6. Pests and diseases
Protecting cassava with pesticide is usually ineffective and hardly ever economic. A range of non-chemical measures help farmers reduce losses while protecting the agro-ecosystem.
Friends and foes of cassava. Top, the whitefly and cassava mealybugs; above, two natural enemies: Coccinellidae beetles and the lacewing
Like other crops, cassava is vulnerable to pests and diseases that can cause heavy yield losses. In some regions, the incidence of pests and diseases is increasing as the crop is grown more intensively over larger areas and planted throughout the year for industrial processing.
Because synthetic insecticide, fungicide and herbicide disrupt the natural crop ecosystem balance, and can exacerbate pest and disease problems, “Save and Grow” seeks to minimize their use through integrated pest management (IPM), a plant protection strategy that enhances the biological processes and biodiversity that support crop production. A range of sustainable, non-chemical measures can help cassava growers reduce losses to insect pests and diseases.
- Bacterial blight, one of the most widespread of the cassava diseases, is transmitted by infected planting material and farm tools. It can be controlled by using varieties with good tolerance, soaking stakes in hot water before planting, sterilizing tools with disinfectant, and intercropping to reduce plant-to-plant dissemination.
- Viral diseases are usually transmitted through infected planting material. In addition, whiteflies are vectors for viruses that cause cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), which can cause total crop failure. Key recommendations for control of CMD and CBSD are strict enforcement of quarantine procedures during international exchange of germplasm, and cultural practices, especially the use of resistant or tolerant cultivars and virus-free planting material.
- Root rots occur mainly in poorly drained soils during intense rainy periods. In Colombia, farmers eliminated root rot by planting stakes taken from healthy mother plants, using ashes and dry leaves as a soil amendment and fertilizer, and intercropping with cowpeas. An effective biological control is immersion of stakes in a suspension of Trichoderma viride, a soil fungus that parasitizes other soil-borne fungi.
- Whiteflies are probably the most damaging insect pest in all cassava-producing regions. Although some farmers use insecticide to control whiteflies, spraying is usually ineffective. Not spraying insecticide, on the other hand, allows biological control by the whitefly’s natural enemies. A two-year experiment in Cameroon found that intercropping cassava with maize and cowpeas was associated with a drop of 50 percent in the adult whitefly population and a 20 percent reduction in the incidence of cassava mosaic disease.
- Mealybugs feed on cassava and inject a toxin that causes leaf withering. A region-wide outbreak of mealybug in sub-Saharan Africa was brought under control with the introduction of a natural enemy from South America, Anagyrus lopezi, a tiny wasp that lays its eggs in the pest (the growing larvae kill their host). A serious outbreak of mealybug in Thailand in 2009 was overcome rapidly by the release of 3 million pairs of A. lopezi across the infested area.
- Cassava mites are a major pest in all cassava-producing regions. The introduction of green mites from Latin America devastated African cassava production in the early 1970s. They were brought under control by a predatory mite introduced from Brazil, which reduced substantially the damage caused by the pest. Cassava mites can also be controlled using resistant or tolerant varieties, and by fertilizing the crop to improve plant vigour.
Some cassava pests and diseases have been accidentally introduced on plant species closely related to cassava, such as Jatropha curcas. Special care must be taken in moving vegetative planting material of related species between countries, and large Jatropha plantations should not be located in cassava-growing regions.
Cassava’s slow initial growth gives weeds a chance to emerge and compete strongly for sunlight, water and nutrients. “Save and Grow” cultural practices to control weeds include encouraging vigorous early growth with fertilization and – in the dry season – drip irrigation, covering the soil with mulch, and intercropping with fast-growing, short-duration crops that can be harvested when the cassava canopy closes and weeds are shaded out.
Hand-weeding at 15, 30, 60 and 120 days after planting has helped achieve cassava root yields of 18 tonnes per ha, only 8 percent less than those obtained when weeds were controlled with herbicides.
However, herbicides are often used on larger farms or when labour is unavailable or is too expensive. Since many products are highly toxic, farmers need to exercise care in the choice of herbicide and follow the advice of local plant protection specialists. Products should be locally registered and approved, and carry clear instructions for their safe handling and use.
Save and Grow: Cassava (FAO, 2013) can be purchased from [email protected]