It was established that although the phytoalexin scoparone is associated with resistance/tolerance of citrus rootstocks to stem cancer caused by Phytophthora citrophthora, it does not dictate tolerance to root rot caused by P. nicotianae. It can therefore not be used as an indicator for tolerance to root rot. Results of the current study show that increase in total soluble phenolic concentrations in citrus roots after inoculation plays a key role in tolerance of citrus rootstocks against P. nicotianae root rot. Elevation of the levels of total phenolic compounds is therefore a part of the mechanism involved in rootstock resistance. As far as could be established, this finding has not been reported before in citrus. Determination of total soluble phenolics in citrus roots can therefore be used as a parameter in the screening of rootstocks for tolerance to P. nicotianae. Application of the systemic fungicide fosetyl-Al increased the total soluble phenolic concentrations in the roots more than infection with the pathogen alone. This provides evidence that the elevation of phenolic levels is involved in the mechanism of action of fosetyl-Al in control of Phytophthora root rot, therefore supporting an indirect antifungal mode of action. A unique chemical compound (U82) has been discovered that is associated only with tolerant rootstocks. If this yellow compound is a viable marker for resistance, it will certainly be a breakthrough in rootstock resistance research. Such a unique compound that is only associated with tolerant rootstocks could potentially be used in developing a more reliable high throughput screening technique for citrus rootstock resistance.
Dissertation (MSc (Microbiology and Plant Pathology))--University of Pretoria, 2006.