Hybrid cassava that has been developed by Professor Mathews Dida (right) of Tom Mboya University that is resistant to Striga weed. [Denish Ochieng, Standard]
Cassava is one of the most important staple food crops in the continent. It is an important food security crop in a number of countries as the last line of defence against famine. Cassava is cultivated not just for consumption but also to provide raw materials for emerging industries that depend on products from the roots, particularly starch. One advantage of growing cassava is its ability to thrive in nutrient-deficient soils. However, diseases remain the biggest threat to the production of this crop. Unfortunately, cassava farmers do very little or nothing to control diseases and pests. Today, we will explore the common diseases that affect cassava.
African Cassava Mosaic
Cassava mosaic disease is caused by a virus. The disease is manifested through leaf discolouration, with a mixture of green and yellow or white patches. These chlorotic patches are indicators of reduced amounts of chlorophyll in the leaves, which affects photosynthesis and therefore yields. The disease spreads largely through the use of infected cutting used as planting materials
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Control
Improved varieties of cassava resistant to mosaic disease have been developed by researchers. Farmers can source these planting materials from Kenya Agricultural and Livestock Research Organisation (Kalro). Agriculture extension agents should help farmers in their operational areas to access desired disease-resistant varieties. Mosaic disease can be controlled effectively through the use of healthy or virus-free planting materials. Farmers must select stems from plants with no mosaic symptoms to produce cuttings.
Remove plants showing symptoms of the disease from a population of healthy ones and destroy them. This will reduce the spread of the disease to other parts of the farm. Regular inspection of plants on a farm is necessary if effective disease control is to be achieved.
Cassava bacterial blight disease
Distinctive symptoms of the disease include the appearance of water-soaked spots or lesions on leaves of infected plants. The spots often start along the veins, margins and tips of leaf blades. As the disease develops, neighbouring spots join together to form large brown patches or blights killing the leaf blade as it expands. The leaf dries or wilts and finally falls. In some susceptible cultivars, creamy or yellowish-brown gummy matter are discharged on leaves or stems but often distinctively on leaf petioles of infected plants.
The bacteria gains entry into plants through wounds or scratches on leaves and stems. Farm tools can easily transmit the disease from farm to farm if the tools are not cleaned after work in an infected field. Use of infected cuttings as planting materials can also transmit the disease. The disease can be controlled by crop rotation, use of disease resistant varieties, and removal of infected plants. If complete outbreaks on whole farms are observed, harvest roots immediately and destroy stems and leaves and other plant debris through burning. Infected plant debris may also be ploughed deep into soil. In both situations, the land must be allowed to fallow for three years minimum before cassava is planted in it. The bacterium responsible for the disease is unable to survive over a long period outside the host.
Cassava anthracnose disease
The disease is caused by a fungus that is also capable of causing diseases on other food crops. It affects leaf and stem production. Severe anthracnose attacks can cause death of stems which can affect the availability of planting materials. The distinctive feature is the appearance of cankers or sore-like lesions on the stem of susceptible varieties. The cankers may be formed at the nodes bearing petioles or along any part of the stem depending on the variety. Spores of the disease causing fungus are spread by wind or rain splashes and may gain entry through wounds on stems to establish the disease. Healthy stems free of cankers and dieback tissues must be selected to provide cuttings for new plantings. Healthy cuttings sprout well and give rise to young vigorous growing plants that are likely to escape early infections. Where canker bearing stems are the only available sources of planting materials, stem cuttings must be dipped in appropriate fungicides recommended by extension agents in their operational areas.
