Purpose

Primary prevention: Minimize fungal infestation and aflatoxin contamination

Cultivation of A. flavus resistant varieties

Control field infection by following appropriate phytosanitary measures Seed treatment and fungicide application

Appropriate scheduling of planting, harvest and post-harvest

Application of soil amendments, e.g., gypsum, farmyard manure, etc.

Lower moisture content of seed after harvest and during storage Add preservatives to prevent insect infestation and fungal contamination during storage Secondary prevention: Eliminate or limit fungal contamination

Limit fungal invasion and toxin production during crop growth Limit fungal inoculum in the field

Limit fungal invasion during crop growth

Avoid drought and other abiotic stresses

Increase soil nutrients (especially calcium) and water-holding capacity; promotes growth of antagonistic native soil microflora

Limit fungal invasion and growth during storage

Limit fungal invasion during storage

Sort contaminated pods and kernels

Redry groundnut pods and kernels Appropriate storage conditions Detoxification of contaminated products

Reduce aflatoxin contamination in final product

Limit fungal growth and toxin synthesis Limit fungal growth and toxin synthesis Chemical inactivation or binding of af-latoxins through the use of clay dietary supplements or ammoniation_

87110 were resistant to seed infection in Niger, Senegal and Burkina Faso (Upadhyaya et al., 2002). Despite considerable efforts at genetic enhancement, the level of resistance in them was not sufficient to protect the crop from aflatoxin contamination under all conditions and they exhibited high GxE interaction effects (Upadhyaya et al., 2004).

Breeding activities continue with the goal of developing high-yielding varieties adapted to different agroecosystems with enhanced levels of resistance to A. flavus infection (resistant to fungal invasion and proliferation, and resistant to drought, soil pests and diseases). Biotechnological approaches to increase host plant resistance through the use of anti-fungal and anti-mycotoxin genes also have begun. This approach received a major boost with the successful establishment of peanut regeneration and transformation protocols (Shar-ma and Anjaiah, 2000), and led to the transformation of popular peanut cv. JL24 with a rice chitinase gene to help prevent invasion by fungal pathogens. These transgenic events have now advanced to the T3 generation, with three events showing good resistance to A. flavus infection (< 10% infection) in in vitro seed inoculation tests (Sharma et al., 2006).

Use of biocontrol agents

Fluorescent pseudomonads and several strains of Trichoderma species inhabit the rhizosphere of many crop plants and have been identified as potentially promising biocontrol agents against A. flavus. Since the beginning of the 21st century, a large number of Trichoderma (> 250) and Pseudomonas (> 100) isolates have been obtained from peanut rhizosphere and evaluated for their antagonism towards A. flavus and their ability to reduce preharvest kernel infection of peanuts (Thakur and Waliyar, 2005; Anjaiah et al., 2006). Significant reduction of A. flavus populations and kernel infection occurred in both greenhouse and field experiments (Desai et al., 2000; Kumar et al., 2002; Thakur et al, 2003). Two Trichoderma isolates, Tv 47 and Tv 23, and two bacterial isolates P. cepacia (B 33) and P. fluorescens (Pf 2), were effective in reducing aflatoxin content in the kernels. Efforts also are being made to identify atoxigenic strains of A. flavus that can be used to alter the population dynamics of toxigenic strains of A. flavus in risk-prone zones (Cotty et al., Chapter 24). The effectiveness of the bio-control agents still needs to be established under African field conditions and simple, cheap and effective formulations developed for use in farmers' fields. Integration of these biocontrol agents with host plant resistance and agronomic management would provide an environmentally-friendly option for the management of aflatoxin contamination in peanuts.

Control through cultural practices

During the growing period several factors influence fungal colonization and aflatoxin production, including the soil type and condition, rate of evapotranspiration, availability of viable spores, end-season drought stress, damage to peanut pods by soil pests, and mechanical damage during harvesting (Mehan et al, 1991a, 1995; Nahdi, 1996; Waliyar et al., 2003a). It is impossible to control all of these factors, but some cultural practices can greatly reduce the amount of fungal infection. Some of these practices include: summer plowing, selecting planting dates to take advantage of periods of higher rainfall and avoiding end of the season drought effects, seed dressing with systemic fungicides or biocontrol agents, maintaining good plant density in the fields, soil amendment with gypsum and farmyard manure, removing prematurely dead plants, managing pests and diseases, timely harvesting, excluding damaged and immature pods, drying pods quickly, controlling storage pests, and only storing pods/seeds with < 10% moisture content. The use of mechanical threshers and seed storage bins also can reduce aflatoxins in peanuts. Although most of these practices are cost-effective and practical under subsistence farming conditions, they remain largely unadopted by subsistence farmers due to various socio-economic constraints including farmers' attention to other revenue generating activities and a lack of appropriate structures for drying and storage.

Control through soil amendments

In studies conducted at ICRISAT research stations in Sadoré (Niger) and Samanko (Mali) from 1999-2001, application of lime (0.5 t/ha), farm yard manure (10 t/ha) and cereal crop residue (5 t/ha) at the time of sowing, either singly or in combinations of lime and farmyard manure, helped reduce A. flavus seed infection and aflatoxin contamination in peanut by 50-90% (data not shown). Lime, a source of calcium, enhances cell wall thickness and pod filling and decreases fungal infection (Rosolem et al., 1997). Organic supplements, such as farmyard manure and crop residues, favor growth of native microbial antagonists and suppress soil- and seedborne infections (Karthikeyan, 1996). These three components also improve the water-holding capacity of the soil, minimizing the effect of end-of-the-season moisture stress, and thereby reduce the fungal colonization and aflatoxin accumulation in the peanut seeds. Lime and farmyard manure are cheap and easily available in most developing countries, including those in Sub-Saharan Africa.

