REVIEW ARTICLE

 

Mycotoxigenic Fusarium species associated with grain crops in South Africa - A review

 

 

Ilze Beukes^I; Lindy J. Rose^I; Gordon S. Shephard^II; Bradley C. Flett^III; Altus Viljoen^I

^IDepartment of Plant Pathology, Stellenbosch University, Stellenbosch, South Africa
^IIInstitute of Biomedical and Microbial Biotechnology, Cape Peninsula University of Technology, Cape Town, South Africa
^IIIGrain Crops Institute, Agricultural Research Council, Potchefstroom, South Africa

Correspondence

 

 

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ABSTRACT

Cereal grains include some of the most important crops grown in South Africa and play a major role in the local economy. Maize, wheat and sorghum are extensively consumed by humans and farm animals, and are also utilised in industrial processes. Grain crops that are grown commercially contribute up to 33% of the country's total gross agricultural production, whereas subsistence farmers grow grains mainly to sustain their families. In rural communities an average intake of maize grain of more than 300 g dry weight per person per day is not uncommon. The production of grains is often constrained by pests and diseases that may reduce their yields and quality. In South Africa, 33 mycotoxin-producing Fusarium species have been associated with grain crops. Mycotoxins, such as fumonisins and deoxynivalenol, have been found in levels exceeding the maximum levels imposed by the US Food and Drug Administration and the European Union and therefore pose a serious public health concern. We provide an extensive overview of mycotoxigenic Fusarium species associated with grain crops in South Africa, with particular reference to maize, wheat and sorghum.
SIGNIFICANCE:
• Mycotoxigenic Fusarium species negatively affect the most important staple food crops grown in South Africa.
• Mycotoxin contamination has a direct impact on food safety and security.
• The genus Fusarium includes some of the most important mycotoxin-producing species.

Keywords: cereals; food safety; fungal contamination; mycotoxins; phytopathology

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Introduction

Grain crops grown in South Africa contribute between 25% and 33% of South Africa's total gross agricultural production.^1,2 The most commonly cultivated grain crops include maize (Zea mays L.), wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), sorghum (Sorghum bicolor L.), oats {Avena sativa L.), millet (Pennisetum glaucum L.) and rye (Secale cereal L.). Of these, maize is considered the most important and wheat the second most important.^2,3 The grains are utilised for food and livestock feed and, to a lesser extent, for malting purposes and bioethanol production.^1,2 Grains constitute the major portion of the total calorie intake of South Africans, across all age groups. The average consumption of maize by people older than 10 years varies from 762 g to 848 g cooked weight per person per day.⁴

The production of grain crops in South Africa is constrained by various abiotic and biotic stresses. Drought, nutrient deficiency, insect damage and diseases all cause a reduction in yield and grain quality.^5,6 One of the more important biotic stresses affecting maize, wheat and sorghum grain in the country involves the fungal genus Fusarium. The Fusarium sp. most commonly associated with these three grain crops is F. graminearum sensu lato (s.l.) Schwabe, also referred to as the Fusarium graminearum species complex.⁷¹⁰ Other Fusarium species affecting maize grain in South Africa include F. verticillioides (Sacc.) Nirenberg (syn. F. moniliforme Sheldon) and F. subglutinans (Wollenweber & Reinking) Nelson, Toussoun & Marasas, with F. proliferatum (Matsushima) Nirenberg occurring less frequently.^8,11,12Fusarium verticillioides is also associated with grain mould of sorghum in South Africa.^9,13 Additional Fusarium species associated with sorghum include F. thapsinum Klittich, Leslie, Nelson & Marasas; F. andiyazi Marasas, Rheeder, Lamprecht, Zeller & Leslie; F. nygamai Burgess & Trimboli; and F. pseudonygamai O'Donnell & Nirenberg.¹³ Fusarium head blight of wheat is associated with several species including F. culmorum (W.G. Smith) Sacc., F. cerealis (Cooke) Sacc. (syn. F. crookwellense Burgess, Nelson & Toussoun) and F. avenaceum (Fries) Saccardo.¹⁰

Infection of grain by Fusarium spp. does not only result in reduced yield and grain quality, but could lead to food safety concerns. Most Fusarium species produce one or more toxic secondary metabolites, commonly known as mycotoxins, in the grain.¹⁴F. graminearum s.l. produces type B trichothecenes (TCT-B) such as deoxynivalenol (DON) and nivalenol (NIV). Another important group of mycotoxins, the fumonisins (FUM), are produced by several Fusarium species (Table 1). Both F. verticillioides and F. proliferatum have been associated with the production of fumonisins in maize and sorghum grains in the country.^8,12,13

The discovery of fumonisins in South African maize grain by Bezuidenhout et al.¹⁵ sparked a significant interest in Fusarium-associated mycotoxins in the country and also worldwide. The objective of the current review is to give an overview of the information available on mycotoxigenic Fusarium species associated with grain crops in South Africa. We furthermore provide an outline on the production of the three most important grain food crops in South Africa: maize, wheat and sorghum.

