A. Integrated Pest Management in Cassava:
Pests, Diseases, Beneficials and Control Strategies
Cassava (Manihot esculenta) is an introduced crop, native of South America. Tubers as well as leaves are used. Other members of this family are the castor plant (Ricinus communis) as crop and Euphorbia tirucalli and Jatropha curcas as hedges. The seed oil of the latter is a promising botanical pesticide. Cassava is a major subsistence crop in the more humid parts of the region (mainly Kahama). It is cultivated on about 40 000 ha in the region, with increasing importance. Yield levels are around 5 - 8 t tubers per ha, but more than 20 t/ha are possible. Cassava is often grown extensively on depleted light soils.
Cassava is vegetatively reproduced by stem cuttings (usually around 15 cm long). The cuttings are planted out in the fields from the beginning of the rains. Recommended planting pattern on ridges is 150 x 75 cm and 120 x 90 cm on the flat.
Vegetation period depends on the variety: it ranges from 9 months (e.g. Alpin Valencia), over 1 year (Binti Athoumani, Ali Mtumba), to 18 months (e.g. Kigoma, Msitu Zanzibar); these are sweet varieties; bitter varieties are Liongo (1 year) and Limbanga, Lumalampuni and Mzimbitala (vegetation period of 18 months). There are new varieties under testing. Unlike sweet potatoes, cassava tubers can be stored in the soil for longer periods; this is true in particular for the bitter varieties.
Cassava roots are eaten either fresh (sweet varieties) or processed: chopped, sundried (Makopa), done with sweet varieties
harvested, soaked for some days in water and sundried (Udaga), done with bitter varieties
The sundried pieces are screened for fibres and then pounded with mortars to flour.
Main pests are nowadays Cassava Green Mite (tanabui) and Cassava African Mosaic Virus (batobato); new strains of Bacterial Blight disease are approaching from Uganda. The problem of Cassava Mealy Bug seems to be solved. Rodents can be a problem in overstaying crops, especially with sweet varieties. Weeds are usually well controlled with a hand hoe (jembe); 2 weedings are sufficient.
2. Pest Description and Biology
2.1. Cassava pests
Common Name:Cassava Green Mite
Scientific Name: Mononychellus tanajoa
The Cassava Green Mite is nowadays the major arthropod pest of cassava in the region. It has been introduced in the 1970s from South America. The mites are sucking out leave cells, which can lead to malformation and shedding of affected leaves, particularly during the dry season. Estimated yield losses are in the range of 5 - 10 % only, since the plant can stand a lot of defoliation (up to 40%). Up to 50 % losses were recorded in Uganda.
The mite is very small (0.5 mm long, difficult to see without lens), greenish in colour and has 8 legs (visible only under a lens). Life cycle is only 12 -14 days, the mites can thus reproduce rapidly under favourable conditions. Reproduction is by eggs. Females can spread from plant to plant by silken threads; otherwise no web is produced by the mites on the leaves. The mites suck out single cells, which are then filled with air and appear white. Symptoms on older leaves are many white spots; young leaves are malformed and the symptom can be confused with that of Mosaic Virus - at closer examination, the white spots are distinguishable.
A classical biocontrol using predatory mites is in preparation.
Common Name:Cassava Mealy Bug
Scientific Name: Phenacoccus manihoti
The Cassava Mealy Bug was some years ago the main pest of cassava. It is an introduced pest from South America and was spreading rapidly in Africa since the late 1970s. Since the successful introduction of a classical biocontrol (Epidinocarsis lopezi wasp). it has lost much of its importance. The mealy bug...