The following text is from Chapter 6 of FAO's Save and Grow Guide to Sustainable Cassava Production. http://www.fao.org/3/a-i3278e.pdf
The first line of defense against crop pests and diseases is a healthy agro-ecosystem. Because synthetic insecticide, fungicide and herbicide disrupt the natural crop ecosystem balance, “Save and Grow” seeks to minimize their use to the extent possible. It promotes instead integrated pest management (or IPM), a crop protection strategy that aims at enhancing the biological processes and crop-associated biodiversity that underpin production Crop losses to insects are kept to an acceptable minimum by deploying resistant varieties, conserving and encouraging biological control agents, and managing crop nutrient levels to reduce insect reproduction. Diseases are controlled through the use of clean planting material, crop rotations to suppress pathogens, and elimination of infected host plants. Effective weed management entails timely manual weeding and the use of surface mulches to suppress weed growth. When necessary, low-risk selective pesticides may be used for targeted control, in the right quantity and at the right time. Since all pesticides are potentially toxic to people and the environment, the products employed must be locally registered and approved, and carry clear instructions on their safe handling and use. Like all major crops, cassava is vulnerable to pests and diseases that can cause heavy yield losses. Their impact is most serious in Africa. Until recently, Asia had few serious pest and disease problems, but this may be changing as the crop is grown more intensively over larger areas and planted throughout the year for industrial processing. When pest or disease management measures become necessary, a strategy of non-chemical control should be considered before any decision is taken to use pesticide. Since cassava is a long-season crop and exposed to pests and diseases for an extended period, a pesticide is usually ineffective and hardly ever economic. That is why insecticide, for example, should be used only in short-term, localized applications in“hotspots” where the pest is first observed, and only when the pest is in its early stage of development. A range of non-chemical measures can help farmers reduce losses to pests and diseases while protecting the agro-ecosystem, planting material should be of varieties with tolerance or resistance to the most important cassava diseases and pests, and taken from mother plants that are free of disease symptoms and signs of attack. As an extra precaution, stakes can be soaked in hot water to kill pests or disease-causing organisms that might be present. In extreme cases, soaking stakes in a solution of fungicide and insecticide may be necessary. However, farmers who do so must have received training in the correct use of pesticide and, in selecting chemicals, should follow the recommendations of local plant protection specialists. Ecosystem-based practices, such as mulching, planting hedges and intercropping, can provide refuges for natural enemies of insect pests. Building up soil organic matter increases pest-regulating populations early in the cropping cycle. During crop growth, applying adequate amounts of mineral fertilizer or manure to the crop can enhance its resistance or tolerance. Insecticide should not be applied to the leaves of the growing cassava plant, as it may kill natural biological control agents that help to keep some major pests and diseases under control. For example, insecticide kills cassava mites’ natural enemies – phytoseiid mite predators – before killing the mites themselves. When natural predators are eliminated, there's usually an increase in the pest population, to which farmers may respond with increased use of pesticide, thereby perpetuating and worsening the cycle of pest damage. Biopesticides, such as extract of neem seed oil, are recommended for controlling whiteflies, mealybugs and variegated grasshoppers. Whitefly and mealybug numbers can also be reduced with sticky traps and by spraying plants with soapy water.
Although the largest number of cassava diseases is found in Latin America and the Caribbean, the plant’s centre of origin, many of them are now also found in sub-Saharan Africa and Asia. Some have evolved separately in Africa and Asia, and have not yet arrived in the Americas.
Bacterial blight is one of the most widespread and serious of the cassava diseases. Caused by the proteobacterium Xanthomonas axonopodispv.manihoti, it is transmitted mainly by infected planting material or infected farm tools. It can also be spread from one plant to another by rain splash, and by the movement of people, machines or animals from infected fields to healthy fields. The bacterium infects first the leaves, which turn brown in large patches and eventually die, then the vascular tissues of the petioles and woody stems. The effect of bacterial blight on yields varies according to factors such as location, variety, weather patterns, planting time and the quality of planting material. In 1974, the disease caused losses of 50 percent in large plantations in Brazil. Bacterial blight can also threaten food security by reducing the production of cassava leaves, which are an important source of vegetable protein in Central Africa. Although potentially devastating, bacterial blight can be controlled effectively with“Save and Grow” practices.
