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(Glomerella manihotis)

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Initial symptoms of the disease are oval lesions (“sores”) on young stems. On older stems, raised fibrous lesions develop that eventually become sunken.

What to do:

• It is not an economically important disease in most cassava growing countries and it does not warrant any intervention.

(Potyvirus – Potyviridae)

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It is particularly serious in coastal areas of Kenya, Zanzibar, Mozambique and Tanzania and lakeshore region of Malawi and in Uganda and is a threat to the whole of sub-Saharan Africa.

The virus is vectored by whiteflies (Bemisia spp.) and also transmitted through infected cuttings. Symptoms include yellowing (leaf chlorosis) and brown streaks in the stem bark (cortex).

Infected tubers have brown streaks (root necrosis) (Field Crops Technical Handbook, MoA, Kenya). It’s a stealth virus, which destroys everything in the field. The leaves may appear healthy even when the roots have rotted away.

What to do:

• Use diseased-free cuttings.
• Use tolerant/resistant varieties (e.g. “5543/156”, “TMS 30572”)
• Remove diseased plants from the field.

(Cercosporidium henningsii)

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Symptoms are restricted to older leaves. Brownish round spots with definite borders appear on the upper leaf surface.

On the lower leaf surface, they are brownish-grey in colour. Infected leaves later become yellow and eventually drop. In wet areas the disease may cause a yield reduction of up to 20%.

What to do:

• Though the disease is widespread in most cassava growing countries, it is not an economically important disease problem and it does not warrant any intervention.

(Xanthomonas campestris pv. manihotis)

Credits: Biovision-Infonet

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.

What to do:

• 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.

Credits: Biovision-Infonet

African cassava mosaic disease is one of the most serious and widespread diseases throughout cassava growing areas in Africa, causing yield reductions of up to 90%.

It is spread through infected cuttings and by whiteflies (Bemisia tabaci).

Symptoms occur as characteristic leaf mosaic patterns that affect discrete areas and are determined at an early stage of leaf development.

Symptoms vary from leaf to leaf, shoot to shoot and plant to plant, even of the same variety and virus strain in the same locality. Some leaves situated between affected ones may seem normal and give the appearance of recovery.

What to do:

• Use disease-free cuttings. If it is not possible to find cassava plants that are completely free from the disease, select cuttings from stem branches instead of from the main stem. Stem cuttings from the branches are more likely to sprout into disease-free plants than stem cuttings from the main stems (James et al, 2000).
• Resistance to ACMD has been successfully incorporated into high yielding cultivars of acceptable quality through breeding programmes at IITA. Use resistant/tolerant varieties (e.g. “SS 4”, “TMS 60142”, “TMS 30337” and “TMS 30572”)