Cassava Pests and Diseases: Save and Grow

Pests and Diseases of Cassava

The following text is from Chapter 6 of FAO's Save and Grow Guide to Sustainable Cassava Production. 

The first line of defence against crop pests and diseases is a healthy agro-ecosystem. Because synthetic insecticide, fungicide and herbicide disrupt the natural crop ecosystem balance, “SaveandGrow” seeks to minimizet their usetothe 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, 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. Buildingupsoilorganicmatterincreasespest-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. Insecticideshouldnotbeappliedtotheleavesofthegrowingcassava 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.


Control of major cassava diseases

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.

Use varieties with good tolerance (many tolerant, high-yielding varieties are now available)
Use healthy planting material from disease-free plants or plants derived from meristem culture, rooted buds or shoots
Before planting, treat stakes by soaking them in hot water at 50°C for about 50 minutes. In extreme cases, and on the advice of local plant protection specialists, stakes may be soaked for 10 minutes in a solution of cupric fungicides
Plant at the end of rainy periods
After using tools in blight-infected plots, sterilize them in hot water or in a dilute solution of a disinfectant, such as sodium hypochlorite Ensure that the plants are adequately fertilized, especially with  potassium
Uproot and burn any diseased plants and infected crop residues
Intercrop cassava with other species to reduce plant-to-plant dissemination of bacterial blight caused by rain-splash (fast growing crops such as maize will also reduce dissemination by wind)
To prevent the carry-over of the disease in the soil, rotate cassava with other crops, or leave the field in fallow for at least six months  between cassava crops

 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 Cassavamosaicdiseaseisendemicinsub-SaharanAfrica.Common symptomsincludemisshapen 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 10p0ercent in parts ofKenyaandUganda.CMDisalsothemostseriouscassavadiseasein IndiaandSriLanka,whereitcanleadtorootlossesofupto9 percent in traditional varieties Cassava brown streak disease causes corky necrosis in roots that rendersthemunfitforconsumption.Thediseasehasbeenresponsible 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 arestrictenforcementofquarantineproceduresduringinternational exchangeofcassavagermplasm,andculturalpractices,especiallythe use of resistant or tolerant cultivars and virus-free planting material. AmajorefforthasbeenmadetoproduceanddistributeCMD-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: ests and Diseases79 and lead to loss of leaves, dieback in stems and shoots, and root deterioration,eitherasthecropgrowsorduringpost-harveststorage. 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 ofTrichoderma viri, a fast-growing species ofsoilfungusthatparasitizesthevegetativetissueofothersoil-borne fungi3, . In experiments in Nigeria, two groups of stored cassava rootswereinoculatedwithfourpathogenicfungi.Onegroupwasalso inoculated with a culture filtrate ofT. viride. Over a period of three weeks, the group withouTt. viridesufferedanincidenceofrotranging from 20 to 44percent; in the group inoculated with the biocontrol agent, there was a drastic reduction in the range and number of the targetfungi,withtheincidenceofrotrangingfromzeroto3 percent after three weeks. Inoculation with T. viriderendered unnecessary repeated spraying with synthetic fungicide Control of major insect pests round 200species of arthropod pests have 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,wheretheyhaveco-evolvedwiththe crop.However,cassavapestproblemsarenotnecessarilymoreserious