 

Grain crops in South Africa

Maize

Maize forms the main staple food for the majority of South Africans, and constitutes a major component of animal feed. In 2014/2015, approximately 56% of the total area under maize cultivation (2 656 500 ha) comprised white maize, mainly used for human consumption, and 44% yellow maize, mostly used for animal feed.² The maize industry, therefore, is an important contributor to the economy of South Africa, both as an employer and generator of income.¹ In addition to its use as food and feed, maize is utilised in the manufacturing of paper, paint, textiles, adhesives, biodiesel, medicine and food.

Advances in maize cultivation practices - such as improved cultivars, effective crop rotation and enhancements in fertilisation and pesticide programmes - have steadily improved the yield per hectare. Whereas the total area harvested in South Africa has decreased from 4 118 000 ha in 1960 to 2 656 500 ha in 2014, yield has increased by 8 225 000 metric tons (Figure 1a).^2,16 The increase in production has ensured that the importation of maize has been minimised, and any surplus can be exported (Figure 1a), thus contributing towards generating foreign currency. The Free State (43%), North West (20%) and Mpumalanga (19.5%) Provinces of South Africa were the main production areas during the 2013/2014 production season for total white and yellow maize harvested.² Maize in South Africa is cultivated during the summer months with ideal planting times in November and December.

 

 

Maize production systems in South Africa can vary from resource-poor subsistence farming to small-scale and intensive commercial farming.^12,17 Chambers and Ghildyal¹⁸ defined a resource-poor farm family as 'one whose resources of land, water, labour and capital do not permit a decent and secure family livelihood'. The Merriam-Webster online dictionary¹⁹ defines subsistence farming as: 'Farming or a system of farming that provides all or almost all the goods required by the farm family usually without any significant surplus for sale.' The average yield per hectare recorded from 2008 to 2012 for non-commercial farmers was a meagre 1.3 tons/ha, while commercial farmers produced an average of 4.6 tons/ha.²

Diseases caused by fungal pathogens - aggravated by the use of inferior seed, monoculture and retaining crop residues - lead to reduced yields and lower grain quality.¹² A survey by Ncube et al.¹² during two production seasons determined that F. verticillioides was the most common Fusarium species associated with maize grain produced in a subsistence farming system, followed by F. subglutinans and F. proliferatum. Maize grain infected with these species was also contaminated with FUM, often at levels much higher than the maximum levels set by the US Food and Drug Administration (FDA) and the European Union (EU). A maize crop quality survey of commercially produced maize is performed annually by the Southern African Grain Laboratory (SAGL) with the financial support of The Maize Trust. Despite the general good quality of commercial maize, high levels of some mycotoxins can be found when weather or other conditions are favourable for fungal infection. Subsequently, mycotoxin contamination levels, in excess of the maximum levels allowed by the EU for maize intended for direct human consumption, have been found in commercially produced maize.^8,17,20 A 2-year survey of two susceptible maize cultivars, collected at 14 localities across South Africa, found a maximum total FUM level of 16 717 µg/kg, with an average of 2542 µg/kg and DON levels as high as 4731 µg/kg (average of 1031 µg/kg). Beauvericin (BEA) was recorded at a maximum level of 1507 ^g/kg (average of 506 µg/kg) and moniliformin (MON) at a maximum of 1530 µg/kg and an average of 604 ^g/kg.⁸ Zearalenone (ZEA) has also been sporadically detected in South African maize. During the 2011/2012 season, only two samples analysed by SAGL tested positive. However, with an average of 249 µg/kg, they exceeded the maximum level of 100 µg/kg allowed by the EU.²⁰ The occurrence of these mycotoxins could be attributed to the presence of F. verticillioides, F. graminearum s.l. and F. subglutinans. The presence of high mycotoxin levels in commercial maize could possibly be attributed to the fact that commercial farmers still consider yield, not disease resistance, the number one criteria when deciding on a hybrid to plant. Fusarium spp. do not only cause ear rot, but can furthermore cause root, crown and stalk rot of maize, thereby causing additional yield losses.²¹

