Genetic diversity of Chrysoporthe cubensis in eastern Africa.

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INTRODUCTION

Plantation forestry, particularly plantations of Eucalyptus spp., contributes significantly to the economy of many countries. It is currently estimated that there are approximately 14 million hectares of commercial Eucalyptus plantations in the world (Turnbull 2000). Furthermore, it is estimated that by the year 2010, approximately 20 million hectares of these trees will have been established globally, if the current planting trends are maintained (Evans 1982, Turnbull 2000). The success of Eucalyptus spp., particularly as non-native plantation species, has been due to their adaptation to a wide variety of environments, rapid growth, easy management as well as their valuable wood and pulp properties (Evans 1982, Sutton 1995, 1999, Turnbull 2000).

Morphology

Sexual structures of Chrysoporthe spp. are characterized by black, valsoid perithecia embedded in bark tissue with perithecial necks covered with amber tissue. (Hodges et al. 1979, Sharma et al. 1985, Conradie et al. 1990, Gryzenhout et al. 2004). Each ascus contains 8 spores that are fusiod to ellipsoid (Hodges et al. 1979, Gryzenhout et al. 2004). Ascospores are hyaline, two celled, fusoid to oval with rounded apices for Chr. austroafricana (Fig. 2) and tapered apices for Chr. cubensis (Hodges et al. 1979, Sharma et al. 1985, Gryzenhout et al. 2004).

Chr. austroafricana

Population diversity studies using VCGs on a South African population of Chr. austroafricana consisting of 100 revealed a very low genotypic diversity (0.095%) within the population (Van Heerden & Wingfield 2001). At that stage, the pathogen was considered to be identical to Chr. cubensis and had been known in South/Central America and Asia for many years, including on native S. aromaticum in Asia. Thus, the emerging hypothesis from VCG studies was that the fungus had been introduced into South Africa, either from Latin America or Asia (Van Heerden & Wingfield 2001). Furthermore, recombination within the South African population was not observed, confirming previous findings that sexual fruiting structures (perithecia) were absent on the cankers (Van Heerden & Wingfield 2001).

Breeding and selection

Canker of Eucalyptus spp. caused by Chrysoporthe spp. has been effectively managed by planting disease tolerant hybrids and clones of Eucalyptus spp. In South Africa and South America, tolerant clones have been identified using natural screening of trees and artificial inoculation trials (Wingfield et al. 1991, Wingfield & Kemp 1993, Van Heerden & Wingfield 2002). Today, in South Africa, the disease can be found only in seedling stands, or in trial plots (Wingfield & Roux 2000).

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Acknowledgements
Chapter One: Literature Review – Taxonomy, host range and geographic distribution of three Eucalyptus canker pathogens previously classified in the genus Cryphonectria
1.0 Introduction
2.0 Chrysoporthe spp
2.1 Taxonomy
2.2 Morphology
2.3 Distribution and host range
2.4 Origin and population diversity
2.4.1 Chr austroafricana
2.4.2 Chr cubensis
2.5 Management
2.5.1 Breeding and selection
2.5.2 Biological control
3.0 Cryphonectria eucalypti
3.1 Taxonomy
3.2 Morphology
3.3 Distribution and host range
3.4 Origin and population diversity
3.5 Management
4.0 Conclusions
5.0 References
Chapter Two: Distribution of Chrysoporthe canker pathogens on Eucalyptus and Syzygium species in eastern and southern Africa.
Abstract
Introduction
Materials & Methods
Collection of isolates
DNA sequence comparisons
Results
Collection of isolates
DNA sequence comparisons
Discussion
References
Chapter Three: Genetic diversity of Chrysoporthe cubensis in eastern Africa.
Abstract
Introduction
Materials & Methods
Fungal Isolates
DNA extraction
Simple Sequence Repeats (SSR) PCR
Genetic Diversity and differentiation
Genetic Distance
Results
Simple sequence repeats (SSR) PCR
Genetic diversity and differentiation
Genetic Distance
Discussion
References
Chapter Four: Development of polymorphic microsatellite markers for the fungal tree pathogen Cryphonectria eucalypti.
Abstract
Introduction
References
Chapter Five: Population structure of the fungal pathogen Holocryphia eucalypti in Australia and South Africa.
Abstract
Introduction
Materials & Methods
Fungal Isolates
Isolations
DNA extraction and SSR PCR
Genetic diversity and population differentiation
Genetic Distance
Results
Genetic Diversity
Genetic Differentiation and gene flow
Genetic Distance
Discussion
References
Chapter Six: Celoporthe dispersa gen et sp nov from native Myrtales in South Africa.
Abstract
Introduction
Materials & Methods
Isolates and specimens
DNA sequence comparisons
Morphology
Pathogenicity tests
Results
Isolates and specimens
DNA sequence comparisons
Morphology
Taxonomy
Pathogenicity tests
Discussion
References