Population structure of phytophthora cinnamomi in South Africa

English: Research presented in this dissertation describes the population structure of Phytophthora cinnamomi in South Africa. Special emphasis is given to the practical implications of population diversity issues for breeding and selection programs against P. cinnamomi. Levels of gene and genotypic diversity of P. cinnamomi populations, their origin, stability over time, variation in levels of pathogenicity and occurrence of sexual reproduction in vivo and in vitro, are considered. The first chapter represents a literature review on the population biology of P. cinnamomi. Special reference is given to the life-cycle of P. cinnamomi, mechanisms of pathogenicity, and control measures. Available genetic markers useful for population genetic studies of fungal pathogens are described. The origin and maintenance of genetic diversity in fungi, with particular emphasis on P. cinnamomi, is discussed. Research concerning the genetic structure of P. cinnamomi populations is summarised, before the overall and specific aims of the research described in this dissertation are presented. This literature review also emphasises the lack of, or limited number of studies pertaining to the population structure of P. cinnamomi. Chapter two considers levels of genetic diversity as determined in populations of P. cinnamomi in South Africa, using isozymes. Differentiation in regional P. cinnamomi populations, stability over time, and occurrence of sexual reproduction in the South African P. cinnamomi population is discussed. Evidence is presented suggesting that P. cinnamomi is introduced into South Africa and has low levels of gene and genotypic diversity. There is no genetic differentiation between regional P. cinnamomi populations and the populations are stable over time. It is also shown that P. cinnamomi rarely reproduces sexually, if at all. The success of breeding and selection programs against P. cinnamomi is dependant on the levels of variation in pathogenicity of the pathogen. In chapter three, significant differences in levels of pathogenicity to E. smithii in the field, were found. Various factors are shown to affect levels of pathogenicity. These include growth rate in vitro of P. cinnamomi, culture age, season of inoculation, and multi locus isozyme genotype characteristics. On the other hand, mating type and geographic origin of P. cinnamomi isolates did not affect the level of pathogenicity. Significantly more variation was found among A2 than Al mating type isolates, demonstrating the clonality of Al mating type isolates. Results of this study provide valuable information on selection of P. cinnamomi isolates for future resistance/tolerance screening assays, of Eucalyptus germplasm in South Africa. Chapter four deals with levels of gene and genotypic diversity in South African and Australian P. cinnamomi populations, using RAPD and RFLP markers. The high degree of similarity between the South African and Australian P. cinnamomi populations is demonstrated. A low level of genetic differentiation (Dm = 0.003) between the populations provides the most striking evidence of similarity in these two populations. A few isolates from Papua New Guinea, possessed unique alleles not found in the South African or Australian P. cinnamomi populations. The similarity of the South African and Australian P. cinnamomi populations provide opportunities to breed and select for resistance/tolerance in Eucalyptus, using either P. cinnamomi population. Evidence for the occurrence of sexual recombination of P. cinnamomi in vitro and in vivo, is limited. The occurrence of sexual recombination in vitro is investigated in chapter five and the first unambiguous evidence for sexual recombination in P. cinnamomi is provided. This has important consequences for breeding and selection programs, as it could change the genetic structure of the population. Levels of pathogenicity of the F 1 progeny was on average lower than that of parental P. cinnamomi isolates. Other factors concerning fitness of the progeny should be investigated to explain the apparent lack of sexual reproduction in vivo in P. cinnamomi. All chapters of this dissertation deal with some aspects of the population structure and genetic diversity within P. cinnamomi populations from South Africa. This is the first comprehensive study on the genetic diversity of P. cinnamomi populations. It sheds light on the origin, occurrence of sexual reproduction, variation in levels of pathogenicity amongst P. cinnamomi isolates, similarity with the Australian population, and the occurrence of genetic recombination in vitro. It also provides important information for the South African forestry industry, particularly in their efforts to breed and select cold tolerant Eucalyptus spp. that are resistant/tolerant to P. cinnamomi.