Good farming practices to prevent or minimise mycotoxin contamination
Grain production in South Africa has undergone major changes over the past 10 years. The major driving force behind these changes has been the deregulation of the grain industries and the demise of the various commodity control boards.
The single desk marketing system has been replaced by a free market system in which grain prices are determined by local supply and demand, international grain prices and the respective exchange rates. Grain trading on the Agricultural Futures Division (SAFEX) of the JSE is generally robust and creates a satisfactory price setting mechanism.
Moving from a production-focussed approach to a market-focussed approach has necessarily held major implications for the producer. One of the most important implications is the need to now produce what the market requires, in other words a safe, nutritious and high quality grain product. The South African grain producer is well aware of the fact that such a product could give him a competitive advantage, either on an individual or collective basis.
There must therefore be no doubt that the objective of every grain producer in South Africa is to produce a safe and healthy product, free of any mycotoxin.
Major diseases and disease control
However, in any extensive and large scale grain production system, certain diseases are bound to take on epidemic proportions under the right climatic conditions and in the presence of the virulent pathogen. This forms the basis of the concept known as the disease triangle:
As has already been indicated by the previous speakers, mycotoxins that are harmful to human and animal health are toxic metabolites produced by grain crop fungal diseases such as Fusarium spp., Aspergillus spp., Stennocarpella maydis, etc. A number of fungi producing mycotoxins occur in South Africa, but by far the two most important are the fungi producing fumonisins in maize and aflatoxin in groundnuts. Deoxynivalenol (DON) and the unidentified toxin causing diplodiosis are however also a concern.
Understanding the principle of the disease triangle is extremely important for the implementation of integrated disease management systems. Without one element of the disease triangle present, the disease cannot develop.
The role of the environment in the disease triangle includes aspects such as macroclimate, microclimate, soil types and properties, plant stress and physical damage. The role of the pathogen involves the organism’s life cycle and build-up in an area. The role of the host in the disease triangle interaction includes the host’s physiological growth stage, host stresses and host resistance. Each variable is equally important in disease development.
The various interactions of the three legs of the disease triangle result in considerable variations in disease incidence and severity which occur seasonally or geographically. This makes disease prediction extremely difficult and therefore also the implementation of control strategies.
There are basically four major disease control strategies available to farmers:
Host plant resistance
Breeding and selection of plant genotypes may result in hybrids and varieties with resistance to specific diseases. However resistance is relative and hybrids/varieties may vary in reaction to a disease between resistant and susceptible as extremes.
Chemical control
The application of fungicides or bactericides to plants for disease control is the basis of chemical control. However, chemicals are expensive and the economics and efficiency of disease spraying programmes must be considered.
Management practices
Tillage:
The effect of tillage practices on plant diseases is variable. Certain tillage practices retain stubble on the surface, which serves as a source of inoculum build-up of certain diseases, specifically also the Fusarium spp. and Aspergillus spp. fungi.
Crop rotation:
The aim with crop rotation is to allow a period for the disintegration of pathogenbearing crop stubble and depriving the pathogen of a susceptible host.
Planting dates:
Planting dates can be adjusted to avoid pathogen peaks or disease-favourable climatic conditions.
Stress management:
Specific stresses affect plant disease in different ways. Stresses may be relieved by adjusting agronomic practices.
Biological Control
The use of biological control agents to control diseases in extensive cropping systems is extremely limited.
Good farming practices
Grain producers are fully aware that to achieve optimum economic disease control, disruption of one or more of the disease triangle interactions is necessary and can optimally be achieved by integrated disease management.
Specifically in the case of the mycotoxin producing fungi, integrated disease management strategies are strongly recommended to and implemented by producers. The following are good farming practices that producers should follow:
1. Varieties
• Choose varieties with resistance and/or tolerance to the graininfecting fungi mentioned earlier. Considerable progress has been made by seed companies and the ARC-GCI in this regard. Consult the Maize Information Guide (MIG) of ARC-GCI for details.
• Use certified or good quality seed.
2. Chemical control
• Currently no feasible option exists to control the problem diseases chemically.
• Seed treatment is however recommended to ensure a strong and viable seedling and plant.
3. Management practices
• Implement and keep to a sensible crop rotation system.
• Where possible, plough stubble into the soil. Not always possible where conservation tillage practices are followed to combat wind erosion.
• Practice water conservation tillage to reduce the risk of drought stress.
• Practice mechanical and/or chemical weed control to minimise competition and drought stress.
• Control and prevent soil compaction by ripping, traffic control, etc. This practice is especially important on the sandier soils.
• Create an optimal seed and root bed for strong plant development.
