Crop Adaptation to Climate Change (Yadav/Crop Adaptation to Climate Change) || Adaptation of Cassava to Changing Climates

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Chapter 19

Adaptation of Cassava to Changing ClimatesHernan Ceballos, Julian Ramirez, Anthony C. Bellotti, Andy Jarvis, and Elizabeth Alvarez

Introduction

Cassava (Manihot esculenta Crantz), along withmaize, sugarcane, and rice, constitutes the mostimportant sources of energy in the diet of mosttropical countries of the world. The species orig-inated in South America (Allem et al. 2001;Olsen and Schaal 2001) and was domesticatedless than 10,000 years ago. With the creation ofthe International Institute of Tropical Agriculture(IITA) in Nigeria and the International Centerfor Tropical Agriculture (CIAT) in Colombia inthe early 1970s, a new era began for cassavawith the implementation of successful breed-ing projects, modernization of cultural prac-tices, and development of new processing meth-ods (Cock 1985; Jennings and Iglesias 2002;Ceballos et al. 2007). National research centersin Brazil, China, Colombia, Cuba, India, Nige-ria, Thailand, Uganda, and Vietnam among manyother countries have also conducted successfulresearch on cassava.

Cassava is an important crop in regions atlatitudes between 30◦N and 30◦S, and from sealevel up to 1800 m above sea level. The maincassava-growing areas of the world were de-scribed by Ceballos et al. (2007). Cassava isa perennial species grown as an annual crop.This feature explains the remarkable plasticity

of cassava to adapt to a great diversity of en-vironments. It can grow in the Amazon basinin areas with more than 3000 mm rainfall peryear as well as in nearby semiarid conditionsof Pernambuco State of Brazil with less than500 mm of rainfall per year. Cassava can becomedormant when environmental conditions are notconducive for growth due to biotic or abioticconstraints. In northeastern Brazil and many sub-Saharan African regions, leaves of the plant fallduring lengthy periods without rains and growthis reinitiated when the rains return. In these con-ditions harvest takes place at the end of the sec-ond period of growth (two years after planting).Another important characteristic of cassava isits vegetative reproduction. The multiplicationrate of cassava is low. A plant typically producesonly 5–10 cuttings. The multiplication rate inmaize, in contrast, can be 1:500. Planting mate-rial is bulky (a truckload of stems are necessaryto plant one hectare) and can only be stored fora few weeks. These characteristics of the com-mercial multiplication of cassava will detrimen-tally influence the impact of climate change onthe crop.

All 98 species of the genus Manihot are nativein the Neotropics from where it was introducedto other regions of the world (Rogers and Appan1973). The origin of cultivated cassava is still

Crop Adaptation to Climate Change, First Edition. Edited by Shyam S. Yadav, Robert J. Redden, Jerry L. Hatfield,Hermann Lotze-Campen and Anthony E. Hall.c© 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.

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412 CROP ADAPTATION TO CLIMATE CHANGE

unclear. Two relevant questions were raised byAllem (2002) regarding the botanical origin(parental wild species that eventually led to theemergence of M. esculenta): the geographic areawhere this emergence took place and the regionwhere it was domesticated (agricultural origin).The prevailing hypothesis is that cultivated cas-sava originated in South America (Allem 2002;Olsen and Schaal 2001). But, the region(s) andmechanism(s) of cassava domestication are stillunder discussion.

Although it is frequently considered a poly-ploid species, analyses conducted during diaki-nesis and metaphase I indicate the presence of18 small and similar bivalents in cassava (Hahnet al. 1990; Wang et al. 2011). In some cases, oc-currence of univalents/trivalents and late bivalentpairing has been observed. Cassava is, therefore,a functional diploid (2n = 2x = 36) (De Carvalhoand Guerra 2002; Jennings 1963; Nassar andOrtiz 2008). It has been suggested that certainportions of the genome may be duplicated and,therefore, cassava may be a segmental allote-traploid (Magoon et al. 1969).

Rogers (1963) listed M. carthaginensis, M.aesculifolia, M. grahami, M. flabellifolia, andM. saxicola as the most closely related speciesto cultivated cassava on the basis of morpho-logical, ecological, and geographical evidence.Allem postulated in 1994 and 1999 that moderncultivated cassava originated directly from wildrelatives of M. escultenta subsp. flabellifolia.This suggestion was further supported by Olsenand Schaal (2001). Nassar and Ortiz (2008), onthe other hand, suggested that cultivated cassavaarose as a result of hybridization of two speciesand proposed that M. pilosa was one of them.The crop may have been domesticated more thanonce (Allem 2002; Nassar and Ortiz 2008).

