Effects of Cultivar, Isolate and Environment on Resistance of Wheat to Septoria Tritici Blotch in Kenya

The research described in this thesis focused on the characterization of some of the factors that influence disease assessment, development and expression of resistance in wheat cultivars to septoria tritici blotch. Earliness appeared to have a strong effect and tallness a small effect on disease severity (DS). A regression equation derived was used to correct the DS in the entries. Another method that gave good disease assessment was to group the cultivars according to their earliness. It appeared that the disease developed in each earliness group at the same rate. The importance of interplot interference in assessing septoria resistance in wheat was studied. There was no indication of interplot interference. The Nitrogen (N) level is another factor that may affect disease assessment.In Kenya there was an increase in DS on cultivars exposed to more N while in The Netherlands there was no similar increase in DS.When an inoculum mixture or single isolates are used for inoculations, the ranking of the cultivars was essentially not affected, indicating that inoculum mixtures can be used effectively in screening wheat genotypes. The correlation coefficients between the DS at the seedling and the adult plant stages was low. Thus resistance assessed at the seedling stage could not fully explain adult plant resistance. Isolates from Kenya and The Netherlands were tested on wheat seedlings. It was concluded that there was variation in virulence (and so in race-specific resistance in the host) of Septoria tritici populations within both countries. The strong cultivar x isolate interactions observed on wheat seedlings was also observed on adult plants in the field.F6 lines of36 crosses were evaluated in the field. Transgressive segregation towards more resistance and or more susceptibility occurred in most crosses. It can be said that a fair number of genes operating in an additive manner and epistasis are involved.