Viral diseases are usually transmitted through the use of infected planting material. In addition, whiteflies – mainly of the species Bemisia tabaci– are vectors for viruses that cause cassava mosaic disease (CMD) and cassava brown streak disease (CBSD).
78 Save and Grow: Cassava Cassava mosaic disease is endemic in sub-Saharan Africa. Common symptoms include misshapen leaves, chlorosis, mottling and mosaic. Plants suffer stunting and general decline, and the more severe the symptoms, the lower the root yield. In the mid-1990s, an unusually severe form of CMD caused yield losses of 80 to 10 p0ercent in parts of Kenya and Uganda.CMD is also the most serious cassava disease in India and SriLanka, where it can lead to root losses of upto9 percent in traditional varieties Cassava brown streak disease causes corky necrosis in roots that renders the mun fit for consumption. The disease has been responsible for total crop failures in parts of Africa’s Great Lakes region. In 2011, FAO warned that none of the cassava varieties grown by farmers in the region seemed to be resistant to CBSD. Even plants produced from clean planting material can become infected through the transmissionofthevirusbyB. tabaciwhiteflies from infected plants in neighbouring plots. Because the symptoms of CBSD may not be evident on the cassava leaves or stems, farmers may not be aware that their crops are infected until they harvest the roots. The lack of above-ground symptoms makes the use of disease-infected planting material more likely. Two key recommendations for control of both CMD and CBSD are strict enforcement of quarantine procedures during an international exchange of cassava germplasm, and cultural practices, especially the use of resistant or tolerant cultivars and virus-free planting material. A major effort has been made to produce and distribute CMD and CBSD-free planting material in the Great Lakes region. January 2012 saw the release in the United Republic of Tanzania of four high- yielding cassava varieties, bred through marker-assisted selection, that are resistant to CMD and tolerant to CBSD. A decade of intensive research at Kerala’s Central Tuber Crops Research Institute identified a Nigerian variety and the wild species, Manihot caerulescen,sas resistant to both the Indian and Sri Lankan mosaic viruses. Researchers have used those two donor parents and crossed them with high-yielding local varieties to produce several promising lines resistant to CMD, one of which has become popular in the industrial cassava belts of Tamil Nadu Root rotsoccur mainly in poorly drained soils during very intense rainy periods, and are common in Africa, Asia and Latin America. They are caused by a wide range of fungal and bacterial pathogens,
Chapter 6: Pests and Diseases and lead to loss of leaves, dieback in stems and shoots, and root deterioration, either as the crop grows or during post-harvest storage. Farm tools and plant residues left in fields post-harvest are often contaminated with disease-causing fungi and are sources of spores that infect new plants. In trials in Colombia’s Amazon region, smallholder farmers eliminated cassava root rot using simple “Save and Grow” practices. They planted stakes taken only from healthy mother plants, used a mixture of ashes and dry leaves as a soil amendment and fertilizer during planting, and intercropped cassava with cowpeas 3. Other cultural practices that control root rots include: ▯ If no disease-free planting material is available, immerse stakes in hot water for around 50minutes ▯ Plant on light-textured, moderately deep soils with good internal drainage ▯ Improve drainage by reducing tillage and using surface mulches ▯ Rotate cassava with cereals or grasses ▯ Uproot and burn diseased plants An effective biological control for root rot is immersion of the stakes in a suspension of Trichoderma viri, a fast-growing species of soil fungus that parasitizes the vegetative tissue of other soil-borne fungi3, . In experiments in Nigeria, two groups of stored cassava roots were inoculated with four pathogenic fungi. One group was also inoculated with culture filtrate of T.Viride. Over a period of three weeks, the group without. vi ride suffered an incidence of not ranging from 20 to 44percent; in the group inoculated with the biocontrol agent, there was a drastic reduction in the range and number of the target fungi, with the incidence of not ranging from zero to 3 percent after three weeks. Inoculation with T. viriderendered unnecessary repeated spraying with synthetic fungicide Control of major insect pests round 200species of arthropod pests has been reported on A cassava. Of these, some are specific to the crop, while others attack other crops as well. The greatest diversity of cassava insect pestsisfoundinLatinAmerica, where they have co-evolved with the crop. However, cassava pest problems are not necessarily more serious
