Browsing by Author "Shaikh, Ayesha"

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    Fusarium verticillioides infection and fumonisin production during maize kernel maturation
    (Stellenbosch : Stellenbosch University, 2021-03) Shaikh, Ayesha; Rose, Lindy J. ; Viljoen, Altus; Van Zyl, Karlien; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Maize (Zea mays. L), a cereal crop readily consumed by humans and animals globally, is subject to infection by fungal pathogens such as Fusarium verticillioides. This pathogen is found wherever maize is grown, causes Fusarium ear rot (FER) and is also capable of producing harmful secondary metabolites known as mycotoxins. Fumonisins are the most important class of mycotoxins. Fumonisin contaminated maize has been associated with irreversible, nocuous effects in humans and animals. For this reason, fumonisins are of great concern to farmers and researchers. More recently, attention has been given to hidden fumonisins. These are fumonisin molecules trapped within the maize kernel that are not detected during mycotoxin screening and are potentially liberated upon ingestion, thus contributing to a greater risk of exposure. Fusarium verticillioides can be managed using cultural practices and controlling maize- feeding insects. However, once conditions that favour the growth of the pathogen occur, these practices are no longer adequate to prevent fungal infection in the field. Not only is the use of chemicals not feasible, but there are also no registered chemicals available for the control of F. verticillioides in South Africa. Incorporating host resistance into locally adapted maize serves as the most environmentally friendly means of managing F. verticillioides. This requires a better understanding of factors that may contribute to disease development and progression. As the physical and biochemical composition of maize contribute to resistance to FER, understanding the relationship between structural and physico-chemical factors and fungal infection as well as fumonisin contamination would provide pivotal knowledge for breeding resistant maize cultivars. In this study, we utilised locally adapted maize inbred lines with known response to FER and/or fumonisin contamination to investigate the role that structural traits, such as husk coverage, silk length, silk detachment and silk browning, may have on FER and/or fumonisin and hidden fumonisin contamination. We also investigated physico-chemical properties of the maize kernel, such as pH, moisture, total carbon and nitrogen, fatty acids and starch in the form of amylopectin. Maize ears were inoculated at 7 days after pollination (dap) while another independent set of maize plants were inoculated 35 dap and grain was subsequently harvested at 7, 28, 42 and 52 days after inoculation (dai). Infection indicators (FER disease severity, F. verticillioides target DNA and fumonisin contamination) as well as hidden fumonisins were correlated with the physico-chemical properties. Fusarium verticillioides growth and fumonisins increased progressively over time after inoculation, reaching a maximum at 52 dai for both inoculation events with significant differences between inoculated and control maize grain. Inoculated grain of resistant lines accumulated lower levels of F. verticillioides target DNA and fumonisins when inoculated 7 dap, however, when inoculated 35 dap resistant lines showed an increase in fumonisin contamination. The susceptible line accumulated high levels of fungal DNA and fumonisins in both inoculation events. Of the physico-chemical properties evaluated, pairwise correlations revealed that carbon and nitrogen had strong significant association with fungal DNA at both inoculation events. Silk browning, carbon, nitrogen and C/N were also significantly correlated with infection indicators. Moisture content had a significant negative association with fungal DNA at both inoculation events. Amylopectin increased over time in Inoculation Event 1 and remained constant in Inoculation Event 2 while no significant associations were observed between amylopectin and infection indicators. The fatty acid profile showed a synchronised increase and decrease over time, however, no significant associations with infection indicators or hidden fumonisins were noted. Hidden fumonisins extracted using alkaline hydrolysis was significantly higher when only free hydrolysed fumonisins was extractedusing a standard fumonisin extraction. Furthermore, hidden fumonisins followed a similar trend as free fumonisins, increasing over time after inoculation and peaking at the mature stages of kernel development. Inoculated resistant maize grain accumulated significantly less hidden fumonisins. Lastly, there were no significant correlations between hidden fumonisins and any of the physico-chemical factors evaluated. This study demonstrated that F. verticillioides growth and fumonisin accumulation in maize grain is dependent on the timing of infection and is not triggered by a specific kernel developmental stage. Response of maize lines should also be assessed by artificially inoculating early stages of kernel maturation to determine accurate plant response in matured grain. Physico-chemical factors, such as carbon, nitrogen and C/N, may serve as indicators of potential resistance to F. verticillioides and/or fumonisins. The maize inbred lines used in this study can now be further classified according to their ability to accumulate both free fumonisins and hidden fumonisins.