Allem and coworkers suggested in 2001 thatthere are three M. esculenta subspecies: escu-lenta (cultivated cassava) flabellifolia, and pe-ruviana. These three subspecies along with theclosest wild relative (M. pruinosa) constitute theprimary gene pool. The morphological charac-

teristics of cultivated cassava are highly variableand there are numerous morphological descrip-tors that can be used for cultivar characterization(Alves 2002). However, within cultivated cas-sava the concept of Varietal Group has not beenused. The secondary gene pool includes M. tri-phylla, M. pilosa, M. brachyloba, M. anomala,M. epruinosa, M. gracilis, M. tripartita, M. lep-tophylla, M. pohlii, M. glaziovii, M. dichotoma,M. aesculifolia, and M. chlorosticta.

Cassava is an interesting case because eventhough relatively little is known about its ge-netics, a molecular map has already been de-veloped (Fregene et al. 1997, 2000; Mba et al.2001). In addition, many studies of genetic vari-ability among and within landraces have beenconducted (Asante and Offei 2003; Elias et al.2001a, 2001b, 2004; Peroni and Hanazaki 2002;Sambatti et al. 2001; Zaldivar et al. 2004). Ingeneral, the vegetative reproduction of the crophas not necessarily led to a drastic reduction ofgenetic variability, particularly when it is usedas a reliable food security staple crop. Cassavanaturally outcrosses, typically generating seedwith high levels of heterozygosity that can beexploited successfully as demonstrated in the dif-ferent articles published by Elias and coworkers(Elias et al. 2001a, 2001b, 2004). However, whencassava is used for industrial processing (suchas in SE Asia), large areas have been plantedto a few successful varieties such as the Thaivariety KU50. Genetic diversity in areas wherecassava is used for processing by different in-dustries tends to be small and depends on therate of development of new commercial varietiesand multiplication schemes used for plantingmaterial.

Expected climatic changesand the cassava models

The quantitative analysis is composed of fivestages. Beebe et al. (this book) have provided de-tailed descriptions of the materials and methodsused in this paper to predict effects of climate



ADAPTATION OF CASSAVA TO CHANGING CLIMATES 413

Fig. 19.1. Expected changes in precipitation and temperatures in cassava-growing areas of the world by2020s for the SRES-A2 emission scenario (average of four GCM patterns).

change on cassava. For the initial part of thisanalysis, current and future downscaled climatesurfaces are the same as described in Beebe et al.(this book). Using these surfaces and the cur-rent distribution of the crop (FAOSTAT 2009),changes in rainfall and temperature were pre-dicted (Fig. 19.1). Average warming in cassavagrowing regions by the 2020s ranged from 0.7◦C(Asia, Caribbean) to 1.3◦C (sub-Saharan Africa,South America), a lower range than for othercrops such as beans, and potatoes, which arealso cropped in highlands where predicted tem-perature changes are greater. Some 24% of thecountries are predicted to have increases in tem-peratures above 1◦C. Subtropical environments(Asia, southern Brazil, northern Argentina, andChile) are predicted to have less temperature in-crease than tropical areas.

Rainfall requirements are a critical issue forcassava, since the roots are particularly sensi-tive to waterlogging, causing rotting and lossof agricultural yields. Increases in precipitationsare predicted for 49% of the growing areas ofwhich 30% show increases >50 mm per year.Many important production areas of the world(accounting for more than 52 million tons in 2007

according to FAOSTAT 2009) are predicted toreceive a substantial increase in rainfall. Liberiaalone produced 550,000 tons in 2007 (FAOSTAT2009) and could experience an average increaseof 137 mm. On the other hand, some 17% ofthe growing areas are projected to experiencedecreases in rainfall, most of them are locatedin Central America, the Caribbean, and in SouthAmerica (Haiti, Dominica, Trinidad and Tobago,Nicaragua, Panama Costa Rica, Peru, Colombia,and Venezuela). Uncertainties given by the co-efficient of variation (CV) remain relatively low,as expected for a period of about 15 years (97%of the areas show less than 15% in CV).

