A constant concern in agriculture is the productivity and health of the crops. Diseases, droughts, and a growing human population are contributors to this concern. This paper focuses on the damage mycotoxins can cause to crops, both pre- and post-harvest, in light of climate change (Magan et al. 2010). Mycotoxins are the toxic secondary metabolites of certain fungi, with varying harmful effects. This paper concentrates especially on three mycotoxins: aflatoxin B, ochratoxin A, and deoxynivalenol (DON). Aflatoxin B is produced by Aspergilius flavus; ochratoxin A is produced by Aspergilius carbonarius; and DON is produced by Fusarium graminearum. Besides crops, mycotoxins can also be harmful to consumers. They are often carcinogenic and highly heat-tolerant which allows them to survive the processing after harvest. The European Union has the strictest laws setting limits on how much mycotoxins crops and their products can contain. In African countries, export crops are regulated; therefore, for residents, the risk to health is high. In Kenya, in 2004, there was an outbreak of aflatoxicosis with 125 deaths Lewis et al. (2005). Climate change is predicted to affect the relationship between mycotoxins and crops by stimulating growth of the mycotoxigenic fungi and changing the physiology of plants so that they are more susceptible to pests and pathogens. Due to climate change, CO2 concentrations are prognosticated to rise 1.5 µmol per year, global temperature is expected to increase 0.03°C per year, and water activity (aw) will decrease significantly in certain areas. Water activity is similar to water availability, however, aw has more to do with the chemical interaction between water and other solutes. The more solutes interact with water molecules, the less water there is available for hydration leading to values lower than 1.0 (pure water). —Daniella Barraza
Magan, N., Medina, A., Aldred, D., 2011. Possible climate-change effects on mycotoxin contamination of food crops pre- and post-harvest. Plant Pathology 60, 150–163.
Magan et al. reviewed the scientific literature to examine influence of climate change on pre-harvest and post-harvest mycotoxin contamination, the relationship between increased CO2, increased temperature, and decreased aw, and whether models based on molecular and ecological data can be used to reliably predict mycotoxin levels of risk. Under each topic, they include studies utilizing different models and systems for analysis.
For pre-harvest contamination, an historical occurrence the impact climate change might have. In the years 2003 and 2004, northern Italy experienced very hot and dry spells. A. flavus and Fusarium verticilloides are two competing fungi pathogens in maize grown in this area, but because of the arid conditions, the more xerotolerant A. flavus was able to gain an advantage. Maize contamination was high and, as a result, cow’s milk became heavily infected with aflatoxins since maize was used as animal feed. This contamination resulted in great economic losses. Other crops that are easily contaminated by aflatoxins because of dry conditions are peanuts and cottonseed. One way to determinate which crops will be at high risk of contamination because of dry conditions is through geostatistics. A three year study used goetstatistics to find the relationship between meteorological data and contamination, indicating that contamination was highest just before harvest. This method can be useful for understanding the spatial (latitude and longitude) and temporal variations in mycotoxin contamination and can be useful in determining the timing of fungicide application and pest control. An agricultural production systems simulator which relates seasonal temperature and soil moisture can also be useful in determining the aflatoxin risk index (ARI). For the mycotxin DON, the DONCAST takes into account weather conditions including number of rain days before anthesis and after ripening, and temperature variations. A systems approach can also be taken for DON prediction which takes a look at the life cycle of F. graminearum. However, these models are very simple and don’t demonstrate complex interactions. They have yet to include many important factors such as levels of CO2 and ongoing changes to the physiology of host species and mycotoxigenic fungi.
Post-harvest storage of crops and processing that occurs to turn them into marketable products can also incur risk of mycotoxin contamination. Crops stored at high temperatures and in damp conditions are at risk and any infection with aflatoxins or ochratoxin A can result in the loss of the entire crop. Fungistatic preservatives are used, but they are likely not completely effective, even applied at the most appropriate concentration. Higher temperatures and damp conditions increase the volatility of these fungistatic preservatives thus cutting their job short.
Atmospheric CO2 concentrations of 800–1000 ppm, triple the current levels, may not have a much effect on fungi. Although they can withstand extremely high levels, atmospheric CO2 will not be the only variable to change. In combination with temperature and aw , CO2 does have an effect. With higher aw, aflatoxin growth is restricted under atmospheres containing 25% and 50% CO2. Atmospheres with 75% CO2 resulted in greater growth restrictions despite changes in aw levels. Elevated CO2 mixed with N2 have a different effect on aflatoxin B. A 70% CO2 concentration with 0.80 aw prevented spoilage of bakery products. Focusing only on temperature and aw, contour maps have been used to predict mycotoxin production at +3°C and +5°C with varying aw. One discovery was that exactly 0.90 aw and temperature +5°C will cause no mycotoxin production.
To relate the genetics of mycotoxins and ecological data, a microarray was developed. A key marker gene under study is aflD which produces aflatoxin B. The microarray established that there are two peaks for mycotoxic production. The first is the exact intersection between temperature and aw which is most conducive to production, and the second peak is when the mycotoxic fungi is under stress and overreaching the demonstrated extremes. Changes in CO2, temperature, and aw can be modeled based on understood gene expressions.
There are still many questions left to answer especially since models need improvement and more studies need to be made. Future questions that need to be addressed should be more specifically determining the impact of climate change on the physiology of host species, on the life cycle of mycotoxigenic fungi, and how to improve storage for post-harvest crops.
Lewis, L., Onsongo, M., Njapau, H., Schurz-Rogers, H., Luber, G., et al., 2005. Aflatoxin Contamination of Commercial Maize Products during an Outbreak of Acute Aflatoxicosis in Eastern and Central Kenya. Environmental Health Perspective 113, 1763–1767.