a substantial difference in sensitivity
to triadimenol, was found to occur prior to exposure of the diploid fusion products to
haploidizing agents with the result that progeny generated from any one fusion product were
either sensitive or resistant to triadimenol. Segregation of additional minor genes occurred
following exposure to haploidizing agents and these modified the level of resistance expression
within the progeny. These analyses provided the first genetic evidence for cross-resistance to
these fungicides in this fungus. Cross-resistance relationships were shown to depend on the
mode of action of the chemicals and also varied within the chemical groups. Interaction was
seen between major and minor gene(s) in the expression of cross-resistance relationships.
Pathogenicity towards wheat and rye was shown to be considerably disrupted by parasexual
recombination. Although most paraexual progeny were not pathogenic a minority of isolates
were able to infect and produce typical disease symptoms on wheat (14%) and / or rye (21%).
Pathogenicity to wheat and rye was not associated indicating that the inheritance of pathogenic
ability towards wheat and rye is genetically separable and that new 'pathotvpes' can be
generated through recombination. Withm the non-pathogenic progeny a subset of recombinant
strains were found which were able to colonise host tissues demonstrating a clear parasitic
ability. The remaining progeny appeared to be deficient in both pathogenic and parasitic
abilities. These three groups of recombinants : pathogenic, parasitic/non-pathogemc and nonparasitic will allow the future investigation of both pathogenicity and parasitism in this fungus.
Parasexual progeny were used to investigate the inheritance of vegetative incompatibility in P.
herpotrichoides. However, detection of vegetative incompatibility between W - and R-tvpe
isolates or between recombinant progeny was not successfull in spite of using a number of
different techniques. Further refinement of the assay system is required.