Calibration of the Ecocrop model for cas-sava was done using data from CIAT (2002)and an iterative process from which the mostaccurate predictive model was selected (the onewhich matched the best known distribution ofthe crop according to several experts on the cropincluding MAS El-Sharkawy, personal commu-nication, March 2008). Duration of the growingseason was set to 240 days, minimum killingtemperature to 0◦C, minimum absolute tempera-ture to 15◦C, minimum optimum temperature to22◦C, maximum optimum temperature to 32◦C,




and maximum absolute temperature to 45◦C.The minimum and maximum optimum rainfallswere set to 800 and 2200 mm, respectively, andthe minimum and maximum absolute rainfallswere set to 300 and 2800 mm, respectively.

According to the Ecocrop model, tropical cli-matic adaptations of cassava are considerablybroad making it suitable across most regionsof the tropics. Highly suitable areas were pre-dicted to be located in central Africa (CentralAfrican Republic, Democratic Republic ofCongo, Congo, Angola), matching the knowndistribution of the crop (Carter et al. 1992;CIAT 2002; FAOSTAT 2009). NortheasternMozambique (Cabo Delgado, Nassa, Nampula,Zambezia, Sofala, Manica), eastern Malawi, aswell as most of western Africa below the Sa-hel were also predicted to be highly suitableareas (Guinea, Liberia, Ivory Coast, southernGhana, Cameroon, Nigeria). Plain areas near theAndes (Central Colombia, western Ecuador, cen-tral Peru, northern Bolivia, and northeasternParaguay) were also predicted to be optimalplaces for growing cassava, and the same waspredicted for the Atlantic coasts of Colombiaand Mesoamerica.

Crop suitability in subtropical regions(Fig. 19.2b) of Brazil (Minas Gerais, Sao Paulo,Parana), eastern Paraguay, very northern Ar-gentina, southern China, northern India, Thai-land, Vietnam, and Myanmar was predicted tobe primarily constrained by temperatures, andthe same pattern was observed throughout cen-tral Africa, Madagascar, southeastern Brazil, andnorthern Mexico. Some areas in Brazil (west-ern Bahia, Piauu, eastern Ceara, Rio Grande doNorte), southeastern Bolivia and its borders withParaguay, Tanzania (Tabora, Singida, Dodoma,and southern Arusha), very northern Uganda,and southern Angola were predicted as beingconstrained by low rainfall (drought).

High rainfall (waterlogging) is predicted to bea constraint in the Amazon (Brazilian, Colom-bian, and Peruvian Amazon), the Guyanas andSuriname, as well as in the coasts of westernAfrica (Guinea, Sierra Leone, Liberia, Nigeria

and Cameroon), and several growing areas inIndonesia, Malaysia, Thailand, and Vietnam. De-spite the broad adaptation of cassava in terms oftemperatures, northern areas of India may expe-rience excessive heat and the large temperatureranges occurring in these areas indicate cassavaproduction also may be constrained by minimumtemperatures within the same growing season.

Future suitability of cassava production intropical regions (Figs. 19.2c and d) is predictedto be, on average, favored by climate change. Aglobal average increase of 2.5% is expected bythe 2020s, and when reductions are predicted,these are only between −1 and −9%. In additionto this limited negative impact, there could besome gains in subtropical environments (south-eastern Paraguay, southern Brazil), and in high-lands in the Andes, where the current croppedlands could expand upward. Significant losses inclimatic suitability are expected only in north-ern India (Uttar Pradesh and Madhya Pradesh),likely due to excess heat.

When the likely benefits of breeding for abi-otic constraints are analyzed, the most significantbenefits may result from increased drought andcold tolerance (Fig. 19.3). However, even theseimprovements are predicted to only bring an in-crease of less than 10% in the highly suitableareas. These traits could become more importantif further temperature increases occur after the2020s. About 0.7 million hectare of current cas-sava fields in the world, mainly located in areaswith low rainfall, are predicted to benefit fromimproved drought tolerance, but the biggest gaincould result from some 14.7 million hectares ofnew suitable area. Similar patterns are predictedfor cold tolerance by the 2020s, only 1.1 millionhectares of currently cropped lands would ben-efit, but some 14 million hectares of new areacould open up for cassava cultivation.

The suitability model only takes into accounttwo major factors: temperatures and rainfall.However, specific physiological responses of thecrop to increased levels of CO2, and interac-tions of these responses with changes in tem-peratures and rainfall, as well as soil conditions,




(a)

(b)

(c)

(d)

Fig. 19.2. (a) Predicted suitability for current conditions overlaid with locations of cassava farms, (b) current climaticconstraint for cassava cultivation, (c) predicted future suitability (2020) using an ensemble (average) of four GCM patterns,and (d) predicted suitability change (future to current) by 2020.