80 Save and Grow: Cassava in Latin America – many harmful insects are kept under control by predatorsandparasitoids,whichhaveco-evolvedoverthecenturies 4, . Whitefliesfeed directly on young cassava leaves and are also a virus vector, making them probably the most damaging insect pest in all cassava-producingregions.InLatinAmerica,1w 1hiteflyspecieshave beenreportedoncassava,includinA gleurotrachelus social,isA.aepim andTrialeurodes variabili,swhich cause most damage. The whitefly Bemisia tabac,i 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.AnotherspeciesA , leurodicus disperse,sorspirallingwhitefly, Bemisia tabacitransmits isfoundinIndia,LaoPDRandThailand,aswellasinAfrica,andcan serious viral diseases to cause serious damage and yield losses. cassava plants Although many farmers use insecticides to control whitefly infestations,sprayingisusuallyineffective–A. socialiswhiteflies,for example,doubletheirnumbersinlessthanfivedays.Notsprayingin - secticide,ontheotherhand,allowsbiologicalcontrolbythewhitefly’s natural enemies, which include many Figure28Meannumberofadultwhiteflies species of parasitoids, predators and oncassavaleaves,Cameroon 40 entomopathogens. Atwo-yearexperimentinCameroon Cassava found that intercropping cassava with Cassava +maize 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 disease (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 percentin thecassava/cowpeasystem,butashigh 13 as 65percent 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”,whennocassavacanbepresentin Source:AdaptedfromFondong,V.N.,Thresh,J.M.&Zok,S.2002.Spatialandtemporalspreadtobreakthewhitefly’s ofcassavamosaicvirusdiseaseincassavagrownaloneandwhenintercroppedwithmaizeand/orcowpea. J.Phytopathology,150:365-374.


Chapter 6P : ests and Diseases81 development cycle (although, this may not be as effective with some species, such asB. tabaci, that have multiple hosts). Recent trials in Colombiaindicatethatplantingdifferentcassavavarietiesinthesame field may reduce herbivore load and increase yields in zones subject to heavyT. variabilisattacks1. Mealybugsfeed on cassava stems, petioles and leaves, and inject a toxin that causes leaf curling, slow shoot growth and eventual leaf withering.Yieldlossininfestedplantscanbeupto60percent of the roots and 100percent of the leaves. Of the approximately 1s 5pecies of mealybug that attack cassava plants, two – Phenacoccus herrini andP. manihoti– cause major damage to cassava in Latin America. In the early 1970s,P. manihotiwas accidentally introduced into sub-Saharan Africa, where it had no natural enemies, and spread Mealybugs have rapidlythroughouttheregion’scassavagrowingareas.Themealybug devastated cassava fields population was brought under control by the introduction of several in sub-Saharan Africa natural enemies from South America. The most effective predator and Thailand wasAnagyrus lopez,ia tiny wasp: the female wasp lays its eggs in the mealybug and the growing larvae kill their host. P. manihotiwas recently introduced inadvertently into Thailand andwithinayearithadspreadthroughoutthecountry.Atitspeak,in May 2009, it affected 230000ha of Thai cassava-growing land. The outbreak devastated the 2010 cassava harvest, which fell to 22m.7il liontonnes, from a record of 30milliontonnes the year before. HowThaiauthoritiesandfarmersrespondedtothe2009mealybug outbreak provides an excellent example of the effectiveness of b- iologi cal pest control. To avoid new outbreaks, farmers were advised not to plant cassava in the late rainy season and early dry season, and to soak stakes in an insecticide solution before planting. They were also warned to avoid spraying insecticides on the plants themselves – Natural enemy of cassava experience had shown that spraying provoked the pest’s resurgence. mealybugs – the tiny Tocontroloutbreaks,researchersidentifiedseveralnativepredators waspAnagyrus lopezi andparasitesbutconcludedtheywereunabletoeffectivelyreducethe mealybug population. They suggested the useA ofagyrus lopez,ithe wasp that had successfully controlled the mealybug in Africa in the 1970s. In September 2009, some 500 adults of A. lopeziwerehand- carried to Bangkok from IITA’s Biological Control Centre in Benin. After quarantine laboratory tests and field trials, the government beganlarge-scalemultiplicationanddistributionofthewasp.ByMay 2012, almost 3millionpairsofA. lopezihadbeenreleasedthroughout


