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Sweetpotato weevil (Cylas spp.) resistance in Africansweetpotato germplasmHarriet Muyinza a , Herbert L. Talwana b , Robert O.M. Mwanga c & Philip C. Stevenson da National Crops Resources Research Institute, Namulonge, PO Box 7084, Kampala,Uganda, (Present address: National Agricultural Laboratories, Kawanda, PO Box 7065Kampala, Uganda)b Faculty of Agriculture, Department of Crop Science, Makerere University, PO Box 7062,Kampala, Ugandac International Potato Centre, Uganda Office, PO Box 22274, Kampala, Ugandad European Centre for IPM, Natural Resources Institute, University of Greenwich,Chatham, Maritime, Kent ME4 4TB, UK and Royal Botanic Gardens, Kew, Richmond,Surrey, TW9 3AB, UK

Available online: 07 Mar 2012

To cite this article: Harriet Muyinza, Herbert L. Talwana, Robert O.M. Mwanga & Philip C. Stevenson (2012): Sweetpotatoweevil (Cylas spp.) resistance in African sweetpotato germplasm, International Journal of Pest Management, 58:1, 73-81

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Sweetpotato weevil (Cylas spp.) resistance in African sweetpotato germplasm

Harriet Muyinzaa, Herbert L. Talwanab, Robert O.M. Mwangac and Philip C. Stevensond*

aNational Crops Resources Research Institute, Namulonge, PO Box 7084 Kampala, Uganda (Present address: National AgriculturalLaboratories, Kawanda, PO Box 7065 Kampala, Uganda); bFaculty of Agriculture, Department of Crop Science, MakerereUniversity, PO Box 7062 Kampala, Uganda; cInternational Potato Centre, Uganda Office, PO Box 22274 Kampala, Uganda;dEuropean Centre for IPM, Natural Resources Institute, University of Greenwich, Chatham, Maritime, Kent ME4 4TB, UK andRoyal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, UK

(Received 27 April 2011; final version received 28 November 2011)

Host-plant resistance could be a useful tool for managing the weevils Cylas puncticollis and C. brunneus, which aremajor insect pests of sweetpotato in Africa. There is currently little information on existing resistance mechanismsagainst Cylas spp. in African cultivars, except where lower levels of weevil damage were attributed to escape due todeep rooting and reduced soil cracking, limiting the exposure of roots to weevils. Here, we evaluate weevil resistancein 134 sweetpotato cultivars and landraces over two seasons in two agroecologically diverse locations. Severalsweetpotato cultivars, including New Kawogo, expressed resistance to Cylas spp. The resistance characteristics havebeen demonstrated in previous laboratory experiments to be quantifiable and thus potentially useful in targetedplant-breeding against Cylas spp. We showed external root and stem base damage to be an accurate quantitativeindicator of internal root damage, offering rapid and accurate evaluation of resistance in field trials for screening.Moreover, weevil resistance can be assessed earlier in plant development, so saving time in the selection of theprogeny from breeding programmes.

Keywords: Cylas brunneus; Cylas puncticollis; host-plant resistance; sweetpotato breeding; root damage

1. Introduction

Sweetpotato (Ipomoea batatas [L.] Lam.) is a staplefood for a large proportion of the population in manyparts of sub-Saharan Africa (Bouwkamp 1985), but itis of greatest importance in the food systems ofUganda, Rwanda, Burundi and eastern Congo whereit forms a major part of the diet (Kapinga et al. 2003;Rees et al. 2003). In this region, sweetpotato is alsogrown as a major income generator for the smallholderpeasant farmers (Andrade et al. 2009). With an annualproduction of 2.7 million metric tonnes, representingalmost 3% of the total world production, Uganda isthe world’s third largest producer of sweetpotato(FAOSTAT 2009). In Uganda, sweetpotato is thethird most important source of carbohydrates afterbanana and cassava (Yanggen and Nagujja 2006) andis grown in all parts of the country, mainly by small-scale subsistence farmers on plots that rarely exceed0.5 ha (Kapinga et al. 2003).

Over the past three decades the area undersweetpotato production has greatly increased by64.4% to 5.9 million hectares, but productivity hasremained static at around 4.4 ton/ha compared to aworld average of 12.7 ton/ha (Yanggen and Nagujja2006). By far the most important production constraintof sweetpotato worldwide is plant damage caused bysweetpotato weevils, viz. Cylas spp. (Rees et al. 2003).