Control through postharvest practices

Although the prevention of preharvest infection in peanuts is the best control strategy, afla-toxin also accumulates during harvesting and postharvest processing. We found in West Africa that aflatoxin accumulates, especially in susceptible cultivars, when pod removal from lifted plants is delayed. The toxin content in peanut kernels increased from 4 ng/g when tested immediately after lifting the plants to 6.8 ng/g two weeks after plant lifting in resistant cultivars 55-437 and J11. In the susceptible cultivars JL24 and Fleur 11, aflatoxin levels were 105 ng/g immediately after lifting and 270 ng/g two weeks after lifting. Farmers traditionally dry lifted plants in heaps, which increases humidity due to poor ventilation and favors fungal proliferation and toxin accumulation. To alleviate these problems, postharvest recommendations include removal of pods soon after lifting of plants. Replacing farmers' traditional practice of drying in heaps with a "batch" drying process (pods facing the sun for rapid drying and facilitating good aeration; cf., Fig. 2) dramatically reduces aflatoxin accumulation. For example, in experiments conducted in farmers' fields at Kayes village (Mali), the mean aflatoxin level was 14.3 ng/g (range 2-21.5 ng/g) for peanuts dried in batches, and 46 ng/g (range 12-60 ng/g) for peanuts dried by traditional heaping method.

On-farm and household storage conditions available to subsistence farmers are inadequate for the safe storage of produce, e.g., lack of clean storage bins and frequent pest infestations. These problems create conditions conducive to the accumulation of aflatoxin even in products that were uncontaminated when harvested. For example, toxin content increased in stored peanut kernels, cv. Fleur 11, from 84 ± 33 ng/g at harvest to 184 ± 54 ng/g and, 255 ± 78 ng/g, when tested after one and two months of on-farm storage, respectively. The development of adequate postharvest storage requires further study, including profiles of storage and practices in various regions, to identify methods that maintain proper temperature and humidity while providing protection from insects and other pests.

Creating awareness and technology transfer

The farming community and general public in developing nations are unaware of the hazards associated with aflatoxin contamination of food and feed and their implications for trade and for human and livestock health. Increased awareness will help increase the adoption of the technologies available to minimize mycotoxin contamination. Various information pathways are being used to increase awareness about the dangers of mycotoxins in food and feed. These include the distribution of flyers and brochures in national languages, e.g., Bambara in Mali and Hausa in Nigeria, a farmer-participatory approach to technology evaluation and dissemination, and conducting training courses and workshops. To strengthen the local capacity to monitor aflatoxin contamination, infrastructure for aflatoxin diagnostics has been established in several countries (Malawi, Mali, Niger and Senegal) and personnel have been trained to manage these facilities locally (Waliyar et al., Chapter 31).

Conclusions and perspectives

Aflatoxin contamination of peanut is widespread in developing countries of the world. Sub-Saharan Africa is particularly vulnerable to this problem due to persistent environmental con-

Figure 2. Postharvest methods for drying peanuts. (Left to right) Traditional heap drying and improved batch drying with pods exposed to the sun.

ditions that favor fungal colonization and proliferation in numerous crops and commodities. Efforts continue to find solutions for reducing the risk of aflatoxin contamination in staple foods, particularly among the resource poor of the world (Ortiz et al., Chapter 35). Our work in Sub-Saharan Africa suggests that a holistic approach combining host plant resistance, crop management practices, postharvest technologies, capacity development and public awareness, can effectively reduce the risk of aflatoxin contamination in peanut. Pre-sowing soil amendment with lime, timely harvesting and improved drying procedures alone can result in dramatic reductions in the levels of aflatoxin contamination. These approaches can be scaled-up in locations where aflatoxin contamination is a chronic problem.

There also is a need to increase genetic resistance of peanuts to A. flavus and to lower af-latoxin accumulation by using biotechnological tools when they are the most practical way to manage the aflatoxin problem. Biocontrol of aflatoxin contamination is a promising technology. However, before it can be implemented, a more thorough characterization of the diversity, abundance, and activities of the microbes in the peanut rhizosphere is needed to understand the mechanisms involved in host-pathogen-biocontrol agent interactions and to determine the suitability of these microbes for commercial production and for application under diverse agroe-cological conditions. Thorough studies also are needed to profile farmers' storage practices in various regions in developing countries, to determine the efficacy of different storage structures and practices that reduce aflatoxin contamination, and to identify storage technologies that minimize the risk of increasing aflatoxin contamination. Strengthening the local capacity to monitor aflatoxins is necessary for developing countries to regain the high-value export peanut trade. Such monitoring requires the availability of cheap, accurate and rapid-testing procedures, such as ELISA-based methods or other technologies that match location-specific needs with the socio-economic profiles of farmers in developing countries.

Establishing a prediction system to forecast the likely risk of aflatoxin contamination before or during the cropping season would facilitate implementation of appropriate management practices by the farmers. Development of these forecasting models requires quantitative data on the relationships amongst environmental and crop management factors, A. flavus infection and pre-harvest aflatoxin contamination. For broad-scale applications these models should be based on remote soil, water and temperature monitoring. The availability of such prediction models will help target improved aflatoxin management technologies on a regional scale.

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Responses

  • wilimar galbassi
    Can amendment of soil with manure and lime reduce aflatoxin?
    2 years ago

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