MYCOTOXINS - toxins produced by maize ear rot fungi

Maize is the staple food commodity in South Africa. This crop is plagued by many ear rot diseases. The fungi infecting maize ears can also produce mycotoxins, which are toxic substances. Mycotoxicoses can cause various diseases in humans and animals. In this article the most commonly found mycotoxins and their causal agents are discussed.

Fumonisins

The fungus Fusarium verticillioides grows on and infect maize ears. The fungus usually appears as a white to pale pink or pale purple coloured growth on the maize kernels (Photo 1).

Fusarium ear rot occurs widespread in maize producing areas of South Africa and the fungus favours drought and high temperature conditions. Rain during flowering can help to spread the fungus to the silks of the maize. The fungus grows down the silks and starts to colonise the kernels.

The disease is also prevalent where stem borer damage is observed. F. verticillioides produces fumonisins and there are three types of fumonisins called FB1, FB2 and FB3. The fumonisin FB1 is the most common and comprises up to 75% of the fumonisin content. These toxins possess cancer-promoting properties and are regarded as Class 2B carcinogens. It has been shown to cause liver cancer in rats and it has been associated with oesophageal cancer in South Africa and China, where people consume a lot of fumonisin contaminated maize.

In horses it causes leukoencephalomalacia (LEM), which is the liquefying of the cerebral hemispheres of the brain and it leads to death. It also results in lung oedema in pigs and immunosuppression in chickens.

Control measures

• Implement production practices that could minimise drought stress.
• Since F. verticillioides infects via the silk, use maize hybrids with tight husks in order to maintain silk integrity for longer. Just remember that tight husks can lead to a greater risk of Gibberella ear rot, but if Fusarium ear rot is of greater concern, use tight husk hybrids.
• Maize that is harvested can be further decayed and contaminated by the already present Fusarium spp. Maize harvested must contain a low moisture content and the area in which you store the maize must be kept dry and clean.

Deoxynivalenol, Nivalenol and Zearalenone

Gibberella ear, crown, root and stalk rots are widespread throughout the South African maize production area. These diseases are caused by 13 fungal species belonging to the Fusarium graminearum species complex.

Gibberella ear rots occur where maize is produced under wet, warm xonditions. The disease has been common in the moderate eastern production areas and has recently been noticed to be on the increase in the western production areas. The pathogen can also infect wheat, oats and barley. Gibberella ear rots is caused by species within the Fusarium graminearum species complex and studies have shown F. boothii to be the only species causing Gibberella ear rots in South Africa.

Rotation of maize with graminaceous crops can increase disease levels in the field depending on the amount of F. boothii inoculum that is carried over from one crop to the next. The disease can also increase in reduced tillage fields due to increased inoculum levels because of stubble retention. Gibberella ear rots can cause yield losses and affect grain quality. Ear infections initially appear white on the ear tips and move toward the base of the ears (Photo 2). The mycelium will later turn red-pink in infected kernels.

The fungi in this complex are known to produce mycotoxins, such as deoxynivalenol, nivalenol and the estrogenic metabolite zearalenone, which are harmful to humans and livestock.

Nivalenol and deoxynivalenol are known protein synthesis inhibitors and consumption of grain contaminated with these mycotoxins can cause anaemia, skin lesions, vomiting, diarrhoea, and damage to hematopoietic (liver) tissues in humans and animals. Zearalenone-contaminated feed can lead to animals developing reproductive problems.

Control measures

• Host resistant or tolerant varieties are the most cost effective and practical means of combating the disease.
• Avoid planting maize at unacceptably high densities as this increases stress.
• Rotate with non-hosts of the Fusarium graminearum species complex such as legumes, cotton and sunflower.
• Harvest early to avoid lodging. Get rid of infected debris to avoid buildup of inoculum.
• Control insects such as stalk borers which may serve as possible vectors, observing the threshold value of 10% infested plants for chemical control.
• In order to prevent ear rot after harvest, store grain under low moisture conditions.

At the ARC-GCI we are looking for Gibberella stalk rot and Gibberella ear rots on maize plants. It will be much appreciated if maize plants with these symptoms can be sent to Aneen Schoeman, ARC-GCI, 114 Chris Hani Street, Potchefstroom, 2520.

Aflatoxins

Aspergillus ear rot causes only little damage to kernels. Only a few kernels are infected and have masses of yellow green spores (Photo 3).

Sometimes no visible spores will be on the surface of the kernels, but the kernels will appear dull and discoloured. The fungus favours areas with high temperatures and drought conditions. Aspergillus flavus and A. parasiticus are the causal organisms that produce aflatoxins. Aflatoxins are compounds that are toxic and carcinogenic.

The most potent and biologically active of these groups is aflatoxin B1 and it usually affects the liver. In general, the toxicity of aflatoxins is more of a concern than their carcinogenicity. Clinical symptoms of aflatoxins are grouped into three classes: primary acute, chronic, and secondary.

Primary acute usually results in the death of the animals. Animals with chronic aflatoxicosis show symptoms of reduced weight gain, haemorrhages, liver damage and capillary fragility. Secondary aflatoxicoses results in decreased phagocytic efficiency and cell mediated immunity in animals. This can result in death of the animals since these conditions make animals more susceptible to other diseases.

Aflatoxin contamination can occur in the field at any time, but is associated with conditions that stress the plants such as drought and temperature stress, nitrogen deficiency and significant insect damage.

Control measures

Use of resistant hybrids is the best and most economical method of control. Use of irrigation to avoid stress of plants. Tillage to reduce inoculum present in the field. Control of insects that feed on the ears. Timing of planting in order to reduce stress during and after pollination.

For any enquiries regarding the abovementioned mycotoxins or the ear rot diseases, the plant pathologists at the ARC-GCI, Potchefstroom, can be contacted at (018) 299-6100 or email Aneen Schoeman at BelgroveA@arc.agric.za.