Fig. 19.3. Potential benefits for cassava production in 2020 from improvedtolerance to drought and waterlogging (top), and improved tolerance to cold andheat (bottom).

and changes in pest and disease dynamics mustalso be taken into account in a cassava produc-tion forecasting model in order to refine the pre-dictions. In addition, changes in growing seasonlength due to deficiencies in temperature or rain-fall levels in certain periods might cause signifi-cant changes and should be further explored.

Our results predict that future (2020s) cli-mates will maintain cassava production in almostall of the current production areas, with the ex-ception of a few isolated sites in Asia (northernIndia especially, which is not a major produc-tion area at this time). Furthermore, a substantialincrease (greater than 20%) in area of cassavasuitability was predicted for subtropical regions

in the southern hemisphere (Brazil, Argentina,and southern Africa).

Abiotic stresses: expected effects

Since cassava is a perennial plant, it is a veryplastic crop that can be grown in very contrast-ing environments. Cassava can naturally with-stand lengthy periods under severe water deficits.Those areas where climate change will result ina reduction of rainfall will require not only achange of the varieties grown by farmers butalso a change in cultural practices. If reductionof rainfall is very severe, farmers may need toextend the growing cycle for two years as it




is currently done in northeastern Brazil. Mostcassava grown in dry environments is affectedby different mite species. Therefore, it is ex-pected that the abiotic stress caused by droughtalso could result in increased mite pressure(and occasionally increased mealy bug pressureas well).

In many other cassava-growing regions, how-ever, there is an expected increase of rainfall.This may benefit many of these regions but mayalso have negative impact. Cassava is frequentlygrown in sloped land where heavy rains can havedevastating effects early in the growing cycle(first two months) when the soil remains unpro-tected and severe runoff may occur (Howeler2002). In these conditions, soil erosion can bevery severe. Changes in cultural practices may,therefore, be needed for these conditions suchas intercropping cassava and using contour andhedgerow plantings to reduce soil erosion. Ex-cess rainfall may result in an undesirable in-crease in incidence of root rots, especially inheavy soils or areas with limited drainage capac-ity. On the other hand, increases in rainfall willhave the positive effect of reducing mite pres-sure and also some insects such as whiteflies ormealybug.

Increase of atmospheric CO2 is one of the ma-jor causes for climate change and has increasedby 40% from a preindustrial revolution baseline.Increased levels of CO2 can be considered as andabiotic stress on cassava (Gleadow and Woodrow2002; Gleadow et al. 2009). These studies indi-cated that increases in atmospheric CO2 concen-tration could result in a reduction in root produc-tion. Concentration of cyanogenic glucosides inthe roots was not affected by increases in CO2.On the other hand, there was a large increase ofglucosides in the leaves of plants grown in higherCO2 concentrations.

Probably the most important impact of cli-mate change in cassava production will be onthe dry matter content (DMC) of cassava roots.Typically, farmers plant cassava at the beginningof the rainy season and harvest it at the end of thedry season (just before the return of the rains).

A key factor in this scheme is to enable DMCof the roots, at least in the varieties grown byfarmers, to reach a maximum at harvest time.This is important because when cassava is usedfor processing (dried chips and pellets for animalfeeding or the starch industry), its price is posi-tively linked to DMC of the roots. A low DMCmeans a longer period in the drying yards (for theproduction of dried chips) or larger amount ofeffluents (for the starch industry). Consequently,low DMC results in higher processing costs. Thisis the reason why cassava breeders have paidso much attention to DMC for the new vari-eties released for industrial purposes (Ceballoset al. 2007; Kawano 2003; Kawano and Cock2005).

