The fungus Fusarium circinatum is the causal agent of pitch canker that was initially discovered in the south eastern United States in 1946. In South Africa this pathogen was first discovered in 1990 in a single nursery in the Mpumalanga Province where it caused significant losses due root rot and collar infections on Pinus patula seedlings. After this initial discovery F. circinatum spread to other parts of South Africa where it apparently remained limited to pine seedling nurseries. In 2006, F. circinatum was discovered for the first time on established plantation trees of P. radiata in the Western Cape of South Africa. Since then the pathogen has been detected in numerous established pine plantations in the country. Although the epidemiology of the pathogen in the pine seedling nursery is relatively well understood, not much is known regarding the emergence of pitch canker and the distribution and spread of F. circinatum in pine plantations. The overall aim of the research presented in this dissertation was therefore to improve current knowledge regarding the population biology of F. circinatum in the plantation setting.
Chapter 1 reviews the molecular basis of vegetative and sexual compatibility in Ascomyceta, which represents two of the most widely used phenotypic properties for studying the population biology of Fusarium populations. Vegetative compatibility enables these fungi to distinguish self from non-self, which in turn allows them to maintain their unique genetic make-up in natural populations. This system is governed by a few so-called heterokaryon incompatibility (het) loci which have been extensively characterized at the molecular level in Neurospora crassa, Podospora anserina, Aspergillus nidulans and Cryphonectria parasitica. Sexual compatibility allows fungi to reproduce by recognising self from non-self. This system is governed by mating type (mat) loci, which have been extensively characterized in Saccharomyces cerevisiae, Schizosaccharomyces pombe, N. crassa, P. anserine, Cochliobolus heterstrophus and F. circinatum. In this chapter, a review is also provided of the methods used to study vegetative and sexual compatibility in the laboratory and the types of population genetic information that can be gained from such studies.
In Chapter 2 the possible origin and reproductive mode of the F. circinatum population responsible for the outbreaks of pitch canker on established plantation trees in the Western Cape province of South Africa were considered. The first aim of this study was therefore to collect isolates of F. circinatum from diseased P. radiata seedlings and plantation trees in the region. To determine the possible origin of these isolates, they were compared to representative isolates obtained from diseased seedlings elsewhere in the country, as well as the USA and Mexico by making use of vegetative compatibility tests and Amplified Fragment Length Polymorphism (AFLP) analysis. The final aim of this chapter was to evaluate the relative importance of the sexual cycle in the reproduction of the F. circinatum populations in the Western Cape. For this purpose PCR-based diagnoses of mating type and standard mating tests were employed to determine sexual compatibility and fertility. The results indicated that the outbreak in the Western Cape was due to one or more separate introductions of the disease into that area.
One of the main findings of Chapter 2 was that the mating type of the F. circinatum populations in the examined Western Cape plantations is skewed towards Mat-1. Although this may be due to the high levels of clonality observed within these populations, other factors related to the sexual process itself cannot be excluded. The research presented in Chapter 3 considered some of the factors potentially contributing to the biased inheritance of mating type in F. circinatum. The first aim was therefore to determine the mating types of F. circinatum originating from laboratory-induced sexual fruiting structures and to compare these data to what is expected under the Mendelian law of segregation. The second aim of this study was to determine whether mating type was linked to fitness attributes such as mycelial growth rate and pathogenicity. For this purpose, standard pathogenicity tests were conducted on P. patula seedlings and cultural growth studies were conducted. There was no association between pathogenicity and growth study, therefore, it is not known what causes this distortion in F. circinatum.