Wheat

Wheat is the second most important grain crop produced in South Africa and is also regarded as an important staple food. It serves as the second main provider of energy in the national diet after maize meal, even though more money is spent on bread annually (ZAR6.7 billion in 2000) than on maize food products (ZAR6.2 billion in 2000).²² The majority of wheat cultivated in South Africa is bread wheat, with minor quantities of durum wheat produced for the production of pasta. Wheat is primarily used for human consumption (bread, biscuits, breakfast cereals, rusks), while the balance is used as seed for re-planting. Poorer quality wheat is marketed as animal feed and other non-food industrial uses such as the production of alcohol for ethanol, absorbing agents for disposable diapers, adhesives and starch on coatings.²³ Approximately 3900 commercial wheat farmers provide job opportunities to almost 28 000 people.²⁴

Wheat production in South Africa can be divided into two different cultivation systems, each with their own adapted wheat varieties. In summer-rainfall areas, wheat is mostly cultivated under irrigation, and planted between mid-May to the end of July (Northern Cape, Free State, KwaZulu-Natal). In the Western Cape, a winter-rainfall area, wheat is mostly planted under dryland conditions between mid-April and mid-June. About 600 mm water per year is required for wheat cultivation and, in dry areas where zero tillage or minimum tillage are practised, stubble mulching is recommended for moisture conservation.²³

The main wheat production areas in South Africa during the 2013/2014 season were the Western Cape (50%), Northern Cape (16%) and Free State (15%). Wheat is predominantly produced by commercial farmers with negligible amounts produced by small-scale and subsistence farmers, mainly because of high input costs and low yields, which results in smaller profit margins. Although the yield per hectare of wheat has shown a steady increase over the past 10 years (2.02 tons/ha in 2004 to 3.73 tons/ha in 2014)², the area harvested has decreased at a higher rate than the increase in yield could support, resulting in an overall reduction in production (Figure 1b). Lower production has led to an increase in the importation of wheat into South Africa to accommodate the drastic increase in domestic consumption (Figure 1b). Production of wheat in South Africa is constrained by several factors. Input costs have increased because of substantial increases in the cost of fertilisers and fuel, competitive international wheat prices and poor climatic conditions, amongst others.²³ Fertiliser costs in the Swartland wheat-producing area of the Western Cape can be as much as 30% of the total input cost and weeds may limit grain yields by approximately 20% annually.

Wheat is susceptible to a range of insect pests and diseases caused by plant pathogenic viruses, bacteria and fungi.^5-7,10 Several Fusarium species are associated with root rot, crown rot and head blight of wheat in South Africa, including F. avenaceum, F. brachygibbosum Padwick, F. cerealis, F. chlamydosporum Wollenweber & Reinking, F. culmorum, F. graminearum s.l., F. incarnatum-equiseti (syn. F. equiseti (Corda) Saccardo), F. lunulosporum Gerlach, F. oxysporum Schlechtendahl emend. Snyder & Hansen, F. poae (Peck) Wollenweber, F. pseudograminearum Aoki & O'Donnell, F. solani (Martius) Appel & Wollenweber emend. Snyder & Hansen and F. tricinctum (Corda) Saccardo.^7,10,25 The presence of some of these species may result in the contamination of the infected grain with mycotoxins such as DON, ZEA, BEA and MON.^14,26,27

Sorghum

Sorghum is the fourth most important grain crop produced in South Africa after maize, wheat and barley, and the third most important food grain crop.^2,28 Barley is mostly used for malting purposes in the production of beer, and is not considered a major food crop in South Africa. Sorghum is indigenous to Africa and is considered a staple food in many rural communities in South Africa. Approximately 90% of commercially grown grain sorghum is used for human consumption in the form of beverages and food (e.g. malt and sorghum meal), while the remainder is used as animal feed.^28,29 Industrial uses of sorghum include wallboards, biodegradable packaging materials and the production of ethanol. The brewing industry is the main consumer of sorghum, and about 55% of the total domestic produce is used for the manufacturing of traditional African sorghum beer. Sorghum flour competes directly with maize meal as a breakfast cereal or as soured porridge, known as 'mabele'.²⁸ However, mabele has been found to have better nutritional value (9.7% protein, 1.6% fat) when compared to super maize meal (7.4% protein, 1.0% fat). In South Africa, sorghum cultivars are divided into three classes: Class GM includes sweet sorghum with a low tannin content, which is especially suitable for malting and milling purposes; Class GL includes sweet sorghum with a low tannin content, which is especially suitable for milling and animal feed purposes; and Class GH includes bitter sorghum with a high tannin content (bird resistant), which is used for industrial malting.²⁸

