Quantitative detection of Fusarium spp. and its correlation with fumonisin content in maize from South African subsistence farmers

A quantitative detection tool was developed to enable the monitoring of fumonisin-producing fungi in food and feed commodities. To this end, a quantitative PCR (TaqMan) was developed that targets a conserved region in the polyketide synthase gene fum1, which is involved in the biosynthesis of fumonisin. Hence, this method specifically detected isolates from the fumonisin-producing species Fusarium verticillioides, F proliferatum, F nygamai and globosum whereas isolates of the fumonisin non-producing species F equiseti, F graminearum, E oxysporum, F semitectum and F. subglutinans that commonly occur on maize were not detected. Moreover, a few fumonisin non-producing E verticillioides isolates did not generate any fluorescent signals and were therefore not detected. The correlation between quantitative PCR and mycotoxin content was determined using field samples collected at homestead farms in South Africa. Among 40 samples from the Eastern Cape collected in 2005 a good correlation (R-2=0.8303) was found between pg fungal DNA and fumonisin content. A similar correlation (R-2=0.8658) was found among 126 samples collected from four provinces in South Africa in 2007. These observations indicate that samples containing >= 40 pg fungal DNA/mg sample are suspected of also exceeding the 1 mg/kg total fumonisin level and therefore do not comply with the European Commission limit for fumonisins B-1+B-2 for maize intended for direct human consumption that applies from 1 October 2007. Combined with the very high maize intake, our results indicate that fumonisin levels in maize from South African homesteads regularly exceed the tolerable daily intake for fumonisins.