Aflatoxins Bi and Gi are the most commonly produced forms in peanut. These toxins are involved in several human diseases, particularly liver cancer and growth defects in children. Aflatoxin interactions with Hepatitis B and C viruses result in relatively high levels of primary hepatocellular carcinoma. Aflatoxins also are toxic to livestock, including ruminants, poultry, birds and fish, when contaminated meal is used in their feed. Due to its human and livestock health implications, aflatoxin contamination has become a major issue in the international trade of peanuts and can directly impact the lives of poor farmers by reducing their income.
Infection of peanut by Aspergillus spp. can occur both pre- and postharvest. Preharvest infection by A. flavus and consequent aflatoxin contamination is important in crops grown under rain-fed conditions in the semi-arid tropics. End-of-season drought and damage to peanut pods by soil pests increases the preharvest aflatoxin levels. Mechanical damage during harvest and postharvest practices, e.g., heaping, increase toxin levels in warm, humid areas. Poor harvest and storage practices may lead to rapid development of the fungi and consequently to higher production of the toxin. Aflatoxin contamination occurs frequently in peanut seeds, with very high toxin levels found in immature and small seeds. Small pods remaining in haulms, damaged and immature seeds often are used as cattle feed. Milk from cattle fed such contaminated fodder contains high levels of aflatoxin Mi.
Field and greenhouse screening methods have been used to increase the efficacy of evaluation of aflatoxin resistance in peanuts. Sick plots with highly aggressive, toxigenic strains of A. flavus, and laboratory inoculation methods for selecting individual resistant seeds now enable the screening of large amounts of germplasm. As a result, sources of resistance to seed infection and aflatoxin production have now been identified and used to breed high-yielding lines with resistance to seed infection and aflatoxin contamination that have been registered and shared with national agricultural research systems (NARS) for further use in their programs, e.g., ICGV 88145, 89104, 91278, 91283 and 91284 in Asia and ICGV 87084, 87094 and 87110 in West Africa.
The estimated heritability for seed colonization ranged from 0.55 to 0.79, for seed infection from 0.27 to 0.87, and for aflatoxin production from 0.2 to 0.47. Thus, the levels of resistance in available sources and in the peanut breeding lines are not very high and do not suffice to effectively protect the crop from aflatoxin contamination under all conditions. Further, the diversity of these lines is very narrow. Hence, ICRISAT researchers have developed protocols for the transformation of peanut to produce transgenic plants with anti-fungal genes, e.g., chitinases, that may increase the resistance to A. flavus (K.K. Sharma, personal communication).
Other options for aflatoxin control in peanut include the use of isolates of Trichoderma and Pseudomonas, which provide biological control of Aspergillus in both field and greenhouse trials, and cultural practices that reduce aflatoxin contamination, e.g., the application of farmyard manure, lime, gypsum or cereal crop residues to the soil. Treatments including lime and farmyard manure can reduce aflatoxin contamination up to 90% in a highly susceptible cultivar such as "Fleur 11". Harvesting pods at the proper maturity, exclusion of damaged and immature pods, improved harvesting practices, the use of mechanical threshers, and proper seed storage bins are other cultural practices that help reduce aflatoxins in peanuts.
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