The area under sorghum cultivation and the total production of sorghum in South Africa has been on a decline since 1986 (Figure 1c).^2,16 While maize and wheat increased in yield per hectare (Figure 1a,b), the same was not observed for sorghum (Figure 1c), for which the yield per hectare has remained mostly unchanged since 1995, fluctuating only with climatic changes.² This observation could be explained by the limited amount of research and development funds available to the sorghum industry, which is relatively small when compared to other major grain crops. A total of only 903 000 tons sorghum was produced in the 5 years from 2009 to 2014, in comparison to 8.9 million tons of wheat and 61 million tons of maize produced during the same period.²

Sorghum is planted from mid-October to mid-December in South Africa.²⁹ The Limpopo Province is the main sorghum-producing province, with limited production in other provinces such as Mpumalanga, North West, Northern Cape, Eastern Cape, KwaZulu-Natal and the Free State.³⁰ As with maize, sorghum farming systems vary from subsistence to intensive commercial farming, depending on farm sizes, production and marketing methods.³⁰ Subsistence farmers consume most of their products without measuring the area under production and yield. The average sorghum yield on smallholder farms is estimated to be 0.8 tons/ ha³⁰, significantly lower than the 2.4 tons/ha produced on commercial farms². The lower yield per hectare for subsistence farmers can be attributed to insufficient fertiliser and pest control programmes as well as soil cultivation and crop rotation practices, amongst others. These factors furthermore favour disease development by fungal pathogens, thus increasing the possibility of mycotoxin contamination.

Fusarium grain mould is a very important biological constraint to sorghum production worldwide, while Fusarium stalk and root rot may result in lodging, causing decreased yields.^13,31-33 Several mycotoxin-producing Fusarium species have been isolated from sorghum grain in South Africa. F. andiyazi, F. nygamai, F. thapsinum and F. verticillioides¹³ are known FUM producers, while species within F. graminearum s.l. are TCT-B and ZEA producers⁹.

The high consumption levels of up to 500 g/person/day³⁴ of inferior quality maize and sorghum by subsistence farmers pose a considerable threat to human health. Case studies have shown that the incidence of oesophageal cancer in areas where grain with high levels of FUM contamination is consumed is much higher than in other populations where FUM-contaminated food is not a staple.^35,36

 

Mycotoxigenic Fusarium species affecting South African grains

The mycotoxin-producing Fusarium species first described from grain in South Africa was F. culmorum, which was isolated from the stems and roots of wheat grown near Stellenbosch, Western Cape, in the 1930s.³⁷ By the end of 1985, a total of 27 Fusarium species, either toxigenic or non-toxigenic, had been reported from a broad range of hosts in South Africa.^32,38 To date, 33 mycotoxigenic Fusarium species have been associated with local grain crops (Table 1). These species include F. verticillioides, F. proliferatum and F. subglutinans, which are commonly associated with Fusarium ear rot (FER) of maize, and F. graminearum s.l. that causes Gibberella ear rot of maize, Fusarium head blight (FHB) of wheat and barley and grain mould of sorghum (Table 1). Certain Fusarium species are associated with FER, FHB and Fusarium crown rot under specific climatic conditions. For instance, FHB of wheat is caused by F. avenaceum, F. culmorum and F. poae in the cooler regions, whereas F. graminearum is predominant in the warmer regions worldwide.³⁹ In South African maize, the FER pathogen F. verticillioides predominates in the warmer dry areas, while F. subglutinans is abundant in cooler areas. The Gibberella ear rot pathogen, F. graminearum s.l., is most prevalent in intermediate climate areas.⁴⁰ Mycotoxin-producing species such as F. polyphialidicum Marasas, Nelson, Toussoun & van Wyk and F. sacchari (E. J. Butler) W. Gams are known to occur on grain crops elsewhere in the world, but have, to date, not been found on South African grains. These two species have, however, been found in soil debris and sugarcane, respectively, in South Africa.^41,42