80 Save and Grow: Cassava in Latin America – many harmful insects are kept under control by predators and parasitoids, which have co-evolved over the centuries 4, Whiteflies feed directly on young cassava leaves and are also a virus vector, making them probably the most damaging insect pest in all cassava-producing regions. In Latin America, white fly species have been reported on cassava, including leurotrachelus social, is. a epim and Trial euro die variable, which causes the most damage. The whitefly Bemisia Tabac, the vector of cassava mosaic disease and cassava brown streak disease, is found in most of sub-Saharan Africa and now in India. It is also present in Latin America but does not feed on cassava. Another species, leurodicus disperses or spiraling whitefly, Bemisia tabaci transmits is found in India, Lao PDR, and Thailand, as well as in Africa, and can serious viral diseases to cause serious damage and yield losses. cassava plants Although many farmers use insecticides to control whitefly infestations, spraying is usually ineffective – social is whiteflies, for example, double their numbers in less than five days. Not spraying insecticide, on the other hand, allows biological control by the whitefly’s natural enemies, which include many Figure28Meannumberofadultwhiteflies species of parasitoids, predators and on cassava leaves, Cameroon 40 entomopathogens. A two-year experiment in Cameroon Cassava found that intercropping cassava with Cassava + maize and cowpeas was associated 30 +cowpea with a drop of 50percent in the adult whitefly population and a 20 percent reduction in the incidence of cassava 12 mosaic diseases (Figure 28) . Research 20 in Colombia suggests that intercrop - ping with cowpeas depresses cassava lef growth, making the plant less a- p petizing to whiteflies. Less vigorous 10 growth did not affect root yields – in fact,yieldlosseswereonly13 percent in the cassava/cowpea system, but a high 13 as 65 percent in the monoculture. 0 Other recommended control me- a 4 6 8 10 12 14 16 sures include imposing a “closed se -a Weeksafterplanting son”, when no cassava can be present in Source: Adapted from Fondong, V.N., Thresh, J.M.&Zok, S.2002. Spatial and temporal spread to break the whitefly’s of cassava mosaic virus disease in cassava grown alone and when intercropped with maize and/or cowpea. J.Phytopathology, 150:365-374.
82 Save and Grow: Cassava Figure29 Area infested by cassava mealy bugin Thailand, 2009-2012(‘000ha) 250 1 Anagyrus wasp importedfrom Benin 200 4 2 Waspundergoestrials 3 Waspreleasedin25villages 3 4 Waspreleasednationwide 150 1 100 50 2 0 14May 8 De 15Jan 24Dec Jan 9 De15 Jan 30 Jul 2009 2010 2011 2012 Source:Rojanaridpiched,C.,Thongnak,N., Jeerapong, L.&Winotai, A.2012. Rapidresponsetotheaccidentalintroductionofthemealybug, Phenacoccusmanihoti, inThailand. FactsheetpreparedforFAO.(mimeo) the infested cassava area.The biological control campaign washighly successful – the infested area was reduced to 17 0000ha in 2010, to 64000ha in 2011 and just 3300ha in 2012 (Figure 29) 1. Current recommendations for the control of cassava mealybugs include: Conserve the population of natural enemies by not spraying ▯ synthetic pesticide ▯ If necessary, treat planting material with a solution using a locally registered and recommended insecticide ▯ Monitor cassava plantations every 2w toe4kstodetectfocalpoints of infestation ▯ Remove and burn the infested parts of plants Avoid the movement of planting material from one region to another ▯ ▯ Minimize the movement of planting material from infested to non-infested fields Cassava mitesare an important insect pest in all producing regions. The cassava green mite,Mononychellus tanajoa , causes the most damagetocassavainLatinAmericaandsub-SaharanAfrica,especially