• Analyse soils to determine their chemical status.
• Fertilise and lime soils according to soil analyses results and plant requirements. Ensure that the soil fertility is maintained or even improved.
• Plant at the optimal date to ensure good plant development and to avoid drought and high temperatures where possible.
• Plant at the optimal density and row width to avoid drought stress as far as possible.
• Practice Integrated Pest Management (IPM) strategies for especially stalk borer control, as well as for other harmful insects. Bt maize also offers potentially strong options in this regard.
• Optimise irrigation, where applicable. Avoid unnecessary irrigation during flowering and especially during the later stages of ripening.
4. Harvest practices
• Harvest at a relatively low moisture content (<14.0%) if possible. Especially in the South African context we have an advantage above especially the USA producers who generally have to artificially dry their maize.
• Ensure harvesters care in good working condition and free of old organic matter at the start of the harvesting season.
• Do not delay harvesting unduly as the mycotoxin content of grain already infected by Fusarium spp could increase significantly.
• Prevent mechanical damage to kernels as colonisation on broken or damaged kernels is considerably faster.
5. Post-harvest practices
• Sieve out broken and damaged kernels to achieve the required grade.
• Ensure transport containers and bins are clean, dry and free of insects and mouldy kernels.
• If drying is required, prevent overdrying and over-heating of the circulated air as this leads to cracked and damaged kernels.
Current situation
On the maize side mycotoxins, such as fumonisins and a number of others, are monitored annually by the South African Grain Laboratory (previously by the Maize Board). Samples are taken at grain silo’s across the whole production region and evaluated. Generally the levels of mycotoxin contamination of commercial maize fall within the acceptable threshold levels.
It is interesting to note that imported consignments of maize have over the years shown a trend of considerably higher mycotoxin contamination than locally produced maize, especially of aflatoxin which is not a problem in locally produced maize. The Department of Health would be well advised to drastically improve their inspection services at all harbour terminals to address this situation.
Obviously strict grading regulations (Agricultural Products Standards Act of 1990) discriminate heavily against fungus-infected kernels. In cases of only moderate infestation, maize, and other grain products, are not accepted for commercial purposes at all.
This obviously leads to huge losses for the producer. From there, his primary motivation for producing a safe and quality product.
Concerns from producer perspective
From a producer’s perspective there are a number of concerns that need to be addressed; viz.
• Grading, phytosanitary and sanitary regulations need to be applied consistently, irrespective of supply and demand situations and especially also for imported grain. Too often inconsistent application of legislation acts as a trade barrier or vice versa.
• The accuracy of mycotoxin level determination in grain samples in this country is suspect. Quick, reliable and inexpensive assays are necessary.
• Sampling problems occur and these lead to considerable variation, even within a single consignment.
• The toxin causing diplodiosis, affecting especially poultry and ruminants, has not been identified as it generally only occurs in southern Africa. More research on the hard issues concerning this mycotoxin are required.
• Decisions regarding mycotoxins must be taken responsibly and objectively. Too much emotion often clouds the scientific debate. For example allergens in groundnuts are probably a far more life-threatening phenomenon than aflatoxin in groundnuts.
• Much, if not most of the fungi infestation and mycotoxin production takes place post-harvest under storage conditions. The situation should be monitored at regular points further down the value chain, as well as just before the product reaches the end consumer.
Conclusion
- A major responsibility rests on the shoulders of all role players in industry to educate producers to proactively control mycotoxin-causing diseases. In this regard the work done by plant pathologists within especially the ARC needs to be recognised. They have made a significant and valuable contribution. This research needs to be supported.
- Producers will increasingly need to move towards applying so-called “Good Agricultural Practices”, as embodied in the EUREPGAP Protocol. To remain competitive nationally and internationally, producers may need to review management and recording practices. This could lead to lower levels of disease infestation and mycotoxin production.
- Nothing in life is without risk. Mycotoxins will from time to time be present in grain samples. The challenge is to react scientifically, responsibly and reasonably, and to keep levels below scientifically determined threshold levels.
- Ironically, so-called GM technology holds major potential advantages if specific disease resistance genes can be incorporated into grain and oilseed crops. This would immediately put a stop to the threat of mycotoxins.
Already it can be argued that Bt maize protects the maize against stalk borer infestation, and thus indirectly against Fusarium spp. infestation and consequently mycotoxins.
Acknowledgement
I would like to acknowlewdge Dr Brad Flett of ARC-GCI for valuable comments and literature on this topic.
Author: Dr John Purchase - Grain SA
(This paper was presented at the AFMA Mycotoxin Workshop on 29 October 2003)
 | The previous article is a special collaboration from AFMA South Africa |
