Characterisation of Fusarium species associated with floral malformation of Syzygium cordatum

Abstract

Inflorescence malformation in Syzygium cordatum and Mangifera indica (mango) is characterised by abnormally enlarged and excessively branched panicles, which are unable to bear fruits. In mango, the malformation disease is caused by at least three Fusarium species in the economically important group of fungi that are collectively known as the Gibberella fujikuroi complex. Previously, a single Fusarium species that also forms a part of this complex has been reported from the malformed inflorescences of S. cordatum. The primary aim of this dissertation was to identify Fusarium species associated with the floral malformation on native S. cordatum in South Africa and agriculturally important M. indica in the Sultanate of Oman. In Chapter 1, a critical overview of the diversity, taxonomy and evolution of Fusarium species in the G. fujikuroi complex is presented. Currently, the complex includes at least 50 Fusarium species and phylogenetic lineages. Of these, 34 represent morphospecies and 10 are sexually fertile biological species that have been described formally. The taxonomic history of the complex and species concepts used to resolve taxonomic predicaments are discussed. From an evolutionary point of view, the species in the G. fujikuroi complex are grouped into three major clades, named after the geographic origins of their respective host plants. The biogeographic hypothesis appears to be true for most of the species, although each clade includes at least one species that does not fit this view. Apart from the aerial arrangement of microconidia, no other morphological character, host/substrate preference and/or mycotoxigenic property uniquely differentiates the members of the three clades from one another. Our understanding of the phylogeography and evolution of these fungi, therefore, will be dependent on the study of Fusarium species that were isolated from indigenous ecosystems. Floral malformations are important diseases of angiosperms that can cause plant sterility. Chapter 2 of this dissertation focuses on the diversity of insects, mites, nematodes, viruses, bacteria, fungi and oomycetes associated with these diseases. Specific examples of inflorescence abnormalities that these biotic groups induce are provided. As the malformations of both M. indica and S. cordatum have been associated with F. suglutinans sensu lato and mites in the genus Eriophyes, special emphasis is given to the Fusarium species and mites that are known to colonise the inflorescences of these plants. Future studies should elucidate the role of these Fusarium taxa in the development of the malformation disease.