Modelling the incidence of fusarium and aspergillus toxin producing species in maize and sorghum in South Africa
Janse van Rensburg, Belinda
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Maize and sorghum are important crops produced in South Africa with 8 million- and 125 000 tonnes being produced respectively, annually. Ear and head rot fungi can negatively affect yield and grain quality of these crops and also produce mycotoxins which negatively impact on human and animal health. This study focussed on the occurrence of aflatoxin producing A. flavus and A. parasiticus and fumonisin producing F. verticillioides and F. proliferatum from commercial maize and sorghum grain. The natural occurrence of fumonisin producing Fusarium spp. and fumonisin contamination of maize was quantified in 29 maize production areas of South Africa over a three year period. Higher fungal biomass and fumonisin concentrations were associated with warmer production areas such as Northern Cape, North West and some areas of the Free State where the average temperatures ranged from 29°C to 32°C. In the cooler areas of Mpumalanga, KwaZulu-Natal, Gauteng and some areas of the eastern Free State, where mean maximum temperatures ranged from 24°C to 27°C, fungal biomass and fumonisin levels were absent or low. High fumonisin levels, in excess of 2 ppm recorded at 10 localities over the three year period, are of concern because of possible mycotoxicoses in animals and carcinogenic effects in humans. Corresponding high fungal biomass values may also indicate infection levels that may reduce yields and cause grain discoloration, physical breakdown of grain structure and reduction of grain nutritional value. Maize cultivars differed in susceptibility to colonisation by fumonisin producing Fusarium spp.. This necessitated a better understanding of the role of environment and the physiology of differential responses of cultivars in relation to infection by fumonisin producing Fusarium spp. at different localities in order to identify maize production areas with a potential high/low risk of fumonisin synthesis. The qRT-PCR method used in this study quantified the biomass of fumonisin producing Fusarium spp. (pathogenic and/or endophytic) in maize kernels more accurately than the plating out method and will replace the plating out method in future research. To determine the effect of aflatoxin producing Aspergillus spp. and fumonisin producing Fusarium spp. and their resultant mycotoxins on sorghum production in South Africa, sorghum grain samples were collected from five cultivars planted at 21 localities in South Africa from 2007-2009. HPLC and qPCR results indicate that Aspergillus spp. and Fusarium spp. and their mycotoxins do not pose a threat to sorghum production in South Africa. Data from Chapter 2, together with meteorological data, were used in the development of a provisional epidemiological model to predict the risk of maize kernel colonisation by Fusarium spp. and fumonisin contamination. Fusarium colonisation of grain and fumonisin levels were related to prevailing weather conditions during early post-flowering and grain development stages, respectively. Both colonisation and fumonisin production were significantly inversely correlated with mean maximum temperature and minimum relative humidity during the critical growth periods. Our models were consistent regarding time of fungal infection and fumonisin production in each respective season (2007-2009), although it did not give consistent prediction values over seasons, which indicates variation that is not accounted for by the selected two weather variables. This is an on-going study and the continuous incorporation of data into this model should improve predictive values over seasons. Since no cultivars/lines have been identified with resistance to Fusarium ear rot of maize in South Africa, the disease remains difficult to control. To date, no fungicides have been registered for the control of ear rots in South Africa and the potential of prophylactic fungicides, generally applied for the control of foliar diseases, to reduce Fusarium ear rot of maize and fumonisin synthesis was investigated. No significant differences between sprayed and control treatments on colonisation of grain by fumonisin producing Fusarium spp. or fumonisin contamination were recorded. This lack of efficacy may be attributed to the timing of fungicide applications relative to the plant growth stages critical to the infection of kernels by ear rot pathogens as determined from our epidemiological model. Further investigation into the possibility of using fungicides for the control of maize ear rots are being undertaken including time of application, application dosage and different fungicide regimes. Such applications must contribute to a reduction in maize leaf diseases as well as maize ear rots and their resultant mycotoxins. Only when fumonisin legislation and incentives are introduced into South Africa, will these fungicide spray programmes be economically justifiable. Robust, field-based models to predict fumonisin producing Fusarium ear rot in maize grain have been elusive due to the complexity of interactions between numerous abiotic and biotic disease factors. The findings in this study could contribute to an understanding of these complex interactions, thereby creating new management strategies to prevent or reduce the growth of F. verticillioides and F. proliferatum at field level as well as reducing contamination of grain with fumonisins.