During the initiation and execution of a rootstock breeding programme to overcome the financially crippling disease, Phytophthora root rot of avocado, various constraints have been identified for both the breeding as well as the screening aspect of the programme. A review of the literature revealed a complex host-pathogen interaction that should be taken into account in the recombination and screening of genetic material. With the detection of beneficial genotypes being the crux of a breeding programme, this dissertation was focused on the screening of rootstock material for tolerance to Phytophthora cinnamomi. Screening should be scientific but at the same time also be time and cost effective. Specific attention was given to (i) the correct medium for screening mass numbers of seedlings, (ii) fast and effective cloning of single selections, and (iii) evaluation of clonal material for tolerance to P. cinnamomi. Soil as a screening medium was compared with three inert hydroponic media as well as one aeroponic system. Only soil was found to be ineffective due to its properties. The other media tested, namely, sand, vermiculite, water and the aeroponic system were equal in performance. The medium to be used will depend on the preference of the breeder as each medium has its own pro's and con's. It was, however, found that the evaluation criterion to be applied depends on the medium that is used. With regard to cloning of single selections, a definite difference with regard to the cloning ability of the different selections was found. An inability to be etiolated was displayed by some of the selections and these could thus not be vegetatively propagated and were not further tested. One of the tolerance mechanisms in the standard cultivar Duke 7, is root regeneration. It was thus expected that this characteristic cloning would give an indication of the rootstock's ability to tolerate P. cinnamomi. This could not be confirmed, but most of the selections did, however, perform better than Duke 7. Comparison of feeder root percentage in non-inoculated and inoculated treatments was not sufficient for facilitating the final selection of candidate rootstocks from a large number of potential clonal selections. Four selections were made, based on the hypothesis that a larger root system will be a better forager and thus enhance the horticultural aspects of the rootstock-scion combination. Valuable information was obtained with regard to various mediums and criteria to be used during mass screening and final screening of clonal selections. This knowledge must be taken into account in the planning of future breeding projects. During this project a total of 38 984 seedlings were screened and four selections were made. For both the nursery and the producer, knowledge of the clonal ability of a potential new rootstock is important from a financial point of view.
Dissertation (MSc (Horticultural Science))--University of Pretoria, 2006.