he African Cassava Mosaic Disease (CMD) is widespread in Africa and causes up to 90 per cent yield losses. It is caused by cassava mosaic geminiviruses and spreads through infected cuttings, whiteflies (Bemisiatabaci), and occasionally, through farm tools. The disease affects leaves at the vegetative growth stage.
Symptoms
Leaves become distorted and growth is stunted. It occurs as a leaf mosaic pattern, which affects discrete areas at an early stage of leaf development. Symptoms vary from leaf to leaf, shoot to shoot and even plants of the same variety may show varying symptoms.
Severely affected plants produce virtually no yield of roots or stems for further propagation.
Yield loss
Yield loss due to CMD ranges from 12 to 82 per cent, depending on the variety and type of infection.
Ways to control the disease
• Use disease-free cuttings: Use cuttings from healthy plants. If it is not possible to find disease-free cassava plants, select cuttings from stem branches and not the main stem, as they are more likely to sprout into disease-free plants.
• Use of resistant cultivars: Resistance to CMD has been successfully incorporated into high-yielding cultivars through breeding programmes. Hybrids can be bought from Kenya Agricultural & Livestock Research Organisation (KALRO) Kenya, Mikocheni Agricultural Research Institute (MARI), Tanzania, and National Crop Resources Research Institute (NaCRRI), Uganda.
• Field hygiene (sanitation): Identify and remove all infected cassava and other host plants from sites to be used for new plantings (roguing). This strategy does not protect them from being inoculated by whiteflies, but research shows that the virus is more aggressive in plants infected from contaminated cuttings than by insect vectors.
• Proper spacing: The wider the space between cassava stands, the higher the risk of disease. Disease incidence has proven to be high at the widest spacing between cassava stands and along footpaths or gaps in the stands. Use uniform dense cassava stands rather than irregular widely spaced ones.
• Planting date: Do not plant in seasons when there are many white flies as this exposes vulnerable young plants to risk of infection. Plant during rainy seasons. l.
• Soil fertility and nutrient status: Though cassava grows in poor soils, this may enhance damage caused by the virus.
• Intercropping: Cassava can be grown with other crops, including banana, sweet potato, cereals and legumes. This improves overall land productivity and may decrease whitefly vector populations, whitefly activity, and virus spread.
• Plant many varieties: Studies have shown that in areas where many varieties of cassava were grown, losses were much less than in areas with one variety.
• Field size and shape: Virus incidence and whitefly numbers tend to be greatest in the outermost rows of plantings, especially those that face the prevailing wind. Plant a resistant variety of cassava around the field margins.
• Burn all the plants attacked by the disease.