Fungi associated with the die-back of Pterocarpus angolensis (kiaat) in South Africa

Abstract

Pterocarpus angolensis (kiaat) is one of 20 species of trees in the pantropical genus Pterocarpus. The species occurs in Southern Africa and has a broad distribution over several African countries, including Tanzania, Democratic Republic of Congo, Angola, Zambia, Mozambique, Namibia, Botswana, Zimbabwe and South Africa. The tree is commercially used in several traditional medicinal applications and is well-known as a source of timber for woodcarving and furniture. A single serious disease known as mukwa has been reported on P. angolensis, and the cause has been attributed to the generalist fungus Fusarium oxysporum. Reports of this disease date back to the late 1950s and appear localized in the Livingstone distinct, between Livingstone in Zambia and Bulawayo in Zimbabwe. Recent studies indicate that mukwa disease has begun to subside. There are more recent reports, however, of P. angolensis trees dying in South Africa, in the Mpumalanga Province. The aim of the research presented in this dissertation was to establish the nature of the disease affecting P. angolensis trees in the Mpumalanga Province in South Africa. Trees were sampled from several areas in the province, both from reported disease areas and from areas sustaining healthy trees. Field observations and fungal isolations provided a means to determine whether the disease was attributable to a biotic factor or the result of environmental factors affecting the tree. Isolations concentrated on obtaining species of the Botryosphaeriaceae and of the so-called ophiostomatoid fungi because these fungi are well-known pathogens of trees in the area, have been fairly well studied and the die-back symptoms suggested that they could be involved. The first chapter of this dissertation presents a review of the literature pertaining to P. angolensis, with a focus on mukwa disease. Aspects of the review introduce the genus Pterocarpus, the ecology of the environment wherein P. angolensis occurs and cover the biology of the species. Commercial aspects of the tree as well as the need for conservation are also addressed, providing the foundation for further studies in this dissertation. Many of the symptoms of diseased trees studied were indicative of infection by the Botryosphaeriaceae, a group of several genera of endophytic plant pathogens. These fungi were thus isolated from P. angolensis trees growing in different areas and were subsequently identified. Pathogenicity tests were used to evaluate the possible role that these species might play in the dieback of P. angolensis. Copyright