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Jabu Felix from Reitz in the Free State writes: I’m a new maize farmer. The one thing I didn’t expect to be dealing with is leaf diseases on maize. My neighbours say it’s quite well-known, but I really need some help in identifying and managing it.
Rikus Kloppers, integrated field science lead, and Vicky Coetzee – both affiliated to DuPont Research – says in a perfect world it would be ideal if we could predict what the season will look like and which problems to expect. However, the incidence of pests and diseases is completely dependent on the environmental conditions during the critical growth stages of the crop.
Disease can only occur when all three the following factors overlap: the host, pathogen and suitable environmental conditions. All of this develops over time and this determines when the infection will occur, how fast the disease establishes and the epidemic develops, and what the damage will be in the end. There are many models that can predict the prevalence and development of certain leaf diseases on various crops, and that can even predict when and whether to use control measures.
There are no new leaf diseases in South Africa on maize that can catch the producer unawares. It is usually the incidence of diseases in areas where they were previously less relevant that causes surprises. They are often the direct consequence of climate conditions changing. It is therefore important for producers to recognise the well-known leaf diseases on maize and also to know when one can expect to see them.
Before they occur, the most important maize leaf diseases have unique needs in terms of environmental conditions. Furthermore, the physiological growth stage of the plant determines when certain diseases appear.
Northern corn leaf blight
The most common leaf disease that occurs in most of the maize producing areas of South Africa, is northern corn leaf blight. The disease is caused by the fungus Exserohilum turcicum.
Environmental conditions that promote the disease are moderate temperatures (18°C – 27°C), moist conditions and long dew periods. The fungus survives on maize leaf residues, and multiple secondary infections develop throughout the season from existing lesions.
The lesions are usually noticed on the bottom leaves first, and as spores are released under favourable conditions from these lesions, the upper leaves are infected and it gives an appearance of a disease creeping up in the plant.
Common rust
Common rust is usually noticed for the first time during the season on the young maize plants. The disease is caused by the fungus Puccinia sorghi.
Environmental conditions that promote common rust are moderate temperatures (16°C – 25°C) and moist conditions (>95%) humidity. Common rust infection is promoted by dewfog conditions – especially during the night when spores on the leaf surface germinate and penetrate the leaf through the stomata.
Early signs of rust infection are visible approximately five days after initial infection as small flecks on the leaves, and proper rust pustules (10 to 14 days) then develop and release spores. On release these spores can be distributed across great distances by the wind and infect neighbouring plants or even the same plant again within the same season.
How to manage leaf diseases
General management guidelines for the risk management of maize leaf diseases include:
- Planting resistant maize hybrids.
- Planting a maize hybrid package and not only concentrating on one vulnerable hybrid with a high yield potential.
- Tillage and crop rotation to reduce disease pressure.
- Preservation tillage practices with all the advantages they offer. Unfortunately, they promote the survival of leaf diseases such as grey leaf spot and northern corn leaf blight, as the fungi mainly survive in the maize plant residues. To enjoy the actual benefits of a conservation tillage system, disease control is critical.
- The judicious use of registered fungicides in a good fungicide spray programme can effectively control most of the fungus diseases on leaves.
- However, the economic justifiability of an expensive chemical input is important and factors such as the maize price, yield potential and disease vulnerability of the hybrid and risk of disease in certain areas are very important. Do not spray injudiciously. It is important to remember that the primary goal of fungicide spraying is disease control. This protects genetic yield potential, and improved standability is secondary and a bonus.
- An integrated management strategy of all the above is the best approach to risk management.
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