When rains return, starch and other nutri-tional compounds stored in the roots are hy-drolyzed and growth is reinitiated. This meansthat, within 2–3 weeks upon the arrival of therains, DMC can be reduced drastically. In anevaluation conducted by CIAT of 1350 geno-types in the subhumid environment of the north-ern coast of Colombia, average DMC beforethe arrival of the rains was 32.4% and afterthe rains arrived, the average had decreased to26.7% (CIAT 2001). This illustrates the drasticimpact that the environment (rainfall) has on akey component of overall cassava productivityand value. One of the ways climate change couldaffect cassava productivity is by more erraticweather conditions. This may already be hap-pening in certain cassava-growing regions. Forexample, Fig. 19.4 illustrates changes of rain-fall patterns at the CIAT Experimental Station inPalmira, Valle del Cauca, Colombia. Harvest typ-ically takes place in April to June each year. Thechanges in the patterns of the rainfall, however,had a drastic impact on DMC of genotypes thatwere harvested 8, 10, and 12 months after plant-ing. The average DMCs were 34.3%, 28.2%,and 25.5%, respectively. The drastic reductionof DMC in the second and third harvests doesnot usually happen and is an example of the ef-fects that erratic rainfalls would have on cassavaproductivity.




250Precipitations (mm/month)

2008–2009

Average

200

150

100

50

0250

Evaporation (mm/month)

2008–2009

Average

200

150

100

50

0Mar May Jul Sep Nov Jan Mar May Jul

Fig. 19.4. Data on rainfall and evaporation in the period March 2008–August2009 compared with historic averages at CIAT experimental station in Palmira,Colombia. The expected dry period during the first 3 months of each year weredrastically altered in 2009.

Biotic stresses: expected effects

There are alternative approaches to overcomemost of the biotic stresses that may affect cas-sava (Alvarez and Llano 2002; Bellotti 2002;Ceballos et al. 2007). A combination of geneticresistance, biological control, and/or simple cul-tural practices such as selection of clean plant-ing materials is enough to achieve reliable andhealthy growth of cassava and competitive pro-duction. Climate change, however, can resultin increased disease and arthropod (insects andmites) pest problems that could result in signifi-cant economic losses. Predicting pest outbreaks

and subsequent crop damage in relation to envi-ronmental and/or climatic changes is a desirablegoal but difficult to achieve. It is already knownthat many arthropod pests, disease vectors, andbeneficial natural enemies can be strongly in-fluenced by climate. Accurate prediction of cli-mate changes linked to increases or declines inpest population size will require long-term mon-itoring of population levels and pest behavior.The identification of particularly sensitive re-gions may provide an important first indicationof a biotic stress response to changes in climate.

The direct and indirect effects of globalclimate change on the population dynamics




of arthropod pest populations, especially insubtropical agroecosystems, will depend onthe relative lengths of the wet and dry seasons,rainfall patterns, and temperature. In subtropicalagroecosystems temperatures, especially duringthe normally “cold” months, could have an effecton cropping patterns as well as on subsequentpest populations during the warmer months(Bale et al. 2002). Cropping patterns will changein response to changing climate and may resultin more or less frequent plantings during a year’scycle. In addition, crop management practices,especially those that might affect pest dynamics,might be altered by changes in temperature andrainfall.

Altered rainfall patterns and temperatureswill affect cropping patterns and crop manage-ment practices and this, in turn, can affect pestpopulation dynamics. This could be especiallyimportant in the lowland tropic areas that havedistinct wet and dry seasons. Planting of acrop species is usually initiated at the onsetof the rainy or wet season, unless irrigationcan be provided. The dry season will be idealfor the increased populations of certain pestspecies (e.g., mites and mealybugs), while itmay be a deterrent for others. Likewise, therainy season, a period that is often conduciveto considerable plant and foliage growth, mayfavor certain pest species (hornworm and somewhitefly species). In addition, some pest speciesmay have their diapause strategies disrupted ifthe linkages between temperatures, moistureregimens, planting dates, and daylengths arealtered. Some pest species may adapt readily tothese changes, while others may not have theinnate ability or genetic makeup to recognizeand respond to these environmental signals.

Crop damage resulting in root yield lossesdue to whitefly, mealybugs, and lacebugs havebeen recorded in southern states of Brazil. Theseincreases in pest populations may be the conse-quence of climate changes in the regions and sub-sequent changes in cassava crop management.Southern Brazil is subtropical and during themonths of June, July, and August, (referred to as

“winter”) temperatures were often low enoughto cause a frost that resulted in crop defoliationand stem damage. In June, cassava producerswould prune plants back to almost ground leveland store stems in protected confines, to be usedas cuttings, for planting in September when thethreat of frost has ceased. The absence of cas-sava foliage in the fields caused pest populationsto dramatically decrease.