Cylas spp. can damage every harvestable part of theplant in a crop, with devastating consequences for poorfarmers (Chalfant et al. 1990; Nottingham and Kays2002) leading to lower income and reduced foodsecurity (Smit 1997; Magira 2003).

The weevil species in Africa are C. puncticollisBohe. and C. brunneus Fab. Sustainable managementof these weevils would boost sweetpotato productionand impact positively on the livelihoods of millionsof poor farmers across sub-Saharan Africa. Onepossible approach is host-plant resistance. In NorthAmerica there is strong evidence for resistanceamong dry-fleshed cultivars (Jackson and Bohac2006) while reduced feeding and oviposition hasbeen attributed to genetic variation (Mao et al.2001). However, identifying resistance to Africanspecies of Cylas that is of a ‘‘dynamic’’ rather thanan ‘‘escape’’ nature (Stathers et al. 2003a, 2003b) hasbeen less fruitful. One East African genotype, NewKawogo, has shown promise as a source ofresistance (Stevenson et al. 2009; Muyinza et al.2010), indicating that a wider survey of other localcultivars and landraces may deliver more usefulgenetic material for breeding. Here, we evaluate 134local cultivars and landraces over two seasons at twolocalities in Uganda, and provide a rapid screeningprotocol for evaluation of weevil damage that could

*Corresponding author. Email: p.c.stevenson@gre.ac.uk

International Journal of Pest ManagementVol. 58, No. 1, January–March 2012, 73–81

ISSN 0967-0874 print/ISSN 1366-5863 online

� 2012 Taylor & Francis

http://dx.doi.org/10.1080/09670874.2012.655701

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be particularly useful for screening progeny frombreeding programmes.

2. Materials and methods

Field experiments were conducted at theNational CropsResources Research Institute (NaCCRI) based atNamulonge, located in central Uganda (08320N, 38350Eat an elevation of 1128 m.a.s.l.), and at the NationalSemi-Arid Resources Research Institute (NaSARRI)based at Serere, located in north-eastern Uganda(3382700E, 18320N, elevation of 1140 m.a.s.l.) which arerepresentative of sweetpotato-growing areas of Uganda(Bashaasha et al. 1995). Namulonge experiences lowertemperatures (Gold et al. 2001) and lower weevilincidence than Serere where it is hotter and drier,conditions which exacerbate weevil pressure (Smit1998).

A total of 134 local and improved sweetpotatocultivars from different agroecological zones in Ugan-da that had been identified by farmers as having somelevels of resistance to sweetpotato weevils (Yada 2009)were evaluated in this study. Two weevil-susceptiblesweetpotato varieties, Tanzania and NASPOT 1, wereincluded in the trial as susceptible checks. The varietieswere initially planted at NaSARRI so as to synchro-nize sweetpotato root availability with a dry seasonpeak, which normally occurs from December toFebruary of each year and exposes the cultivars tothe highest natural weevil infestation pressure. Eachrespective cultivar was planted in a plot of three hand-hoe-made ridges, each 1.5 m long and 15 cm wide, laid1 m apart (distance between the middles of the ridges).Apical vine pieces ca.30 cm long were used for plantingat a spacing of 30 cm apart within the ridge (giving 5plants per ridge). A randomized complete blockexperimental design with three replications was used.

To ensure an infestation from the outset, each ridgein the plot was artificially infected with 9 adult (1–2weeks old) Cylas spp. weevils in the ratio of 2 : 1 C.brunneus :C. puncticollis and maintaining 5 : 4 female tomale weevil sex ratio, at 9 and 13 weeks after planting(WAP). The ratios used were based on earlier observa-tions made of emergent weevil adults from naturalpopulations in infested sweetpotato roots collected onfarms around the experimental sites (Muyinza et al.2007). Theweevils used in the trials were obtained fromaculture maintained on sweetpotato roots at NaSARRI,as used previously by Stevenson et al. (2009). Hilling-upof the ridges and weeding was done three times with ahand hoe at 4, 8 and 12 WAP which reduced soil crackswhich are the means of access to roots for weevils.