FUM-producing Fusarium species, such as F. verticillioides and F. proliferatum, are often associated with maize and sorghum in South Africa (Table 1). Maize samples collected from 2001 until 2013 tested positive for FUM, sometimes at levels in excess of the maximum levels allowed by the EU.²⁰ More FUM and FUM-producing Fusarium species were found in maize grain produced commercially in warmer production areas of the Northern Cape, North West and Free State Provinces¹⁷ than in the cooler production regions. Although FUM contamination of small grain cereals has been reported⁴³, this mycotoxin has not been found in wheat and barley in South Africa when employing a multi-mycotoxin screening method using ultra-performance liquid chromatography mass-mass spectrometry⁴⁴. ZEA and TCT-Bs, however, have been found in both maize and wheat in the country,^20,44 but at higher levels and more frequently in maize than in wheat. The TCT-Bs and ZEA are primarily produced by Fusarium species within F. graminearum s.l.^45,46, and are commonly associated with Gibberella ear rot of maize, FHB of wheat and grain mould of sorghum⁷¹⁰.

The highly toxic mycotoxin, T-2 toxin, has until recently not been recorded in South Africa. T-2 toxin is most commonly produced by F. sporotrichioides, a fungus well-adapted to survive in colder countries.⁴⁷ Some T-2-producing Fusarium species, such as F. poae and F. chlamydosporum (Table 1), have periodically been isolated from wheat with FHB and maize with FER symptoms in South Africa.^35,48,49 The presence of T-2 toxin in local maize grain, recently reported by the SAGL²⁰, as well as its association with F. verticillioides and F. graminearum^50,51, requires further investigation.

Information on mycotoxin contamination of oats, sorghum and millet in South Africa is limited. Sorghum is affected by Fusarium species^32,33,52,53 that produce BEA, FUM, MON, TCT-B and ZEA, such as F. avenaceum, F. chlamydosporum, F. nygamai and the F. solani species complex (Table 1). Some of the same Fusarium species have also been associated with oats, millet or other less important grains such as amaranth. F. verticillioides, a common producer of FUM, also produces mycotoxins of lesser importance such as BEA, Fusarin C and MON (Table 1). The lower toxicity of these mycotoxins, and the relative complexity of multi-mycotoxin analysis,⁵⁴ limits the amount of data available on their occurrence in South African grains.

 

Role of mycotoxins in plant disease development

The role of mycotoxins in the interaction of fungi with plants is not always clearly understood. Some have, however, been shown to benefit the fungus.^55,56 The TCT-Bs, for instance, are phytotoxic and act as virulence factors on sensitive hosts, allowing the fungus to progress in plant tissue.⁵⁷ This effect was demonstrated by non-TCT-producing F. graminearum mutants that were pathogenic, yet caused less disease in maize than did wild-type TCT-producing isolates.^58,59 The virulence of F. graminearum and F. culmorum was also closely correlated with their DON and NIV deposition in wheat grain.⁵⁶ Adams and Hart⁶⁰, in contrast, reported that DON was not a virulence or pathogenicity factor for F. graminearum on maize, following virulence trials with non-toxic protoplast fusion F. graminearum strains.

FUM has been shown to be phytotoxic to maize seedlings, but its role in phytotoxicity, virulence and pathogenicity is unclear. The phytotoxicity of FUM was demonstrated by Williams et al.⁶¹ and Arias et al.⁶² who reported that FUM had a direct inhibitory effect on root growth, root hair development and other functions within the plant, whereas van Asch et al.⁶³ reported the mycotoxin to be phytotoxic to maize callus in culture. Symptoms were further induced when seedlings were watered with high concentrations of FUM in the absence of the pathogen.⁶² Glenn et al.⁶⁴ demonstrated that FUM production by F. verticillioides is necessary for the development of foliar disease symptoms on maize seedlings. Desjardins et al.⁶⁵ acknowledged that FUM could play a role in virulence, but argued that it is not essential for pathogenicity to maize seedlings. These authors compared the offspring of a fum1+ field strain of F. verticillioides with a high degree of virulence and that of a fum1^- field strain. They found that progeny with high levels of virulence were associated with FUM production, while highly virulent FUM-non-producing progeny were not observed. However, a highly virulent FUM-non-producing wild-type isolate was also identified, indicating FUM is not required for virulence. FUM non-producing mutants of F. verticillioides, generated by the disruption of the FUM5 gene, have been as virulent on maize ears as their wild-type predecessor strains.⁵⁹

