Occurrence and distribution of aspergillus flavus and aflatoxins in Asian peanuts

• Department of Agriculture, Thailand

Aflatoxins produced by the common fungi Aspergillus flavus and, to a lesser extent, A. parasiticus are a major problem as food contaminants in tropical Asian countries. They downgrade crop quality, prevent access to export markets, and can be fatal in both man and domestic animals. In humans, the poisons usually act insidiously to cause liver damage, and later liver cancer. In young domestic animals and poultry the effect is often acute, resulting in high mortality rates.

An earlier ACIAR project (8806) demonstrated that aflatoxin levels in Thai peanuts and maize can be more than 25 times higher than limits set by most western countries. Peanuts in particular are widely grown and eaten in Thailand, and have great export potential if the aflatoxin problem can be reduced.

This project is a logical extension of the earlier project. The Thai Department of Agriculture is urgently seeking collaborative assistance to understand and combat the aflatoxin problem. Currently, no regulatory or monitoring system is in place to guarantee peanuts or maize low in aflatoxins, and these commodities cannot be exported to western countries.
A similar situation exists in Indonesia.

Australia is in a strong position to initiate the necessary research. Scientists at CSIRO's Food Research Laboratory in Sydney have been studying the complex interaction between the peanut plant, the developing nut and A. flavus for more than 10 years. Glasshouse studies have been necessary because a variety of environmental factors, especially temperature and water stress, cannot be controlled under field conditions. A major finding has been that A. flavus can invade peanuts systemically. It appears possible to control the fungus with fungicide, or by competitively inhibiting it with a non-toxigenic strain of the same species.

Specific objectives are to:

. monitor A. flavus in peanut crops from the time of planting - through nut development, harvest and storage - to establish when it enters plants and nuts;
. assess the development of aflatoxins in nuts during development, harvest and storage;
. identify and determine the comparative distribution of A. flavus, A. parasiticus, and the related species A. nomius in Thai peanuts and soils;
. assess Australian techniques for reducing A. flavus invasion under Asian field conditions and, if such experiments are successful, plan field trials of one or more promising techniques; and
. carry out preliminary studies on A. flavus and A. parasiticus invasion in maize.

Most of the work will be carried out in Thailand. Collaborators from the Thai Department of Agriculture will study crops in three peanut growing areas - near Khon Kaen, Chiang Mai and Lopburi. Samples from each district will be examined for fungi at field stations in those cities, and aflatoxin assays will be performed at the central departmental laboratory in Bangkok. It is hoped that sampling can be arranged at least fortnightly throughout the growing seasons. Mycotoxin assays will be necessary only from near harvest time. Information on soil physical factors, soil moisture and crop rotation will also be obtained.

Data from the field studies will be sent to the Food Research Laboratory in Sydney for analysis. Here also, research on possible methods to reduce A. flavus invasion of peanut plants and developing nuts will be integrated with the existing research program for Australian peanuts. Studies will include the effectiveness of selected fungicides on small and more mature plants, and the development of a suitable non-toxigenic A. flavus strain to act as a competitive inhibitor of A. flavus. Preliminary studies on maize will also be done in Australia.

The present project, by itself, will provide little direct economic benefit, but the potential for benefit, contingent on the results of the project, is high both in economic and medical terms. The project will improve understanding of the causes and extent of the aflatoxin problem in Asian countries. It will also permit the planning of effective research, both in the glasshouse and in the field, to reduce levels of aflatoxins in peanuts and maize.

Staff in the Thai Department of Agriculture will benefit from the training they receive in research method, mycology and toxicological assay techniques. Australian scientists will benefit from obtaining valuable data to help unravel the complex interactions between A. flavus, A. parasiticus and the crops involved.