2.1. Assessing weevil damage in sweetpotato vines andstorage roots

At crop maturity (20 WAP), the number of survivingplants in each plot was recorded. Five vines from each

ridge (one vine per plant if all the five plants on theridge survived) were randomly selected for weevildamage assessment. A basal segment (15 cm from thecrown) was cut off from each vine, visually assessedand scored for external weevil damage using a scale of1 to 5; where 1¼ 0–20% of the basal segment damaged;2¼ 21–40%; 3¼ 41–60%; 4¼ 61–80% and 5¼ 81–100% (Stathers et al. 2003a). Thereafter, each piecewas split lengthwise using a knife to reveal internalweevil damage, which was also scored using the samedamage assessment scale. After scoring for weevildamage, all the vines in each plot were collected andweighed (recorded as fresh vine weight).

Weevil damage to sweetpotato roots was alsoassessed. Storage roots from the surviving plants oneach ridge were carefully dug up, collected and weighedto obtain the overall root weight per plot. Theharvested roots were sorted into weevil-damaged andundamaged roots. A root was considered to be weevil-damaged if it bore characteristic dark scarred spots onthe surface of the root – a typical symptom of weevilpenetration and feeding (Stathers et al. 2003a). Thoseroots lacking any surface damage were considered tobe uninfested. The weevil-damaged roots were furthercategorized for damage. For example, each of the rootswith slight damage was cut to remove all of thedamaged portions, which were then combined with thecompletely damaged roots, the combination beingweighed to obtain the total weight of roots with anydamage. This weight was, thereafter, expressed as apercentage of the overall fresh weight of all the roots(clean and damaged) per plot. Additionally, a ratiobetween overall fresh root weight and fresh vine weightper plot was calculated per plot as the root–vine weightindex.

2.2. Data analysis

Simple (Pearson) correlation analysis was performedon the number of surviving plants, external stemdamage index, internal stem damage index, percentagedamaged roots and root–vine weight index to explorethe respective interrelationships. Principal Compo-nents Analysis (PCA) based on these correlationcoefficients was run using the PRINCOMP procedureof SAS (SAS 2001; SAS system for windows v8, SASInstitute Inc., Cary, North Carolina) to determine themost important sweetpotato weevil damage variables.Based on absolute values of the Eigen vectors from thePCA (SAS Institute Inc.), internal and external stemdamage indices, and percentage root damage wereselected as the most important variables for sweet-potato weevil damage. Based on these variables, 30least-damaged, 4 moderately damaged, and 6 highlysusceptible sweetpotato cultivars/landraces were se-lected from the original 134 cultivars initially plantedfor further field evaluation at NaCCRI and NaSARRI.These 40 selected cultivars were planted during May

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2006 and assessed as above in November 2006, andagain planted during October 2006 and then assessedin April 2007.

Further, Spearmans’s correlation analysis wasapplied to the following variables: the number ofsurviving plants, external stem damage index, internalstem damage index, percentage damaged roots androot–vine weight index of the 40 selected cultivars, toenable us to explore the respective interrelationships.Thereafter, PCA was run, based on Spearman’scorrelation coefficients using the XLSTAT2010 proce-dures (Addinsoft Inc.) followed by the VarianceMaximization rotation (Varimax) procedure to obtaina clear pattern of factor loadings for each variety.Correlation bi-plots/coefficients of the varietal factorloadings were drawn to reveal groupings among thevarieties using the automatic method of XLSTAT2010Running cluster analysis using the Paired GroupAlgorithm, with the Bray–Curtis Similarity Measurefurther elucidated the groupings.

3. Results

Table 1 shows the simple (Pearson) correlationcoefficients among the five sweetpotato damage vari-ables on the initial 134 varieties, namely the number ofsurviving plants, external stem damage index, internalstem damage index, percentage damaged roots androot–vine weight index. The Eigen vectors of the PCAbased on these correlation coefficients are shown inTable 2. From PCA the most important parameters ofsweetpotato weevil damage were internal and externalstem damage indices and percentage root damage, withEigen vectors 0.61, 0.60 and 0.49, respectively, in theprincipal component 1 which accounted for 49.4% of

the variation observed at NaSARRI when all cultivarswere evaluated (Table 2). These three indices weresubsequently used to select the 40 cultivars for furtherfield evaluation from the original 134.