Mycotoxins could also be involved in reproduction, fungal development and the colonisation of plant tissue. Disruption of a cyclin-like (C-type) gene, FCC1, resulted in reduced FUM B₁ synthesis and sporulation.⁶⁶ FUM is also believed to provide a competitive advantage to the fungus as it inhibits the mycelial growth of other fungal species in vitro.⁶⁷ The oestrogenic mycotoxin ZEA enhances perithecial production in F. graminearum, therefore the sexual development of the fungus is suppressed when ZEA synthesis is inhibited.⁵⁵

 

Impact of mycotoxins on human and animal health

The mycotoxins most commonly found in South African grains include DON, FUM and ZEA.^8,12,20 DON, also known as vomitoxin because of its strong emetic effects after consumption, is one of the most widely distributed TCTs found in grain. When consumed by livestock, DON can lead to food refusal, vomiting, decreased weight gain and less effective feed utilisation.^68-70 These disorders then cause anorexia in pigs and other monogastric animals. Ruminants and poultry appear to be resistant to DON.⁷¹ In humans, ingestion of DON-contaminated food has been associated with nausea, vomiting and diarrhoea.⁷² Outbreaks of acute DONassociated gastrointestinal illness in humans have been reported in China in 1984/1985 and in India in 1987.⁷³ The ingestion of NIV, which is considered more toxigenic than DON, has resulted in decreased feed consumption, lower feed conversion efficacy and decreased liver weights when fed to chickens.^74,75 NIV and NIV-producing F. graminearum s.l. species have, however, been less frequently associated with South African grains.^7,8

High levels of FUM in maize grain has been associated with leukoen-cephalomalacia. Leukoencephalomalacia is a fatal condition that causes the softening of brain tissue as a result of vascular insufficiency or degenerative changes in horses and rabbits.^76-78 FUM has also resulted in fatal pulmonary oedema in pigs and high tumour formation incidences in rats.^79-81 FUM was discovered following the association of F. verticillioides-contaminated maize grain with a high incidence of oesophageal cancer in the Transkei region (Eastern Cape Province) of South Africa.^35,80,82,83 Since then, the mycotoxin has also been associated with human oesophageal cancer in China and Italy and with prenatal birth defects and higher HIV transmission rates.^84,85

ZEA is one of the most widely distributed Fusarium mycotoxins globally. Despite its relatively low acute toxicity, ZEA is biologically potent⁸⁶ and may cause reproductive disorders in farm animals^45,86,87. ZEA-containing feed and fungal cultures fed to chickens and turkeys have resulted in significantly reduced egg production.^88,89 In humans, ZEA has been linked to hypoestrogenic syndromes and is believed to be an eliciting factor for advanced puberty development in girls.^90,91 The potential of ZEA to stimulate the growth of human breast cancer cells has also been demonstrated in vitro.⁹²

The EU and FDA established maximum allowable levels for certain food contaminants, including mycotoxins, with the aim to reduce their presence in foodstuffs to the lowest levels reasonably achievable by means of good manufacturing or agricultural practices.^73,93 In addition to the USA and countries within the EU, more than 100 other countries have established mycotoxin regulations for at least aflatoxin B₁, mostly produced by Aspergillus spp., to aid in minimising food safety concerns.⁹⁴ Fewer countries regulate Fusarium mycotoxins, and in South Africa no restrictions for maximum allowable levels of any of the Fusarium-related mycotoxins in food and feed are governed by legislation.

 

Management of mycotoxigenic Fusarium species

Good Agricultural Practice is a collective set of international codes of practice which forms part of the Codex Code of General Principles on Food Hygiene.⁹⁵ These codes are concerned with all aspects of primary food production, including environmental protection and sustainability, economics, food safety, food quality and health security. It also complements the Hazard Analysis Critical Control Point food management system designed to limit food safety concerns, including food poisoning by mycotoxins.^95,96 The Good Agricultural Practice codes recommend practices for primary production of foodstuffs including fruits, vegetables, grains and legumes. Adherence to these codes of good practice does not only impact on food safety locally, but also influences international trade. Great attention should thus be given to these codes when deciding on an integrated disease management strategy to control Fusarium species and their associated mycotoxins in different grains produced in South Africa.

