Cassava bacterial blight (Xanthomonas campestris pv. manihotis)
It is a major constraint to cassava cultivation in Africa. Infected leaves show localised, angular, water-soaked areas. Under severe disease attack heavy defoliation occurs, leaving bare stems, referred to as "candle sticks". Since the disease is systemic, infected stems and roots show brownish discolouration. During periods of high humidity, bacterial exudation (appears as gum) can readily be observed on the lower leaf surfaces of infected leaves and on the petioles and stems. The disease is favoured by wet conditions.
This disease is primarily spread by infected cuttings. It can also be mechanically transmitted by raindrops, use of contaminated farm tools (e.g. knives), chewing insects (e.g. grasshoppers) and movement of man and animals through plantations, especially during or after rain. Yield loss due to the disease may range from 20 to 100% depending on variety, bacterial strain and environmental conditions.
- Use clean planting material. This can reduce disease incidence in areas where cassava bacterial blight is already widespread.
- In cases of sporadic occurrence of the disease, collect cuttings only from healthy plants and from the most lignified portion of the stem, up to 1 m from the base. Check visually the cuttings for vascular browning. Disinfect tools regularly.
- Intercrop cassava with maize or melon. This been reported to reduce cassava bacterial blight significantly.
- Practise crop rotation and fallowing. These practices proved very successful when the new crop was planted with uninfected cuttings. Rotation or fallowing should last at least 1 rainy season.
- Remove and burn all infected plant debris and weeds. Alternatively plough them into the soil.