In recent years, according to cassava produc-ers in this southern region, temperatures duringthis “winter” period have been warmer with lessprobability of frost. This has had an effect oncassava crop management practices. Farmers nolonger prune all stems back to ground level, leav-ing growing stems and foliage in the field. Thisprovided a food source for the aforementionedcassava pests. The life cycle was not severelydisrupted and active pest populations occurredduring this “winter” period (Bellotti, personalobservation). Therefore, pest populations werepresent in the field when regrowth occurred inSeptember and when cuttings in the subsequentcrop cycle germinated and young, tender leavesemerged. Pest populations, especially whiteflies,can migrate to and infest new growth and youngplants. Whitefly populations can increase rapidlyand high populations can cause considerableyield loss. In addition, the warmer tempera-tures and more frequent rainfall have resultedin more frequent or staggered plantings of thecrop. This has resulted in having the cassava cropat varying ages in the same field or plantation.These staggered planting provide an ideal sce-nario for an increase in cassava pests, especiallywhiteflies.

Temperature is probably the single mostimportant environmental factor influencing in-sect behavior, distribution, development, sur-vival, and reproduction (Bale et al. 2002). Ata higher temperature, certain pests would havean advantage and their feeding on crop foliageand consequently crop damage would increase.If this increase in temperature were accompa-nied by an extended dry period, pest activityand crop damage would intensify. For example,




cassava mites (Mononychellus spp. and Tetrany-chus spp.), mealybug (Phenacoccus herreni,P. manihoti, and Ferrisia virgata), thrips(Frankliniella williamsi), and lacebugs (Vatigaspp., Amblystira machalana) are important peststhat are favored by higher temperatures and pro-longed dry periods. Other pests, such as white-flies (several species), would increase in inten-sity, populations, and frequency in occurrence,in those regions where more frequent rainfallleads to staggered planting of the cassava crop,thereby providing a continuum of young, vig-orous foliage that is preferred for oviposition.In addition, hornworm (Erinnyis ello) frequentlyoccurs at the onset of the rainy season when thereis considerable new growth and young leaves.Studies have also shown that generalist feederssuch as the burrower bug and white grubs prefersoils with higher soil moisture content.

One of the major problems that farmers willface if climatic conditions change drastically isthat the new environmental conditions will re-quire new adaptations from cassava cultivars thatare lacking in the locally available clones. Asstated above, there are cassava cultivars adaptedto most growing conditions, but with changes inthe environment there will most likely be a needto switch varieties currently grown in one regionto another and vice versa. In this regard, thereare huge logistic problems involved in replac-ing one cassava variety by another. Transportingplanting material from one region to another isdifficult and expensive because of its bulkiness,need of careful handling, and the relatively shortperiod that it can be stored. The alternative of lo-cally growing planting material of an introducedclone faces the problem of slow multiplicationrate.

Technical solutions

Cultivars with more stable DMC

The most immediate impact that changes in theclimate will have on cassava production is ex-pected to be the reduction of DMC as a result ofchanges in rainfall patterns. This will affect in-

come of farmers who will receive a lower pricefor their product and the operations of process-ing facilities, which will have higher operationalcosts and enhanced logistic problems. For a fewyears, several cassava-breeding projects world-wide have been paying attention to achieve highDMC of roots when harvested on the optimaldate (when DMC reaches a maximum). A rec-ommended new approach for the future, how-ever, would involve placing additional emphasison identifying clones whose DMC (unavoidably)drops with the arrival of the rains but quickly re-turns to acceptable commercial levels. Breedingevaluation schemes will have to be altered soeach genotype is harvested not only at optimalconditions (end of the dry season) but also afterthe arrival of the rains. There is some prelimi-nary evidence that there is genetic variation forthe capacity for certain clones to recover quicklywith respect to DMC after the return of the rains(CIAT 2001).

Efficient systems for rapidmultiplication of planting materials

Another important impact of changes in the cli-mate will be the need for dynamic and more fre-quent changes in the varieties grown by farmers.The relatively predictable environmental condi-tions prevailing in cassava growing regions couldchange to more erratic conditions.

One of the characteristics of cassava is itslow multiplication rate. Developing systems thatwill overcome this problem is important to facethe erratic climatic patterns that cassava farmersmay suffer in the future. This strategy is veryrelevant because of two major reasons. The firstreason is that the stems can be stored only fora short period after harvest (typically, no morethan 1 month). This is the reason why farmerswill harvest their cassava fields at the end of thedry season just before the expected arrival of therains. Doing this means a short storage periodfor the stems and, therefore, optimal physiolog-ical status of the planting material for the fol-lowing season. However with changes in rainfall




patterns (as illustrated in Fig. 19.4) there wouldbe unexpectedly long storage periods for the stemthat would result in losses in viability and conse-quently chronic problems of limited availabilityof good quality planting material.