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


Chapter 6P: ests and Diseases83 inlowlandareaswithaprolongeddryseason.Itfeedsontheunderside of young leaves, which become white-yellow, deformed and smaller. The mite can cause root yield losses of up to 80 percent. Another greenmitespecies,M. mcgregor,iwasrecentlyreportedinCambodia, China and Viet Nam. Although it may not be as aggressive a Ms . tanajoa, it could cause serious damage owing to the lack of primary natural enemies. The introduction of green mites on cassava imported from Latin Americaintheearly1970sdevastatedAfrica’scassavaproduction.To bring the mite under control, entomologists at IITA and CIAT first identified its area of origin in South America and its natural enemy, anothermite,fromBrazil.TheBrazilianmitessurvivedinAfricabut their diffusion was very slow. Thesolutionwasanotherpredatorymite T,etranychus arip,owhich spread rapidly in African farmer’s fields and did not have a voracious appetite for green mites – an advantage, since it allows enough green mites to survive and prevent the predatory mites from dying out. As Other natural enemies of insect pests worth wellasreducingthedamagecausedbygreenmitesthroughoutAfrica, protecting: Coccinellidae T. aripo has contributed substantially to the science of biological beetles (top) and the control and to the knowledge of how mites work in complex food African lacewing systems 1. Many species of red spider mites have been observed on cassava in all three cassava-producing regions. It is the most prevalent dry season pest of cassava in Asia, where the most common species are Tetranychus urticaeand T. kanzawai.Yield losses range from 18 to almost50percent. Red mites feed mainly on the underside of leaves, but attack old leaves at the base of the plant, causing considerable webbing. Further research is urgently needed to identify the most effective natural enemies of red spider mites. Current recommendations for the control of cassava mites include: ▯ Plant resistant or tolerant varieties, if available Inendemicareas,treatstakeswitharecommended,locallyapproved ▯ insecticide ▯ Promote good establishment by planting early in the wet season ▯ Apply adequate and well-balanced fertilizers to improve plant vigour ▯ Apply foliar sprays with water at high pressure to reduce mite populations ▯ Strictly enforce quarantine regulations


84 Save and Grow: Cassava Other important peststhat are found only in Latin America are the cassava hornworm, burrowing bugs, leaf-cutter ants, shoot flies and fruit flies. Great care needs to be taken to avoid accidentally introducingthosepestsfromLatinAmericatoAfricaandAsia,where they have no natural enemies and could, therefore, do great damage. A newly identified menace in Asia – found in Cambodia, Lao PDR, the Philippines, Thailand and Viet Nam – is witches’ broom disease, which is thought to be caused by a phytoplasma. Some cassava pests and diseases have also been accidentally introduced on other plant species closely related to cassava, such as Jatrophacurcas , which is used as “living fences” in Asia and has become popular recently as a source of biofuel. Special care must be takeninmovingvegetativeplantingmaterialofrelatedspeciesbetween countries, and largeJatrophaplantations should not be located in cassava growing regions. Weed management omparedtomanyothercrops,theinitialgrowthofcassavaisslow. C That, combined with the wide spacing between planted stakes, gives weeds a chance to emerge and compete for sunlight, water and nutrients. In the first four months after planting, cassava can easily be overwhelmedbycompetitionfromnarrow-leafgrassyweedsandfrom broad-leaf weeds, which include many leguminous plants. In East Africa, weeds are often a more serious production constrain17than insect pests or diseases and can reduce yields by about 50percent In Nigeria, farmers spend more time on weeding than on any other aspect of crop production. Once the cassava canopy has closed, it will shade out most weeds and keep the field almost completely weed-free9,. Six to eight monthsafterplanting,whencassavastartstoshedmanyleaves(es-pe cially during the dry season), weeds may reappear, but this generally do esnotseriouslyaffectyields.Excessivelateweedgrowthmaymake harvestingmoredifficult,butcanalsoprotectthesoilfromerosionif post-harvest rains are heavy.


