Diversity of Fusarium Link and mite species associated with malformed Syzygium cordatum Hochst. Ex Krauss (water berry) inflorescences

Abstract

Syzygium cordatum is an evergreen indigenous, water-loving tree species in the Myrtaceae family in South Africa. It is usually found close to water sources such as near streams, on forest margins or in swampy spots and grows best in moist to wet soil. The tree has many functional uses; the fruits are a source of food, the bark is used in traditional medicines for treating stomachache, diarrhea, headaches, tuberculosis and diabetes among others. Recently, floral malformations have been observed among S. cordatum trees. The disease causes abnormal development of vegetative shoots and floral panicles resulting in phyllody and hypertrophy similar to disease symptoms in mango malformation which is associated with Fusarium and eriophyoid mite species. The genus Fusarium Link is one of the most important fungal genera with a worldwide distribution and contains at least 300 phylogenetically distinct species/species complexes. The species in this genus are broadly distributed in soil, root and plant tissues, and other organic substrates. This ascomycete genus is among the world’s most economically destructive plant pathogens, capable of causing diseases of almost all economically important plants. Some of the Fusarium species are also opportunistic human pathogens, causing infection of cornea, nails and other diseases. In addition, Fusarium fungi can produce a diversity of toxic secondary metabolites, such as trichothecenes, zearalenone, and fumonisins, posing significant threats to food safety and human health. Many of the plant pathogenic and mycotoxin producing fungi are within the Fusarium fujikuroi species complex (FFCS) which also includes several species that are linked to mango malformation disease and species that were only isolated from malformed S. cordatum inflorescences. Eriophyoid mites (Acari Eriophyoidea) are phytophagous arthropods which form intimate relationships with their host plants. They can cause malformation by causing galls and blisters. They may also cause malformations indirectly by transmitting plant pathogens such as viruses, phytoplasmas and fungi. Against this backdrop the overall aim of this thesis was to characterize Fusarium and mite species associated with malformed inflorescences in S. cordatum.

Chapter 1 of this thesis represent a comprehensive review of the current knowledge on the causes of floral malformation. It focuses on information regarding the biotic and abiotic factors, the developmental aspects of the abnormalities and specific examples of the type of malformation they cause. The review also emphasizes Fusarium and Fusarium-mite interactions as they are most commonly associated with malformations of many economically important plant species. Considering the importance of Fusarium species in floral malformation disease in different plants, the aim of Chapter 2 was to identify Fusarium species associated with malformation in S. cordatum inflorescences by making use of phylogenetic information of DNA sequences of the translation elongation factor 1 alpha (TEF1) gene. The research focus on the differences in Fusarium species diversity between malformed and healthy inflorescences. Results obtained in this study provides valuable information regarding the diversity of species within the Fusarium genus. In addition, it provides a blueprint for the possible causes of S. cordatum floral malformation. This Chapter has been published as: Mkandawire, R.I., Yilmaz, N., Steenkamp, E.T., Wingfield, M.J. and Fourie, G. (2022). Diversity of Fusarium species associated with healthy and malformed Syzygium cordatum inflorescences in South Africa. European Journal of Plant Pathology 162, 907–926 https://doi.org/10.1007/s10658-021-02447

Chapter 3 of this thesis describes a novel species residing within the Fusarium incarnatum equiseti species complex (FIESC). The chapter uses a polyphasic approach by employing a multi-locus phylogenetic, morphological and biological approaches to describe the novel species. It is the first time that a novel species in the Fusarium genus has been described from S. cordatum.

In chapter 4 the diversity of mites associated with floral malformation in S. cordatum is determined. The research compares the diversity of mites in healthy and malformed inflorescences. In addition, it uses the cytochrome c oxidase subunit 1 gene region to identify mite species and establish a DNA database that could assist in future identification of these species. For the first time, this study provides valuable phylogenetic information of mites associated with malformed inflorescences in S. cordatum