Cultivars with damaged root scores between 0%and 12.9% such as New Kawogo, RAK865, LIR302,MBR551A, HMA519, MKN1190 and IGA1017 wereconsidered resistant. Cultivars with percentage damagebetween 13% and 24% including MBR552 (Kahunge-zi), MBR600 (Kisakyamaria) and MSD403 werecategorized as moderately resistant (Table 3a). Culti-vars with percentage damage between 25% and 35%including KBL625 (Mulelabaana), KBL620 andSRT11 were categorized as moderately susceptible.LIR296, KMI84 (Kampala), NASPOT 1 and Tanza-nia, which had root damage scores above 36%, werecategorized as susceptible (Table 3b).

Further screening of the selected sweetpotatocultivars for differences in susceptibility to weevilinfestation and damage at NaCCRI and NaSARRIfor seasons 2 and 3 showed that cultivars such asAnamoyito and New Kawogo were significantly(P5 0.001) the least damaged, and Tanzania andNASPOT 1 were the most damaged cultivars at bothNaSARRI and NaCCRI (Tables 3a, b and 4a, b,respectively). PCA here too retained three damageindices – percentage damaged roots, external stemdamage index and internal stem damage index – as themost important damage assessment indices (Table 5).Principal Component 1 (PRIN 1) accounted for 49.4%and 64.8% while Principal Component 2 (PRIN 2)accounted for 22% and 23% of the observed variationin the data at NaCRRI and NaSARRI, respectively.PRIN 1 was constituted mainly by Eigen values ofexternal stem damage, internal damage and percentage

Table 2. Eigen values of the Principal Component Analysis using five weevil damage indices of 134 varieties assessed 5 monthsafter planting at NaSARRI.

Eigen values of Principal Components

Weevil damage index Component 1 Component 2 Component 3

Number of surviving plants 0.17 0.95 0.27External stem damage index 0.60 70.21 0.33Internal stem damage index 0.61 70.17 0.29Percentage damaged roots 0.49 0.16 70.86Percentage of observed variation 49.37 22.01 19.2

Table 1. Simple (Pearson’s) correlation coefficients and sweetpotato weevil damage indices of 134 varieties assessed 5 monthsafter planting at NaSARRI.

Damage assessment variable External stem damage index Internal stem damage index Percentage damaged roots

Number of surviving plants 0.03 0.06 0.22*External stem damage index 0.85** 0.50**Internal stem damage index 0.50**

Correlation coefficients significant at *P 5 0.05 and **P 5 0.001.

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root damage while PRIN 2 was constituted by Eigenvalues of root–vine index and mean number ofsurviving plants. Cumulatively, these two principalcomponents accounted for the larger part of theobserved variation in the data at NaCRRI andNaSARRI, respectively (Table 5).

A plot of varietal factor loadings for PRIN 1 versusprincipal component 2 confirmed distinct groupingsamong cultivars. The highly susceptible NASPOT 1and Tanzania were clustered together, while New

Kawogo, LIR302, MBR551A and ARA228 (whichwere resistant) were also clustered together at bothNaCCRI and NaSARRI (Figure 1a and b). Similarly,cluster analysis of the second field evaluation (of 40varieties) grouped LIR302, New Kawogo, RAK865,PAL143, HMA519, ARA244, APA356, KMI88,APA325, MBR551A and MBR551B as resistantcultivars; the moderately susceptible group includedSRT17, ARA230, ARA228, KM188, MBR551B andMLE194, and a highly susceptible group included

Table 3b. Varieties evaluated as resistant over 2 seasons’ plantings at NaSARRI and assessed 6 months later.

NaSSARI season 1 (2005–06) NaSSARI season 2 (2006–07)

VarietyRoot damage

(%)Internal vinedamage index

External vinedamage index

Root damage(%)

Internal vinedamage index

External vinedamage index

KMI 88 18.2 2.2 2.2 11.5 1.8 1.7MSD 400 16.9 2.5 2.4 7.7 3 3APA 325 16.6 2.5 2.2 9.4 3 3ARA 248 15.7 2.1 2.3 10.9 2 2ARA 244 11.4 1.7 1.6 0.5 0.8 0.8KRE 723 9.3 2.5 2.5 0 1.9 1.5ARA 230 8.2 2.7 2.6 3.5 2.2 2.3MBR 551 7.9 2.2 2.5 6.4 1.6 1.7MLE 194 7.6 2.2 2.4 3.3 2.5 2SRT 36 6.5 2.4 2.6 0 2 2APA 356 4.9 1.8 1.8 0 1.6 2RAK 865 4.1 1.8 1.9 0 1.9 1.8NEW KAW 1.4 1.5 1.9 1.6 2 2LIR 302 0.9 1.6 1.6 0 1.6 1.6ARA 228 0.5 1.8 2 2.8 2 2MBR 551 17.8 1.9 1.9 14.7 2.2 2.3

NEW KAW ¼ New Kawogo.

