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.
