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Charcoal rot: a threat to staple food crops in South Africa

Charcoal rot is a relatively unknown disease causing yield losses in crops across South Africa, including maize. Shutterstock

Charcoal rot is caused by a fungus that invades various agricultural crops and gives them a charred appearance. The disease is becoming more widespread in South Africa – which is worrying, since it can dramatically affect crop yields which drives up prices and hits farmers’ incomes.

Charcoal rot attacks crops that are the source of staple foods, like maize and sorghum, as well as sunflower and soybean which are important sources of animal feed. Other crops affected include common beans, canola, cotton, tobacco, strawberries and some vegetables.

The disease is common in many countries in Africa, Asia, Europe, South and North America. It can cause yield losses of up to 90% in sunflower and 50% in soybean. Maize crop losses of up to 70% have been recorded across Africa.

The disease can be particularly devastating in countries like South Africa which has a hot and dry climate. But its importance is overlooked due to the lack of information on its effect on yields and food security. Maize is the most affordable staple for most households in South Africa. The country, usually a maize exporter, was forced to import maize during the 2015/2016 season, the driest on record since 1904.

Charcoal rot has the potential to become an even bigger problem, particularly in maize production. Controlling it is critical yet efforts to deal with it in South Africa have been hampered by a lack of information. A number of initiatives are underway to close the knowledge gap. One is a research project at the University of Pretoria which is looking at some of the aspects of the disease related to soybean and sunflower. The aim is to provide a picture of how charcoal rot is affecting production in the country.

A pernicious fungus

The fungus, Macrophomina phaseolina, infects plants either at seedling stage or at flowering. When the plants are under stress they are more susceptible to charcoal rot. It reduces the crop stand when the seedlings die. It can also lead to significant yield losses.

In some crops like sunflower and soybean it can affect the composition of the oil. This decreases the oil’s quality and nutritional value.

Charcoal rot, caused by a fungus that invades plants giving them a charred appearance. Estiene Jordaan

There is very little information available about yield reduction for crops affected by charcoal rot in South Africa. The little information there is comes from discussions with farmers. What is known is that it is usually a problem during the very dry seasons. And it could become more prevalent as extreme weather events like droughts become more common.

Reports of charcoal rot have been increasing in South Africa, especially in warmer parts of the country, like Mpumalanga, the Free State and the North West. This is attributed to soybean and sunflower production expanding into the country’s warmer regions. Erratic rainfall, attributed to climate change, has resulted in water becoming a scarce resource, limiting irrigation.

Difficult to treat

There is no reliable way to control charcoal rot. The use of crop rotation is limited because of the wide host range of the pathogen. On top of that a minimum of a three year rotation is required.

Fungicide treatment of the seeds has proven to be ineffective. Irrigation can be effective as moist soils aren’t a favourable environment for the fungus to grow in.

But there are steps that can be taken to help reduce the incidence of the fungus. These include:

  • shifting planting dates to avoid excessively high temperatures or drought during flowering stage;

  • crop rotation which can sometimes decrease the amount of the pathogen in the soil. To prevent the pathogen increasing crops used in rotation shouldn’t be hosts of the pathogen, and;

  • increasing planting densities. This creates an unfavourable environment for the pathogen by producing more shade, keeping soil temperatures low and increasing relative humidity in the crop canopy.

There is no known true resistance to charcoal rot. Cultivars showing resistance or tolerance to other fungal root diseases are only somewhat tolerant to charcoal rot.

South Africa needs to look at control methods used in other countries, like resistance breeding, and then develop an integrated disease management programme that is sustainable and economically feasible for conditions in the country.

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