Table 3a. Varieties evaluated as susceptible in at least 1 of 2 seasons’ plantings at NaSARRI and assessed 6 months later.

NaSSARI season 1 (2005–06) NaSARRI season 2 (2006–07)

Variety % damageInternal

damage indexExternal

damage index

Categoryplant

response % damageInternalindex

Externalindex

*Category plantresponse

NASPOT 1 82.6 2.9 3 S 78.1 3.2 4 STanzania 58 3 3.1 S 81.4 3 3.4 SMSD 403 50.4 2.4 3.2 S 34.6 2.3 2.9 MRMLE 191 41.3 2.8 3 MR 25 3.3 3.33 MRLIR 296 34.8 3.3 3.4 MR 62.7 3.1 3.4 SMBR 552 27.3 2.9 3.1 MR 32.3 2.4 2.4 MRIGA 995 20.9 2.6 2.7 MR 73.7 3.3 3.3 SPAL 143 20.5 2 2 MR 14.7 2 2 MRMBR 600 (a) 18.6 2.2 2.4 R 32.9 2.8 2.9 MRSRT 36 21.4 2.1 2.5 MR 0 2 2.3 RMBR 600 (b) 29.2 2.5 2 MR 12.4 2.3 2.4 RSRT 17 27.3 2.7 2.8 MR 6.1 2 2 RIG1 017 20.2 2.8 3.1 MR 4.3 2.4 1.4 RKBL 625 30.9 2.4 2.2 MR 17.6 2.6 2.1 MRSRT 11 27.2 2.9 2.9 MR 17.5 2.2 1.9 MRKBL 620 26.6 1.9 2.1 MR 21 1.9 2.1 MRMSK 1073 12.1 2.5 2.8 R 16.7 2.4 2.4 MRPAL 145 19.7 1.9 1.9 R 17.5 1.9 2.4 MR

R ¼ resistant; S ¼ susceptible; MR ¼ moderately resistant.

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Tanzania, NASPOT1, MSD403 and LIR296 across allseasons at NaCRRI and NaSARRI (Figure 2a and b).At NaSARRI the resistant cultivar New Kawogo,LIR302, MBR551B and APA356 were all furthest

from the highly susceptible NASPOT 1, Tanzania,LIR296 and MSD403 varieties. These were thus at thefurthest distance, on the proximity scale compared to0.3 for the most susceptible cultivars at NaCRRI(Figure 1a); and NaSARRI (Figure 2a and b). Themoderately susceptible cultivars, at NaSARRI andNaCRRI respectively, included MBR552, MBR600,KLB625, MKN1190, HMA519, PAL143, KBL620 andPAL145 (Figure 1a and b).

4. Discussion

This study demonstrated significant differences in levelsof susceptibility and resistance to sweetpotato weevilsamong Ugandan sweetpotato cultivars and landraces.For example, New Kawogo, RAK865 (Dimbuka),HMA519 (Kyebagambire), and LIR302 (Anamoyito)were highly resistant, and thus provide valuablematerial for breeding programmes, whereas cultivarsincluding MSD403, LIR296, NASPOT 1 and Tanzaniawere consistently susceptible at both NaCRRI andNaSARRI. The consistency among the cultivars overtwo seasons and two sites ensures the robustness of theevaluation reported and the methods used. Infestationand damage can be evaluated reliably and effectivelyusing external and internal stem damage indices andstorage root damage, as demonstrated in this work.The external stem damage was strongly linked tointernal root damage. Thus large-scale field screeningof new varieties or for the field evaluation of the largenumbers of F1 products from breeding programmescan be screened effectively and rapidly for resistance toweevils using external stem damage indices, therebysimplifying the evaluation process. However, using thestem damage indices may be complicated by thepresence of other stem-damaging and shoot-feeding

Table 4b. Re-evaluation of susceptibility in 40 varietiesplanted in two consecutive planting seasons at NaCCRI andassessed 5 months later.

