Distribution and quantification of Fusarium verticillioides in South African maize and its effect on grain quality and toxicity

Abstract

Maize is the most important cereal crop produced in southern Africa. Worldwide, Fusarium verticillioides and F. proliferatum are the most commonly reported fumonisin producing fungi to infect maize. Both are important producers of fumonisins that are associated with animal mycotoxicoses. F. verlicillioides-infected maize has also been associated with human oesophageal cancer in South Africa, northern Italy and Iran, Increased incidence of liver cancer has also been reported from certain endemic areas of the People's Republic of China due to ingestion of F. verticillioides-infected maize. The carcinogenic risk that fumonisins pose to humans was evaluated by the World Health Organisation's - International Agency for Research on Cancer (WHO-IARC). They were classified as Group 2B carcinogens, which means that they are probably carcinogenic to humans. This potential threat highlights the necessity for screening human and animal foodstuffs for fumonisins. A primary concern in evaluating potential health risks associated with mycotoxin-contaminated foodstuffs is the reliability of fumonisin detection methods, A number of detection methods have been developed, but results are not consistent when compared. Substantial mycotoxin research has been carried out but high variation in mycotoxin results and species identification confounded statistical analysis of data. The aim of this study was to identify and address sources of variation in the quantification of species identification and fumonisin quantification. Chapter 1 provides a general overview of the importance of maize as a primary crop for subsistence, resource-poor and commercial farmers and the potential threat fumonisins pose to the safety of humans and animals. The objective of chapter 2 was to determine whether fumonisin levels in milled maize samples increase or decrease over time prior to testing for fumonisins, Grain samples from Sannieshof, Ventersdorp and Lichtenburg was evaluated for fumonisin levels every two months for a year. High variation in fumonisin levels quantified with the same samples was observed over time, indicating sources of variation which needs to be studied. Chapter 3 addressed sources of variation associated with sampling and improving sampling procedures to reduce this variation. Maize kernel samples were selected from five localities with high fumonisin levels. These samples were used for investigating the following four sources of variation, namely 1) subsample size (increasing from 25 g to 1000 g, 2) establish variation of fumonisins within a single maize sample, 3) number of replications (5 to 25) using prescribed 25 g subsamples and modified 250 g subsamples and 4) variation between laboratories and techniques for fumonisin detection. In chapter 4 the incidence of F. verticillioides, F. subglutinans, F. proliferaturn and fumonisins in maize from warmer production areas of South Africa was evaluated to determine 1) the incidence and geographic spread of Fusarium pathogens from maize grain silos in South Africa, 2) to study the relationship between isolation frequency, fumonisin incidence and ergosterol concentration and 3) to establish the relationship between fumonisin tevels and weather parameters. Chapter 5 aimed to 1) determine Fusarium spp, variation in maize samples, 2) compare the accuracy of Fusarium spp. identification on rose bengal-glycerine-urea (RbGU) as selective and identification medium with split plates containing standard Carnation Leaf (CLA) & Potato Dextrose Agars (PDA) as identification media and 3) detect and quantify potential bias among enumerators (inter-enumerator reliability) in direct microscopic identifications of F. verticillioides, F. proliferaturn and F. subglutinans. Reducing variation in quantification of fumonisins and improving Fusarium spp. identification on maize kernels, would increase confidence and accuracy in this technology. This will improve the value of research on this topic. Improved accuracy in detection and quantification of fumonisins will also contribute directly to the establishment of realistic and safe tolerance levels for this toxin in basic and processed foodstuffs. This in turn is important in ensuring food safety for humans and animals.