Ceratocystis and Ophiostoma species infecting wounds on hardwood trees, with particular reference to South Africa

Abstract

This thesis concerns the study of selected Ceratocystis species and Ophiostoma species infecting wounds on broad-leaved trees, particularly those occurring in Africa. However, two chapters also deal with these fungi from Australia, Norway, Sweden and Austria. The dissertation is comprised of a literature review, followed by four research chapters, addressing the occurrence of Ceratocystis spp. and Ophiostoma spp. in Africa, Australia and Norway. The first chapter of the dissertation is a review of Ceratocystis spp. and Ophiostoma spp. with particular reference to Africa, hardwood tree species and wound infections. The review highlights the importance of wood and trees, especially on the African continent, and discusses the threat of deforestation. This is despite efforts by many African countries to establish forests of non-native tree species to address the demand for wood and wood products on the continent. Reforestation is associated with increased risks of the introduction of pests and pathogens, including species of Ceratocystis and Ophiostoma. The taxonomic history of these two fungal genera is summarized, and the review further focuses on the economically important species in these genera, particularly those infecting hardwood tree species on the African continent. The lack of information regarding Ceratocystis spp. and Ophiostoma spp. on hardwood trees in Africa is thus highlighted. Furthermore, the review summarised the dispersal mechanisms of these pathogens, highlighting dispersal too and infection of wounds. Ceratocystis pirilliformis was described in 2003 and it is the only species in the Ceratocystis fimbriata species complex that has pear-shaped ascomatal bases. This fungus was first described from Australia where Eucalyptus spp. are endemic. It was later reported from South Africa on Eucalyptus grandis trees. Chapter two of this dissertation attempts to address questions regarding the geographic distribution, impact and origin of C. pirilliformis in South Africa. This was in line with the fact that it has been suggested that the fungus is likely native to Australia. To address this question, surveys were conducted in many Eucalyptus planting areas in South Africa and the genetic diversity of the fungus in the country was investigated using microsatellite markers previously developed for C. fimbriata. C. pirilliformis was found in three Eucalyptus-growing areas of South Africa, which has considerably increased the known geographic range of the fungus in South Africa. The gene diversity as well as the genotypic diversity for the fungus was found to be very low in the country and the population is apparently clonal. Results thus support the view that C. pirilliformis was accidentally introduced into South Africa. In chapter three of this dissertation, O. quercus is reported for the first time from wounds on non-native Acacia mearnsii in Uganda. In addition a new Pesotum sp., P. australi prov. nom. is described from wounds on native A. mearnsii in Australia. This fungus resembles other Pesotum anamorphs of Ophiostoma in many ways, especially species of the O. piceae complex. However, it can be distinguished from these species by many morphological traits and also based on phylogenetic inference. The closest phylogenetic neighbor of P. australi prov. nom. is O. quercus. The fact that it was isolated from A. mearnsii in Australia indicates that it is probably a native fungus in that area. In chapter four, two Ceratocystis spp. and one Ophiostoma sp. are described as new to science, from wounds on native broad-leaved tree in South Africa. Three other Ophiostomaspp. are also reported in this study. Until recently, very little research has been done with regard to Ceratocystis spp. and Ophiostoma spp. occurring on native tree species in Africa. However, results presented in this chapter strongly suggest that these fungi are common on native trees in Africa and many other species, including potential pathogens await discovery. Chapter five of the dissertation reports, for the first time, Ophiostoma catonianum, O. pluriannulatum and O. quercus from native broad-leaved trees in Norway. It also reports O. catonianum for the first time from Austria and O. quercus for the first time from Sweden. In the past, very little research has been undertaken to explore the diversity of these fungi on hardwood trees in the Nordic countries or other parts of Europe, where most research has been focused on Ceratocystis spp. and Ophiostoma spp. associated with conifer-infesting bark beetles. This chapter represents a preliminary study with important discoveries. It indicates that these fungi are common on wounds on hardwood trees in Europe and emphasizes the importance of expanding these studies in the Nordic countries, to include more hosts and geographic areas. Such studies will almost certainly reveal more species and possibly new species of Ceratocystis and Ophiostoma.