A study of Chrysoporthe and Cryphonectria species on Myrtales in southern and eastern Africa

Abstract

Considerable changes have occurred in recent years, regarding the taxonomy and ecology of Eucalyptus fungal pathogens previously treated in the genera Cryphonectria and Endothia. Cryphonectria cubensis now resides in Chrysoporthe with two species, which are very distinct from Cryphonectria. The fungus previously known as E. gyrosa was moved to C. eucalypti and will soon be known as Holocryphia eucalypti. It is very likely that C. eucalypti and Chr. cubensis were introduced onto the African continent, but the hypothesis remains to be tested, while Chr. austroafricana seems native to the African continent. The aim of studies contained in this thesis was to consider the distribution, taxonomy and diversity of Chrysoporthe spp. and Cryphonectria eucalypti on the African continent. This was achieved through surveys in southern and eastern Africa, of both Eucalyptus spp. and native tree species belonging to the Myrtales. The intention was that the results of the studies in this thesis should aid in a better understanding of the taxonomy, origin, distribution, host range, as well as pathogenicity of various Cryphonectria and Chrysoporthe species in eastern and southern Africa. Various new hosts, new areas of occurrence and taxonomic changes have occurred for species of Cryphonectria sensu lato, previously known only on Eucalyptus spp. Chapter one of this thesis presented an overview of the most recent findings regarding the taxonomy, host range and distribution of C. cubensis sensu lato and C. eucalypti. The background to the description of a new genus, Chrysoporthe Gryzenhout&M.J. Wingf. and three new species namely; Chr. cubensis, Chr. austroafricana and Chrysopothella hodgesiana, previously considered to represent C. cubensis was also considered. Furthermore, the wide host range of Chrysoporthe spp. has been reviewed. The fungi are known on various genera in the order Myrtales in both tropical and subtropical areas, worldwide. Emphasis was placed on these Eucalyptus pathogens in Africa. Chrysoporthe cubensis and Chr. austroafricana, collectively known as Cryphonectria cubensis in the past, are important canker pathogens of Eucalyptus spp. worldwide. In chapter two of this thesis I have shown, for the first time, that Chr. cubensis occurs in Kenya, Malawi and Mozambique on non-native Eucalyptus spp. and Chr. austroafricana occurs in Mozambique, Malawi and Zambia on non-native Eucalyptus spp. and native S. cordatum. I was also able to show that Chr. austroafricana causes cankers at the base and higher up on stems of Eucalyptus trees in South Africa and Malawi, which is contrary to prior knowledge. Likewise, the sexual state of this fungus has been shown to be equally abundant as the asexual state in countries north of South Africa, contrary to the situation in southern Africa where the asexual state predominates. The known distribution range of Chr. austroafricana within South Africa was also expanded through this study. Chrysoporthe cubensis is an important fungal pathogen of Eucalyptus spp., worldwide. The fungus is also known on many other hosts all residing in the order Myrtales. Previous surveys conducted in eastern and southern Africa to assess the distribution of Chrysoporthe spp. in this region, revealed the occurrence of Chr. cubensis on Eucalyptus spp. in Kenya, Malawi and Mozambique. In chapter three of this thesis, the population structure of Chr. cubensis isolates from Eucalyptus spp. from Kenya, Malawi and Mozambique was considered for the first time. This represents a first attempt to consider the genetic structure of the fungus from eastern Africa. Results show that there is a very low genetic diversity within the populations of Chr. cubensis from Kenya, Malawi and Mozambique, implying that the fungus is probably newly introduced in these areas. Based on phylogenetic analyses, the origin of eastern African Chr. cubensis is most likely Asia. In chapter four of this thesis, polymorphic microsatellite DNA markers were developed from a single spore isolate of C. eucalypti collected from Eucalyptus stem canker in South Africa. Markers were obtained using the enrichment technique known as FIASCO (Fast Isolation by AFLPs of Sequences Containing Repeats). Ten polymorphic markers were isolated, of which 2 were discarded due to their high polymorphism in the flanking region. These markers will consequently provide useful tools for future investigations considering the population biology and especially the global spread of C. eucalypti. Cryphonectria eucalypti is a fungal pathogen considered opportunistic in South Africa, while in Australia it has been associated with sporadic but serious disease problems. Chapter five of this thesis presents results on the population structure of C. eucalypti from South Africa, eastern and western Australia. Nei's gene diversity (H) showed that the eastern Australian population is most genetically diverse and the western Australian populations from Corymbia and Eucalyptus somewhat less diverse. The South African population displayed the lowest genetic diversity. The high genetic diversity in the Australian populations supports the view that C. eucalypti is native to that region. This is consistent with the fact that Eucalyptus species are also native to the Australian continent. In chapter six of this thesis, I have shown that the fungus isolated from H. canescens, S. cordatum and T. granulosa in South Africa represents a new genus and species related to, but distinctly different from Chrysoporthe. Celoporthe dispersa gen. et sp. nov. is, therefore, described to accommodate this fungus. This description was supported by both morphological characteristics and DNA sequence data. These have clearly shown that isolates of C. dispersa form a clade distinct from Chrysoporthe, Holocryphia and other taxa, which it resembles morphologically. Pathogenicity tests showed that C. dispersa is not pathogenic to H. natalensis, but a potential pathogen of Eucalyptus and Tibouchina spp. The collection of studies included in this thesis demonstrated that Chrysoporthe spp. occur in Malawi, Mozambique, Zambia, Kenya and Tanzania on both Eucalyptus and native Syzygium cordatum trees. This significantly expands the geographical distribution of these important pathogens. The studies have also shown that Chrysoporthe cubensis has recently been introduced on the continent. It is my hope that new knowledge emerging from studies in this thesis will aid in quarantine measure to control the spread of these important fungal pathogens including the new species Celoporthe dispersa.