Pre-harvest control

Field preparation and cultivation practices play a central role in the management of Fusarium diseases and their associated mycotoxins.⁹⁵ The burial of residue plant material from a previous planting season by deep ploughing can reduce the primary inoculum that causes infections.⁹⁷ This is especially important when crops are affected by the same Fusarium species, such as F. graminearum s.l. on maize, wheat and sorghum grown in rotation. While minimum tillage has significantly decreased stalk rot and increased grain yield of sorghum in South Africa³¹, it also has increased inoculum build-up of mycotoxigenic fungi in maize cropping systems⁹⁵. Crop rotation with legumes, brassicas and potato could also significantly reduce F. graminearum s.l. levels.⁹⁸ Limiting plant stress to increase plant vigour by adhering to optimum plant dates, preventing drought stress and the optimal use of fertilisers have reduced Fusarium infection in a number of grain crops.^99-101 Control of alternative hosts for Fusarium species, which include grasses and weeds, can also reduce unwanted inoculum build-up.^95,102,103

No fungicides are registered for the control of Fusarium grain diseases on maize, wheat or sorghum in South Africa.¹⁰⁴ Triazole fungicides such as metconazole and tebuconazole, however, have been shown to control FHB and DON contamination in wheat.¹⁰⁵ Control of mycotoxigenic Fusarium species in maize with fungicides, however, is difficult as ears are covered by tight husks which prevent contact with ear rot pathogens. Field trials in South Africa have reported no significant differences in the colonisation of maize grain by F. verticillioides or FUM contamination after application of protective fungicides such as the strobilurins, triazoles and benzimidazoles.¹⁰⁶ Chemical elicitors also failed to reduce FER and FUM contamination in maize.¹⁰⁷ As strict regulations on chemical pesticides and fungicide use are implemented to reduce human exposure and prevent environmental pollution, biological control has become more popular.¹⁰⁸ Non-pathogenic fungal antagonists such as Phoma betae A.B. Frank and Trichoderma spp. Persoon have reduced FHB and DON contamination under greenhouse conditions, but field results were variable and often failed.^99,109,110 In Ethiopia, 100% disease suppression of Fusarium root and crown rot of sorghum was reported after application of Bacillus spp. under greenhouse conditions.¹¹¹

Disease resistance is the most proficient and environmentally safe management practice to reduce Fusarium diseases in grain crops. Several quantitative trait loci (QTLs) that underlie resistance to FHB have been mapped in wheat, and can be used for marker-assisted selection.^112-114 In South Africa, commercial wheat cultivars and breeding lines containing resistance QTLs derived from Sumai 3 lines with low levels of FHB and DON content were identified under field conditions.¹¹⁵ The resistance of maize cultivars grown in South Africa to FER and FUM contamination are uncharacterised but resistant maize inbred lines were identified.¹¹⁶ These inbred lines could be used as sources of resistance within maize breeding programmes. Mapping studies have previously identified QTLs associated with resistance to FER and FUM contamination in maize.^117,118 However, studies to identify possible QTLs for resistance to grain mould in sorghum were less frequent, but have shown some success.^119-121

Unconventional methods to control plant diseases are becoming more common. Maize hybrids genetically modified with crystal (Cry) genes from the bacterium Bacillus thuringiensis, known as Bt-maize, reduced the feeding of stem borers and resulted in lower infection by F. verticillioides and F. proliferatum and subsequently reduced FUM contamination.¹²² FUM detoxification has also been achieved by genetic modification of maize with a degradative enzyme originating from Exophiala spinifera and Rhinocladiella atrovirens.¹²³

Post-harvest control

FUM and DON levels in grain do not increase significantly when grain is harvested at <14% moisture and when optimal moisture and temperature conditions and control of insect pests are maintained during storage.^124-126 The removal of mouldy, broken and underdeveloped kernels can also significantly lower mycotoxin levels in cereal grains. FUM levels of maize were reduced between 26.2% and 69.4% by sieving (<3 mm), and by 71% by separating mouldy from healthy kernels of maize produced by subsistence farmers in the former Transkei.^127,128 An 86% reduction of FUM was also achieved by the removal of F. verticillioides-contaminated maize kernels by flotation in water and sodium chloride, as these were less dense than sound kernels.¹²⁹ The separation of smaller, underdeveloped and shrivelled wheat kernels by means of the Carter dockage tester resulted in a 6-19% reduction of DON.¹³⁰

