Cassava

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.

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

Birds, rodents, monkeys, pigs and domestic animals (cattle, goat and sheep) are common vertebrate pests of cassava.

Measures that help to manage damage by these pests include:

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%.

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.

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.

This mite is green in colour at a young age turning yellowish as adult. Adult females attain a size of 0.8 mm. They appear as yellowish green specks to the naked eye. They occur on the lower surface of young leaves, green stems and auxiliary buds of cassava. Damage initially appears as yellowish "pinpricks" on the surface of young leaves.

Symptoms vary from a few chlorotic spots to complete chlorosis. These symptoms are somehow similar to African cassava mosaic disease, and should not be confused. Heavily attacked leaves are stunted and become deformed. Severe attacks cause the terminal leaves to die and drop, and the shoot tip looks like a "candle stick". Green spider mites are major pests in dry season. Severe mite attack results in 20-80 % loss in tuber yield.

Predatory mites (mainly Typhlodromalus aripo and T. manihoti) introduced from South America, the home of the cassava green mite, have given effective control of the cassava green mites in Africa (Yaninek and Hanna, 2003).

It is a mussel-shaped scale with an elongated silvery-white cover and about 2-2.5 mm long. This scale may cover the stem with conspicuous white secretions, and eventually the leaves. This scale sucks from the stem and dehydrates it. The leaves of attacked plants turn pale, wilt and drop off. Severely attacked plants are stunted and yield poorly. Scale attack can kill cassava plants, in particular plants weakened by previous insect attack and drought. Stem cuttings derived from infested stem portions normally do not sprout.

Grasshoppers such as the variegated grasshopper (West to East Africa south of the Sahara), and the elegant grasshopper (Z. elegans) (Southern Africa and East Africa) are brightly coloured grasshoppers.

Adults are dark green with yellow, black and orange marking on their bodies. Nymphs are black with yellow markings on the body, legs and antenna and wing pads. Female grasshoppers lay many eggs just below the surface of the soil in the shade under evergreen plants, usually outside cassava fields. Eggs are laid in masses of froth, which harden to form sponge-like packets, known as egg pods, which look like tiny groundnut pods. Eggs start to hatch at the beginning of the main dry season.

Grasshoppers attack a wide range of crops mainly in the seedling stage. They feed on cassava plants, chewing leaves and stems and may cause defoliation and debark stems. This is particularly severe in fields next to the bush when the dry season is prolonged.

The larger grain borer has been found infesting cassava chips in storage particularly during the rainy season in West Africa. This beetle is currently the most serious pest of dried cassava in storage. Weight losses as high as 70% after 4 months of storage have been reported elsewhere.

The cassava mealybug is pinkish in colour. Its body is surrounded by very short filaments, and covered with a fine coating of wax. Adults are 0.5-1.4 mm long. This mealybug does not have males. Females live for about 20 days and lay 400 eggs in average. The lifecycle from egg to adult is completed in about 1 month at 27&degC. It reproduces throughout the year and it reaches peak densities during the dry season. Mealybugs are dispersed by wind and through planting material.

The cassava mealybug strongly prefers cassava and other Manihot species; other host crops and wild hosts are only marginally infested. It sucks sap at cassava shoot tips, on the lower surface of leaves, and on stems. During feeding the mealybug injects a toxin into the cassava plant causing deformation of terminal shoots, which become stunted, resulting in compression of terminal leaves into "bunchy tops". The length of internodes is reduced, and stems are distorted. When attack is severe plants die, starting at the plant tip, where most mealybugs are found.

Mealybug attack results in leaf loss and poor quality planting material (stem cuttings) due to dieback and weakening of stems used for crop propagation. Tuber losses have been estimated up to 80%. The pest-induced defoliation reduces availability of healthy leaves, which are consumed as leafy vegetables in most of West and Central Africa. After the pest cripples plant growth, weed and erosion sometimes lead to total destruction of the crops. In general, yield losses depend upon age of plant when attacked, length of dry season, severity of attack and general conditions of the plant. Mealybug damage is more severe in the dry than in the wet season.

The cassava mealybug was accidentally introduced to Africa from South America. After the first reports in the 1970s, the insect became the major cassava pest within a few years and spread rapidly through most of the African cassava belt. The outbreak led to famine in several countries where cassava is a staple crop and particularly important in times of drought. In an attempt to control this pest natural enemies, mainly parasitic wasps and ladybird beetles, were introduced from South America.

The most effective has been the parasitic wasp (Apoanagyrus (=Epidinocarsis) lopezi), which has kept this mealybug at low levels, resulting on significant reduction of yield losses in most areas in Africa (Neuenschwander, 2003).

Some fungi growth on cassava chips, usually when the moisture content of cassava chips exceeds 14%, making them unfit for feed and food. A survey of cassava chips processing areas of West Africa has indicated that the most common fungi were Rhizopus sp. and Aspergillus sp. (IITA, 1996).

Several species of red spider mites also occur on cassava, mostly on the older leaves. Adults are about 0.6 mm long. Initial symptoms are yellowish pinpricks along the main vein of mature leaves. Spider mites produce protective webbing that can be readily seen on the plant. Attacked leaves turn reddish, brown or rusty in colour. Under severe mite attack, leaves die and drop beginning with older leaves. Most damage occurs at the beginning of the dry season.

A number of beetles feed on dry cassava causing post harvest losses. In Benin Republic the most common are Dinoderus sp., Carpophilus sp., the coffee bean weevil (Araecerus fasciculatus), the lesser grain borer (Rhizopertha dominica), and more recently, the larger grain borer (Prostephanus truncatus).

Infestation by these insects is heavier in the rainy season than in the dry season, is more prevalent in the humid zone than in the savannah, and is found more in large chips than in smaller ones. Maximum infestation was found after 6 to 8 months in storage, at which time chips would fall into dust when squeezed (Bokanga, IITA, FAO).

It is a whitish mealybug with two longitudinal dark stripes, long glassy wax threads and two long tails. It attains a length of 4 mm. The stripped mealybug occurs on the underside of leaves near the petioles and on the stems. It sucks sap but does not inject any toxin into plants. Severely attacked plants show general symptoms of weakening but do not show distortion.

It is a minor pest of cassava, and no control is usually required as is controlled naturally by natural enemies.

Different species of termites damage cassava stems and roots. Termites damage cassava planted late or in the dry season, in particular when the crop is still young at the peak of the dry season. They chew and eat stem cuttings which grow poorly, die and rot. The may destroy whole plantations. In older cassava plants termites chew and enter the stems. This weakens the stems and causes them to break easily.

Several species of whiteflies are found on cassava in Africa. Feeding causes direct damage, which may cause considerable reduction in root yield if prolonged feeding occurs. Some whiteflies cause major damage to cassava as vectors of cassava viruses. The spiralling whitefly (Aleurodicus dispersus) was reported as a new pest of cassava in West Africa in the early 90s.

The adults and nymphs of this whitefly occur in large numbers on the lower surfaces of leaves covered with large amount of white waxy material. Females lay eggs on the lower leaf surface in spiral patterns (like fingerprints) of white material secreted by the female. This whitefly sucks sap from cassava leaves. It excretes large amounts of honeydew, which supports the growth of black sooty mould on the plant, causing premature fall of older leaves.

The tobacco whitefly (Bemisia tabaci) transmits the African cassava mosaic virus, one of the most important factors limiting production in Africa. The adults and nymphs of the tobacco whitefly occur on the lower surface of young leaves. They are not covered with white material. The nymphs appear as pale yellow oval specks to the naked eye.