Botryosphaeriaceae associated with native Acacia species in southern Africa with special reference to A. mellifera

Abstract

Tree species belonging to the genus Acacia have a significant impact ecologically and economically in southern Africa. Together with the African baobab, these trees are recognized as icons of the African landscape. They are widely distributed in this area and extensively used by local communities as sources of energy, stock feed, medical remedies and building material. There is still a substantial lack of knowledge regarding the ecological association between these plants and other living organisms such as fungi. This is, however, not new to the African continent where fungi are generally poorly studied and collected, and it is envisaged that many new fungal species will be discovered as scientists focus their efforts more on this geographical niche. An example of the lack of knowledge on the fungal biodiversity in Africa is reflected in the limited reports of members of the Botryosphaeriaceae, described to date from Acacia spp.. A review on phytopathogens in South Africa by Crous et al. (2000) indicated no records of the Botryosphaeriaceae associated with native Acacia spp. Despite the importance of many species within the Botryosphaeriaceae as pathogens, knowledge about the true diversity and taxonomy of species in this family is limited, especially where native plant communities are concerned. This dissertation attempts to contribute to knowledge on the associations between members of the Botryosphaeriaceae and indigenous Acacia trees in southern Africa, and the possible role they may play in diseases of these trees. Chapter 1 represents a literature review that focuses on fungi previously associated with Acacia spp. on the African continent. Information provided in this chapter refers to available reports on pathogens and saprophytes occurring on Acacia spp. that are both native and nonnative to Africa. Special reference is made to those occurring in southern Africa. Due to the concern of the introduction of new pathogens in areas where native and non-native plants are co-existing, emphises is also placed on the possibility of pathogen-host jumps between native and non-native Acacia spp. The potential threat they might pose to the future biosecurity of these important trees is discussed. Acacia mellifera, also known as the blackthorn, is one of the native African Acacia spp. that has been extensively studied. This tree is threatened by a serious die-back disease with symptoms similar to the die-back typically caused by members of the Botryosphaeriaceae. In an effort to understand the association of the Botryosphaeriaceae with native Acacia spp. in southern Africa, a study was undertaken to search for the presence of these fungi on especially A. mellifera in Namibia and the Pretoria area in South Africa. Other Acacia spp. were also sampled in cases where they were present in the same areas as A. mellifera. These results are presented in chapter 2. In a previous study, the fungal diversity of native trees and plant species in the Northern Cape Province of South Africa was studied. This resulted in the isolation of a number of fungi that resembled the morphological characteristics of the Botryosphaeriaceae. In chapter 3, these fungi were further identified to species level based on morphological and phylogenetic characteristics. In chapter 4 an attempt was made to compare the Botryosphaeriaceae that are associated with important native trees with those occurring on non-native trees. To accomplish this, a pilot study was done to investigate the presence of the Botryosphaeriaceae on A. mearnsii in the Gauteng Province of South Africa. Results from chapters 2 and 3 on native Acacia spp. from Namibia and South Africa served as the bases of comparison for this chapter. Results of previous studies that investigated diseases of plantation grown A. mearnsii were also included for comparison. Lastly, a summary is included to review the results of this study and also the significance and impact these results made, not only on the taxonomy of the Botryosphaeriaceae, but also understanding the fungal biodiversity of indigenous tree species in southern Africa. To date, this is the most extensive study of the Botryosphaeriaceae associated with native African Acacia spp. and it is also the first study that resulted in the describtion of so many new species in this group of fungi from a single host. Results from this study indicated that there is a significantly greater diversity in the Botryosphaeriaceae associated with native Acacia spp. in southern Africa than was previously thought. This dissertation attempts to form the basis for future studies to finally understand the interactions between the Botryosphaeriaceae and their native hosts as well as their role and threat as pathogens to indigenous and economically important plants. Copyright