Mycotoxins are mostly heat stable; however, the preparation of South African traditional maize porridge through normal household cooking can reduce FUM by 23%.¹³¹ The washing of barley with distilled water has reduced DON levels by 69% and ZEA levels by 2%. In maize, DON levels were reduced by 65% and ZEA levels by 61%. A further reduction in DON and ZEA was achieved by using 1 M sodium carbonate solution for the first wash.¹³² The conversion of mycotoxins into non-toxic products can also be achieved through physical or chemical processes. Chemical degradation of DON has been achieved by ammonia, calcium hydroxide, chlorine, hydrochloric acid, ozone, sodium bisulfite, and sodium hydroxide.^133-136 However, the large-scale application of these methods are hampered by costs, safety concerns and the negative impact on grain quality.¹³⁷ Biological detoxification, defined as the enzymatic degradation of mycotoxins or modification of their structure that leads to less toxic products, offers an alternative method to reduce the mycotoxin content in food and feed products.¹³⁸

 

Discussion

Mycotoxigenic Fusarium species negatively affect the most important staple food crops grown in South Africa by reducing their yield and quality, and by contaminating the grain with harmful mycotoxins. These effects pose a serious threat to food safety and security for a rapidly expanding population. Efforts to manage harmful Fusarium species and their associated mycotoxins, both in commercial and subsistence farming systems, should therefore be made to sustain food production, to reduce health risks to humans and other animals, and to safeguard competitive international trade. A first step in achieving this aim could be the introduction of maximum levels for Fusarium mycotoxins in South African food and feed - a directive which has been conspicuously overlooked by the Departments of Health and Agriculture in the country.

A policy brief was compiled in 2009 to¹³⁹:

assist national stakeholders in government and industry, as well as commercial and emerging farmers, in understanding and implementing a united monitoring programme for the prevention and control of mycotoxins in foods in South Africa.

This brief recommended that DON and FUM be added to existing South African regulations in order to align with the guidelines adopted by most other mycotoxin-regulating countries.^94,139 Recently, South Africa has amended regulations regarding the tolerances for fungus-produced toxins in foodstuffs by limiting DON in grains to 2000 and 1000 µg/kg before and after processing, respectively. Maize grain, intended for further processing, is limited to 4000 µg/kg FUM while processed products, ready for human consumption, may not contain more than 2000 µg/kg of FUM.¹⁸⁶ Maximum levels for South Africa should be established by determining the general toxicity, haematotoxicity and immunotoxicity of the different mycotoxins as well as considering consumption levels of grain in the country. Incidences of mycotoxicoses, such as the outbreaks of DON-associated acute gastrointestinal illness in humans in China in 1984/1985 and in India in 1987⁷³, should also be taken into consideration. The biggest limiting factors in this undertaking would be the costs involved in an extensive regulation programme of foodstuffs, such as the laboratory analyses and the monitoring of revised mycotoxin legislation by health inspectors.

Health workers should be trained to identify symptoms exhibited by humans and animals in cases of mycotoxicoses. A serious call should be made on government to support mycotoxin research and to implement legislation on the levels of the different toxins present in foodstuff. The high intake of grains, in terms of both portion size and frequency, as staple foods by the majority of South Africans should be considered when determining allowable levels of contamination. Coordinated efforts should furthermore be made to launch public awareness campaigns through the distribution of educational information, in a responsible manner, without evoking public fear. These efforts should be particularly focused in subsistence farming communities, in which mycotoxicoses pose a genuine public health threat, as a high incidence of oesophageal cancer in the Transkei region of South Africa has been directly linked to high FUM contamination.⁸²

Managing the incidence and severity of mycotoxin contamination in grains, to reduce human and animal health risks and to safeguard competitive international trade, requires continuous efforts to understand and subsequently control the Fusarium species responsible for the production of these mycotoxins. South Africa, with its internationally recognised track record in mycotoxin research, possesses the skills, expertise and motivation to continue to work towards food safety and security for all people.

 

Acknowledgements

Mycotoxin research at Stellenbosch University is funded by the Maize Trust, the Winter Cereal Trust and the Technology and Human Resources for Industry Programme (THRIP) of the National Research Foundation of South Africa. We declare no conflict of interest.

Authors' contributions

I.B. was responsible for the design and content of the work; for the collection, analysis and interpretation of the data; and for drafting the manuscript. A.V. was responsible for the conception and design of the work. L.J.R., G.S.S., B.C.F. and A.V. critically revised the manuscript. All authors approved the final version.

 

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Correspondence:
Lindy Rose
lindym@sun.ac.za

Received: 20 Apr. 2016
Revised: 23 Sep. 2016
Accepted: 30 Sep. 2016

 

 

FUNDING: National Research Foundation (South Africa); The Maize Trust; Winter Cereal Trust