Stepwise Screening of Natural Substances for Control of Postharvest Aflatoxigenic Fungi in maize and Peanut Agroecosystems

Abstract

Studies in typical and new Argentinean peanut areas showed that toxigenic Aspergillus section Flavi strains are widely distributed in soil and seeds, with high probability of being transferred to the storage ecosystem. Mycological analyses of soil showed that Aspergillus section Flavi population were present in the two areas at similar counts (3.2x102 cfu g-1). Within this section, two fungal species were frequently isolated with isolation percentages of 73 and 90% for A. flavus and 27 and 9% for A. parasiticus in soil samples from traditional and new areas, respectively. The percentages of the different A. flavus phenotypes from both peanut-growing areas showed that L strains was recovered in the highest percentage and represents 59 and 88% of the isolates with variable ability to produce aflatoxins (AFs). Peanut kernels collected at harvest time from different localities of Córdoba and Formosa provinces showed A. flavus and A. parasiticus contamination. The 42.8 and 70% were classified as type L and the percentages of aflatoxigenic A. flavus strains were 68.6 and 80.0% in samples from traditional and recent peanut-growing areas, respectively. Highly toxigenic A. flavus S strains were isolated with major frequency from soil a kernel samples coming from traditional peanut-growing area. Aflatoxin contamination was detected in peanut kernels from typical peanut growing area. Harvested peanut were stored during 5 months in three storage systems (big bags, wagons of conditioning and drying and stockpiled warehouse) and mycological population succession was analyzed. Fungal isolation was greater from pod (95%) than from kernel tissues. The most common fungi identified included Penicillium, Aspergillus, Eurotium and Fusarium spp. Within Aspergillus genus, the section Flavi had the greatest mean counts of 1.4x104, 9.4x102, 5.2x102 cfu g-1 for big bags, wagon and warehouse, respectively. A. flavus and A. parasiticus strains with variable ability to produce AFs were isolated from peanut kernels stored in the three system at all sampling period in the order of 1.5x102 and 2.3x102 and 4.5 cfu g-1, respectively. .A. flavus S and L strains contributed to silo community toxigenicity during all storage period. Total AF levels ranging from 1.1 to 200.4 ng g-1 were registered in peanuts conditioned at the higher aW values (0.94?0.84 aW) and stored in big bags. Despite the water stress conditions registered in the stockpiled warehouse throughout the storage period, AFB1 levels ranging between 2.9 and 69.1 ng g-1 were registered from third sampling. Therefore, the interaction between biologic and abiotic factors and substrate may promote the Aspergillus contamination and the subsequent AF accumulation in peanut from sowing to storage, highlighting the need to promote good practices in order to avoid the risk of these metabolites contamination in peanut food chain.