The second reason for improving and de-ploying advanced systems for rapid multiplica-tion is that there could be a need for more fre-quent changes in the varieties grown by farmers,as new environmental conditions occur. Diffu-sion of new varieties through conventional veg-etative multiplication may require as many as10–15 years to cover an area of about a millionhectare. This is too slow for the needs that farm-ers are likely to have. Fortunately, different ap-proaches for rapid multiplication schemes havebeen developed from simple micro-stakes sys-tems to the use of somatic embryogenesis tissueculture techniques relying on RITA R© or similarsystems (DeVires and Toenniessen 2001; Fre-gene et al. 2002; Segovia et al. 2002).

Herbicide tolerance

Direct planting of crops into mulches without in-version plowing provides many advantages thatare particularly relevant for cassava and the con-sequences of climate change. Perhaps the mostimmediate advantage of direct planting is thereduction of production costs. Direct planting,however, can also reduce the detrimental effectsof cassava cultivation on the environment. Directplanting can result in the soil surface not beingexposed to the environment as long as there issufficient protection by a mulch of dead (and/oralive) vegetation. This is a key approach to re-ducing soil erosion that could increase if moreintense rains occur in cassava-growing regionsof the world. Mulches also could increase water-use-efficiency since there is less run-off and morewater infiltrates into the soil and is maintained forlonger period of time because of reduced evapo-ration from the soil surface. Nutrients also couldbe more efficiently used and maintained, and soilstructure can progressively improve under theseminimum tillage systems.

However, a major drawback of direct plant-ing is the frequently unmanageable problem ofweeds. As desirable as direct planting is, inpractice, it only has developed quickly when itis supported with the availability of herbicide-tolerant crops. In 2008, herbicide tolerance de-ployed in soybean, maize, canola, cotton, and al-falfa occupied 79 million hectares of the globalbiotech area of 125 million hectares (ISAAA2008). These data refer to herbicide toleranceobtained through genetic transformation partic-ularly for tolerance to glyphosate. A protocolfor the genetic transformation of cassava ex-ists and is used routinely by different researchlaboratories in search of different traits (Tayloret al. 2004).

There are, however, other alternatives that ex-ploit natural or induced variation for herbicidetolerance in different crops (Sherman et al., 1996;Tan et al. 2005; Tan et al. 2006; Tan and Bowe2008). In most cases, tolerance to imidazolinonesarises from changes in the gene codifying for ace-tohydoxyacid synthase (AHAS). The resistanceagainst ciclohexanedione found in maize is reg-ulated by the carboxilase Acetyl-CoA and thatagainst triazine originates in the psbA gene that isrelated to photosynthesis (Tan et al. 2006). Thesediscoveries have led to development of herbi-cide tolerance in different crops such as maize,rice, wheat, canola, sunflower, lentils, sugar beet,cotton, soybean, lettuce, tomato, and tobacco.CIAT has initiated two aggressive approaches toidentifying herbicide tolerance in cassava. Thefirst approach involves self-pollinating cassavagermplasm to produce S1 genotypes, which ex-poses potential recessive sources of toleranceto herbicides. The genotypes thus produced canthen be subjected to different herbicides to detectphenotypes expressing tolerance. The second ap-proach is through the use of molecular markersfor the application of TILLING or Eco-TILLING(Guang-Xi et al. 2007; Till et al. 2003). This ap-proach is greatly facilitated by clear understand-ing of the genes that need to be mutated andthe recent availability of the sequenced cassavagenome.




A better understanding of cassavagenetics and production ofgenetic stocks

The conservation and exchange of cassavagermplasm is difficult and slow. Typically, invitro plantlets, certified to be disease-free, areused when germplasm is to be moved from onecountry to another. This has high costs and strictquarantine limitations. An alternative approachfor germplasm exchange, conservation and uti-lization, is currently under way to develop par-tially inbred genetic stocks to be used as sourcesof specific traits.