Variety

Rootdamage(%)

Internalvine

damageindex

Externalvine

damageindex

Category ofhost-plantresponse toCylas spp.

NASPOT 1 78.1 3 2.7 STanzania 63.3 3.4 3.3 SMSD 403 59.6 3 3.9 SLIR 296 50.4 2.1 2 SKBL 620 (a) 29.5 2.1 2 MRKBL 620 (b) 29.5 2.1 2 MRMLE 191 20.2 3 2.8 MR

MR ¼ moderate resistance; S ¼ susceptible.

Table 4a. Re-evaluation of resistance in 40 varieties plantedin two consecutive planting seasons at NaCCRI and assessed5 months later.

VarietyRoot damage

(%)Internal vinedamage index

External vinedamage index

MBR 600 19.3 3.4 3.3IGA 1017 18.1 2 2MKN 1190 17.8 2.8 2.9KRE 723 17.5 2.5 2.6IGA 1017 17.1 2.7 2.5KMI 84 14.9 3.1 2.7MLE 194 14.1 3 2.6SRT 17 13.8 3.7 2.9KMI 74 11.1 2.1 1.8SRT 11 11.1 2 0.3KMI 88 10.9 1.9 1.8ARA 214 10.3 1.9 2KBL 625 10.2 3.1 2.6MSK 1073 8.3 2.1 1.9ARA 248 7.3 1.5 1.5RAK 847 6.6 2.4 2.4HMA 519 6.6 1.7 1.4MSD 400 6.29 3.2 2.7MBR 551 6.1 1.9 1.5PAL 145 5.7 2.8 2MBR 556 5.2 2.3 2.4MBR 600 5 2.7 2.6MBR 551 B 5 2 2PAL 143 4.4 3.3 2.9SRT 38 A 3.6 1.9 1.8APA 325 3.5 0.7 1ARA 244 3.3 1.7 1.7ARA 230 3 1.3 1.7ARA 228 2.1 1.5 1.7RAK 865 1.7 1.3 1.5APA 356 1.7 1.8 1.9MBR 552 1.2 2.6 2NEW KAW 0.4 1.3 1.2LIR 302 0 1.3 1.6LIR 302 0 1.3 1.7

Table 5. Eigen values of the Principal Component Analysisusing five weevil damage indices of 40 varieties assessed 5months after planting at NaCCRI and NaSARRI (data of2006 and 2007 combined).

Eigen values of PrincipalComponents

Weevil damage indexComponent

1Component

2

NaCRRINumber of surviving plants 0.14 70.55External stem damage index 0.60 70.10Internal stem damage index 0.60 0.07Percentage damaged roots 0.50 0.16Root–vine weight index 0.12 0.81Percentage of observed variation 49.4 22.0NaSARRINumber of surviving plants 0.21 0.55External stem damage index 0.57 0.06Internal stem damage index 0.57 0.04Percentage damaged roots 0.52 70.16Root-vine weight index 0.20 70.27Percentage of observed variation 64.8 23.0

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Figure 1. (a) Clustering of representative varieties based on Principal Component Analysis at NaCRRI. %Damage¼ percentage damaged roots; Plants¼mean number of surviving plants; Int. stem damage¼ internal stem damage.Variety codes: LIR302¼Anamoyito; ARA230¼Andinyaku; SRT17¼Kawogo1; MBR600¼Kisakyabikilamaria; LIR296;KMI88¼Opaade; MBR551A¼OrurengoA; MBR551B¼OrurengoB; RAK865¼Dimbuka; APA325¼Adok Adok; NewKawogo¼New Kawogo. (b) Clustering of representative varieties based on Principal Component Analysis at NaSARRI. %Damage¼ percentage damaged roots; Plants¼mean number of surviving plants; Int. stem damage¼ internal stem damage.(Internal stem damage omitted because damage parameters were multi-collinear with internal damage eliminated on the basis ofEigen values.) Variety codes corresponding to local names where known: LIR302¼Anamoyito; ARA230¼Andinyaku;SRT17¼Kawogo1; MBR600¼ Kisakyabikilamaria; KMI88¼Opaade; MBR551A¼OrurengoA; MBR551B¼OrurengoB;RAK865¼Dimbuka; APA325¼Adok Adok.

