Biology, pathogenicity and diversity of fusarium oxysporum

Abstract

Fusarium oxysporum is a ubiquitous soil-borne fungus that includes pathogenic and non-pathogenic members. The pathogenic members are best known for causing Fusarium wilt diseases of many economically important agricultural crops. One such a crop is banana (Musa spp.), which is affected by a special form of the fungus known as F. oxysporum f.sp. cubense (Foc). Fusarium wilt was responsible for devastating losses of Gros Michel export bananas in Central America during the first half of the 20th century, and is now, once again, threatening world banana production that is primarily based on the sweet Cavendish varieties, both in the tropics and subtropics. To effectively manage Fusarium wilt, adequate knowledge of the pathogen, plant and environment is required. With this thesis I hope to contribute to the current knowledge available on the pathogen. Previous studies investigated the phenotypic and genotypic diversity, the spread and distribution, and the phylogeny of Foc. Some aspects related to the biology, physiology, diversity and pathogenicity of Foc, however, appeared to be unresolved. These aspects are important in order to develop a sustainable management strategy for Fusarium wilt to ensure continued banana production. Chapter 1 depicts a general review on F. oxysporum as the causal agent of Fusarium wilt of various fundamental crops, and gives a broad overview of the biology, taxonomy, physiology and pathogenicity of the pathogen. Through the application of modern molecular genetic techniques, a lot of progress has been made in the identification of genes and processes involved in the biology and pathogenesis of Fusarium wilt pathogens. The review concludes that some work, however, still needs to be conducted before topics such as race designation and pathogenesis in Foc are fully understood. Temperature, pH and nutrition are all factors contributing to the pathogenesis of F. oxysporum. The different factors can either favour or suppress the pathogen, or they can have a stimulating or inhibiting effect on the host plant. In Chapter 2 the pathogenicity and phenotypic characteristics of a genotypically uniform population of Foc was investigated. Physiological studies included determining the minimum, maximum and optimum temperatures and pH at which Foc grows in vitro, and what nitrogen sources stimulate and inhibit growth of Foc. Knowledge on these aspects could contribute to the management of the pathogen in the field. Differentiation among species of Fusarium can be problematic. To resolve questions related to the nomenclature in Fusarium, our research focus has shifted to the use of molecular tools for identification and determination of evolutionary relationships among and within species. In the past, phylogenetic studies on Foc were conducted using molecular tools such as sequencing, Restriction Fragment Length Polymorphisms, Random Amplified Polymorhic DNA and DNA Amplification Fingerprinting, with varying amounts of success. In Chapter 3 the usefulness of Amplified Fragment Length Polymorhism (AFLP) analysis to study diversity inFoc isolates was investigated. Of the 21 vegetative compatibility groups (VCGs) of Foc identified around the world, only VCG 0120 is found in South Africa. Chapter 4 aimed to identify an AFLP polymorphic DNA fragment unique to VCG 0120, and to develop a molecular marker of this fragment. Such a marker would be extremely valuable to distinguish between VCG 0120 and other isolates of F. oxysporum in terms of identification and confirmation of Fusarium wilt of banana in South Africa. Several pathogenicity-related genes have been identified in F. oxysporum. In Chapter 5, the presence of three pathogenicity-related genes (fmk1, pg1 and xyl3) in F. oxysporum isolates pathogenic and non-pathogenic to banana were verified by means of PCR amplification. The value of pathogenicity genes such as fmk1 and pg1 in comparative phylogenetic analysis was further substantiated.