If a given genotype is to be used as source ofa desirable trait, its value is in the trait itself, notin the whole genotype. CIAT has proposed thatself-pollinations of elite genotypes (S0) shouldbe made to produce S1 genotypes that are ho-mozygous for the trait identified in the originalgenotype. There are three different possibilitiesfor identifying homozygous genotypes amongthe S1 (segregating) progeny: (a) If the trait is re-cessive (i.e., amylose-free starch), only homozy-gous recessive genotypes will express it and se-lection can be done using the phenotype of the S1

genotypes; (b) If the trait is dominant and molec-ular markers are available, codominant markerssuch as SSR can be used to identify homozy-gous S1 genotypes; (c) If the trait is dominantand no molecular marker is available, a secondself-pollination would be necessary to identifyS2 progenies that do not segregate for the traitindicating that the progenitor S1 genotype washomozygous.

The availability of these partially inbred ge-netic stocks provides several advantages:

(a) Breeding value (for the trait) of these ho-mozygous S1 genotypes doubles when theS0 progenitor is heterozygous. The selectedS1 genotypes could then be registered as asource of the desirable trait;

(b) The S1 genotype could be self-pollinated toproduce S2 seed that would also be homozy-gous for the desirable gene(s). The storageand exchange of these S2 botanical seeds

would be considerably less expensive andfaster than maintaining germplasm in vitroor in the field. Phytosanitary restrictions forthe exchange of botanical seed are less lim-iting compared with the shipment of in vitroor vegetative cuttings;

(c) Finally, crosses of S1 genotypes homozy-gous for different desirable traits can bemade to produce new S1 genotypes combin-ing more than one desirable trait in a ho-mozygous condition. Genetic stocks com-bining germplasm developed by the IITA,CIAT, EMBRAPA, and other national pro-grams in Africa, Asia, and Latin Amer-ica could then contribute to a more dy-namic exchange of germplasm and a moreefficient exploitation of cassava geneticresources.

The relevance of the availability of geneticstocks of cassava as sources of desirable traits isthat it will accelerate the exchange of germplasmand the assembly of new varieties combining de-sirable traits such as resistances to abiotic andbiotic stresses and adaptation to specific envi-ronments and end-use requirements.

Integrated pest and diseasemanagement approaches

The long growing cycle of cassava and the factthat it is grown in environments where thereare no winters that break the cycles of pestsand diseases, have determined the strategies tolimit their economic impact. Integrated pest anddisease management (IPDM) is fundamental forcassava production and relies on the efficient ex-ploitation and combination of sources of geneticresistance and techniques that favor the estab-lishment and growth of populations of biologicalcontrol agents.

Wild Manihot germplasm provides a wealthof useful genes for cultivated cassava (Hahn et al.1980; Nassar and Ortiz 2008). A source of toler-ance to PPD has been identified in M. walkerae(Bertram 1993) and introgressed into cassava




(Morante et al. 2010; Cuambe 2007). The onlysource of resistance to the cassava hornworm anda widely deployed source of resistance to cassavamosaic disease were identified in fourth back-cross derivatives of M. glaziovii (Chavarriagaet al. 2004; Jennings 1976; Jennings and Iglesias2002). M. glaziovii and M. melanobasis havebeen identified as sources of resistance to thecassava brown streak disease (Jennings and Igle-sias 2002). Moderate to high levels of resistanceto whiteflies and the cassava green mite havebeen found in interspecific hybrids of M. escu-lenta subsp. flabellifolia. Resistance was recov-ered easily in F1 interspecific hybrids, suggestinga simple inheritance of the trait. These sourcesof resistance contribute to those already found incultivated cassava for whiteflies (for instance inthe landrace MECU72) and thrips (in cultivarswith pubescent young leaves).

Biological control methods for pests that af-fect cassava have also been widely reported(Bellotti 2002). Baculovirus pesticides arehighly effective in controlling hornworms; andPhytoseiidae predator mites and the fungalpathogen Neozygites help to control mites feed-ing on cassava. Several natural enemies, includ-ing three parasitoids, contribute to the biologicalcontrol of whiteflies. Perhaps one of the mostremarkable success stories related to biologicalcontrol was the introduction of Anagyrus lopezito control mealybug species in Africa (Herrenand Neuenschwander, 1989). IPDM include cul-tural practices. For example, intercropping cas-sava with Crotolaria sp. reduces damage ofthe burrower bug (Cyrtomenus bergi). This bugcan also be affected by the fungus Metarhiziumanisoplae and nematodes such as Steinernemacarpocapsae and Heterorhabditis bacteriophora(Bellotti 2002). Success in dealing with pests anddisease will, therefore, rely on the efficient com-bination of various biological assets.

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