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insects and disease symptoms that may disguisedamage symptoms of Cylas spp. Additionally, it hasbeen found that sweetpotato weevils can activelydifferentiate between sweetpotato plant parts, and/orhave preference for some varieties over others at hostselection for feeding and oviposition when presentedwith such a choice (Muyinza et al. 2007).

Stem bases and foliage provide a food source forthe weevils at the beginning of the sweetpotato growingseason, when the storage roots are not yet developed(Raman 1991; Smit 1997). Adult Cylas weevils areknown to feed on foliage, and the larvae to feed in thestem or the leaf and to pupate inside sweetpotato vines(Smit 1997). Since low damage of stems is correlated

Figure 2. (a) Dendrogram of variation in field resistance levels of sweetpotato cultivars at NaSARRI. (b) Dendogram ofvariation in field resistance levels of sweetpotato varieties at NaCRRI. Variety codes corresponding to local names:LIR302¼Anamoyito; ARA230¼Andinyaku; SRT17¼Kawogo1; MBR600¼Kisakyabikilamaria; KMI88¼Opaade;MBR551A¼OrurengoA; MBR600¼Kisakyabikilamaria; MBR551B¼OrurengoB; RAK865¼Dimbuka; APA325¼AdokAdok.

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with low damage to roots, it is possible that the samemechanism governs resistance.

Although the root–vine index was not found to bean important factor in Cylas damage assessment, therewas a weak correlation between this and percentageroot damage (P4 0.05, r¼ 0.265). It is possible undercertain field conditions that thick vine cover with theresultant high vine weights could result in low rootdamage by the weevil. The dense foliage may help thesoil to retain moisture at the surface, and thus reducesoil cracking which would otherwise expose the rootsto weevils. This relationship could explain farmers’observations that varieties with dense vine cover tendto have less damage compared with the high rootyielders with lower vine coverage (Stathers et al.2003a).

The presence of clear differences in weevil damageand the categorization of cultivars into resistant andsusceptible groups provide material for further study.Especially where more insight is required on theheritability of resistance to Cylas spp. in sweetpotatoand on the effects of root chemistry on weevilbiology. This could provide evidence to show thatreduced infestation and damage among the resistantsweetpotato varieties as demonstrated in this study ismore than simply escape, but is active, quantifiableand manageable for breeding purposes. Stevensonet al. (2009) reported a laboratory evaluation ofsweetpotato which showed that under equal levels ofweevil infestation, the roots of New Kawogo sufferedlower feeding damage and resulted in less frassproduction and fewer eggs laid than adults onTanzania. These varieties were shown in the presentstudy to be resistant and susceptible, respectively, inthe field and laboratory. Thus it is clear that theresistance in New Kawogo is not simply escape andit is likely that other varieties shown here alsoexpress active resistance. Furthermore, Stevensonet al. (2009) reported that resistance in New Kawogowas governed, at least in part, by levels of hydro-xycinnamic acid esters in the latex of New Kawogo.As discussed above, since these compounds occur inthe latex and the latex in turn occurs throughout theplant, these compounds may be responsible for theresistance effects at the stem base as well as theroots. Thus the field evaluation of progeny fromvarietal improvement programmes may be achievedby chemical analysis and at a much earlier point inthe plant’s life than evaluating stem base or rootdamage after 20 weeks. Combined with analysis ofquantitative trait loci (QTLs) chemical analysis willascertain whether there is scope for their use asmarkers of resistance in breeding programmes forimproved cultivars. Using molecular markerslinked to the QTLs will speed up the process ofidentifying and selecting plants that have resistanceto weevils.

Acknowledgements

This work was supported by the McKnight FoundationCollaborative Crop Research Programme for the develop-ment of high yielding multiple resistant sweetpotato germ-plasm, under the National Sweetpotato Programme of theNational Agricultural Research Organisation in Uganda(Project Number: 03-1112). We acknowledge all the supportobtained from staff of the National Sweetpotato Programme.The research team also acknowledges Philip Ragama andBrian Isabirye for their support during data analyses.

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