Chapter 6P : ests and Diseases 85 “SaveandGrow”culturalpractice csanprovideaneffectivedefence against weeds. While cultural controls may not be 10p 0ercent effec- tive, they do help in reducing weed competition, and thus the need 21 for mechanical or chemical weeding . Cultural control begins with selectionofhigh-qualityplantingmaterialfromvarietieswithvigorous early growth and tolerance or resistance to important diseases and pests.Highplantingdensityandthecorrecttypeandrateoffertilizer, applied in short bands next to the planted stakes, can stimulate early crop growth and rapid canopy closure. Planting in the dry season under drip irrigation can also encourage the growth of cassava but not that of weeds. To prevent weed emergence, the soil should be covered with a thick layer of mulch, such as rice straw or maize residues. Another “Save and Grow” recommendation is to intercrop cassava with fast- growing plants, such as melons, squash, Figure30Effectofhandweedingonfreshcassavaroot pumpkins, common beans, groundnuts, yield280daysafterplanting,Colombia(t/ha) soybeans, mungbeans and cowpeas. As 25 those are short-duration crops, they can be harvested after about 3 to 4 months, when 20 the cassava canopy closes and weeds are shaded out. While intercrops may reduce 15 cassava root yields, they markedly reduce weedgrowth,andofferaneco-friendly–and lessexpensive–alternativetosprayingwith 10 herbicides. A study in Nigeria of legume 5 covercropsinamixedcassava/maizesystem reportedsignificantimprovementsincassava rootyieldswhenvelvetbeansweregrownto 0 18 0 1 2 3 4 Chemical suppress weeds . Numberofhandweedings control Many smallholder cassava farmers use Source:AnnexTable6.1 mechanical control measures. Most co -m monly, they remove weeds by hoeing, starting about 15 days after planting, or after emergence if the cassava is planted horizontally. ResearchinColombia(Figure30)foundthatwithhand-weedingat15, 30, 60 and 120 days after planting, cassava root yields were 1tonnes per ha, only 8percent less than those obtained when weeds were controlled with herbicides. When weeds were not controlled at all, yieldsfelltojust1.4 tonnes. Weeds growing between the rows can also be incorporated into the soil using an oxen- or buffalo-drawn cultivator or, where


86 Save and Grow: Cassava available, tractors equipped with cultivator blades. In the absence of both machinery and draught animals, farmers in Thailand use a manually-drawn cultivator, known as a “poor man’s plough”. In VietNam, farmers use a contraption made from the handlebar and front wheel of a bicycle, with a cultivator blade attached behind the wheel.Thisoperationisusuallyfollowedbyhandweedingwithahoe between the plants in the row. On larger farms or when labour is unavailabl or is too expensive, weedsareoftencontrolledwithherbicides.Manyherbicidesarehighly toxicand,beingwatersolubleandpersistentintheenvironment,can be washed away to contaminate ground and surface water. Farmers need to exercise care in the choice of the herbicide to be used and follow the advice of local plant protection specialists. Pre-emergence herbicides do not kill existing weeds. Instead, they preventweedseedsinthesoilfromemergingor,atleast,reducetheir rateofgrowth.Pre-emergenceherbicidesareeitherincorporatedinto thesoilbeforeplantingorappliedonthesoilsurfacewithaknapsack sprayerimmediatelyafterplanting.Pre-emergenceherbicidesthatare selective for cassava can be applied over the vertically planted stakes without affecting cassava sprouting or yield. Theapplicationofpre-emergenceherbicidescanmaintainacassava field almost weed-free for 6 to 8 weeks after planting. Farmers may applyamixtureoftwoherbicides–onethatcontrolsthegrassyweeds and one the broad-leaf weeds. A lower dosage is recommended on light-textured soils, while a higher dosage may be needed in heavy soils,suchasloamyclays.Specialcareneedstobetakenwhencassava is grown in association with other crops, because the pre-emergence herbicides normally used for cassava may harm the intercrop. At about two months after planting, weeds may need to be controlled again to reduce competition with cassava. This is usually done by hoeing or using an animal- or tractor-mounted cultivator, dependingontheheightofthegrowingcassavaplantsandtheextent of canopy closure. When most of the weeds are grassy species, it is also possible to apply a selective post-emergence herbicide, which kills grasses but does not affect the cassava plant. Post-emergence herbicides can be used about 4 tom 5onths after planting, when some bottom leaves start to drop off. They should only be applied on windlessdaysandwithanozzleshieldtopreventsprayfromreaching the cassava stems or leaves.