DR BELINDA JANSE VAN RENSBURG, ARC-GRAIN CROPS INSTITUTE, POTCHEFSTROOM
F. verticillioides can cause ear, root and stalk rot of maize and can lead to economic losses due to yield loss and grain quality reduction (Photo 1).
F. verticillioides has the ability to produce mycotoxins (myco = fungi and toxin = poison) named fumonisins. The consumption of maize contaminated with fumonisins may cause mycotoxicosis in animals such as leucoencephalomalacia in horses and pulmonary oedema in swine.
Fumonisin infected maize has been statistically associated with human oesophageal cancer in South Africa, Northern Italy and Iran. A strong correlation between the consumption of fumonisin-contaminated tortillas and neural-tube defects in humans in Mexico has been reported in literature. The potential carcinogenic risk of fumonisin B1 to humans was evaluated and classified by the World Health Organisation’s International Agency for Research on Cancer (WHO-IARC) as “Group 2B carcinogens” which means they are probably carcinogenic to humans.
To date there are no legal limits set locally for fumonisin levels and when necessary, we refer to the guidelines of the Food and Drug Administration (FDA) of the United States of America which is set at 2 ppm for human intake and 5 ppm for horses, pigs, rabbits and pet animals.
F. verticillioides is more common in regions with hot and dry growing conditions especially before or during pollination. F. Verticillioides grows well at temperatures above 26°C and according to literature, the calculated optimal temperature for growth is 31°C.
F. verticillioides has a saprophytic as well as pathogenic stage and may infect maize at all stages of plant development, either via the silk channel, infected seed, or wounds. F. verticillioides can be transmitted to uninfected plants by inoculum from field stubble or airborne conidia (micro- and macroconidia) which are abundant in maize fields during a growing season. The most commonly reported method of kernel infection is through airborne or water-splashed conidia that land on the silks.
Fusarium ear rot symptoms can vary depending on genotype, environment and disease severity. One symptom type noted in the field is the growth of white-pink cottony mould on kernels alongside stalk borer channels. Similar symptoms are often associated with other insect or bird damage on ears. F. Verticillioides can also infect individual (Photo 2) or groups of kernels (Photo 3) scattered randomly on the ear. Another symptom type is a pink discolouration of undamaged kernels, but this must not be confused with a slight pinking of certain maize white hybrids where the discolouration is superficial.
The responses of cultivars tested by the ARC-Grain Crops Institute (ARC-GCI) to date to F. verticillioides infection over localities and seasons, is inconsistent. This means that adequate and reliable resistance is not available to maize producers. Emphasis therefore, needs to shift to management strategies in the field to ensure the quality and safety of maize food and products.
This is complicated by the complexity of interactions between numerous abiotic and biotic factors, our need to understand them, and their manipulation to prevent or reduce the growth of F. verticillioides and thereby reducing contamination by fumonisins.
To date, no fungicides have been registered for the control of ear rots in South Africa and the efficacy of available chemicals still needs to be determined. Feeding activities of lepidopterous insects may spread F. verticillioides spores to silks, kernels, stems and feeding channels, increasing colonisation by the fungus.
Bt-transformed maize contains genes from Bacillus thuringiensis encoding for insecticidal crystal proteins. Reduced insect damage on Bt maize stalks can reduce infection by Fusarium spp. through plant injuries and reduce fumonisin levels as a result. Birds that cause physical injury to stalks and ears are also suspected to promote infection by Fusarium spp.
The most plausible solution seems to be prevention in the field through crop techniques that are able to guarantee less favourable conditions for F. verticillioides development and subsequent fumonisin production. Cropping practices such as N fertilisation, timing of sowing and harvesting, insect control and plant density influence fumonisin contamination in maize grains. Preliminary studies carried out in a field trial by the ARC-GCI, during the 2011/2012 planting season, indicated that higher plant populations had no effect on fungal infection, but an increase in fumonisins was observed. This observation must be confirmed with another two season’s data.
These significant higher fumonisin levels observed at higher plant densities could be attributed to “stress” factors on the plants caused by competition for water and nutrients. It has been indicated in literature that low temperature and water stress reduce fungal growth of F. verticillioides, but an increase in water stress increased FUM 1 expression which is the first step in fumonisin synthesis. This could possibly explain higher fumonisin levels in the presence of higher plant densities although fungal biomass was not significantly increased.
It is evident from literature and this study that maize cultivated at high plant populations, may exhibit a significant increase in fumonisin risk in maize grain. The correct plant population in different maize production areas will depend on local climatic conditions, soil fertility, crop techniques and the genotype used. Maize producers are therefore encouraged to enquire about optimum plant populations suited to their area and production practices, as this could be critical in reducing fumonisins in the maize crop.