African Crop Science Journal, Vol. 10. No. 4, 2002, pp. 317-324
Reaction of sweetpotato clones to virus disease and their yield performance in Uganda
E. Byamukama E. Adipala, R. Gibson1 and V. Aritua
Department of Crop Science, Makerere University, P.O. Box 7062, Kampala,
(Received 16 January, 2002; accepted 16 August, 2002)
Code Number: cs02030
Sweetpotato virus disease (SPVD) casused by dual infection of sweetpotato chlorotic stunt virus (SPCSV) and sweetpotato featherly mottle virus (SPFMV) is a major constraint to sweetpotato production in Uganda, whose infestation often necesitates instituting control measures. Although among the available control measures use of resistant and high yielding varieties is the cheapest and effective means of controlling the disease, reaction of several sweetpotato clones to the disease is largely unknown. A study was conducted in three geographical regions (Namulonge, Kachwekano, Bulegeni, Mbarara and Serere) in Uganda to evaluate reaction to sweetpotato virus disease (SPVD) and yield of 15 clones. The study was laid out in a completely randomised block design with 4 replications per clone. Mbarara and Namulonge had high incidence of SPVD (area under disease progress curve, AUDPC = 0.77, 0.47, respectively), while Serere had the lowest incidence (AUDPC = 0.13). Clones obtained from the International Potato Center (CIP) were highly susceptible to SPVD, while Ugandan lines were tolerant to SPVD even under high disease pressure. Clones Zapallo, Kemb 37, Mugande and Araka red were the most infected by the disease (AUDPC = 1.17, 1.04, 0.96, 0.77, respectively), while clones 93-523, 93-1927, 93-493 and 93-319 each had a consistently lower AUDPC across all locations (AUDPC = 0.086, 0.112, 0.114, 0.118). Clone 93-52 was high yielding across all locations (17.7 t ha-1) followed by 93-1927, Kemb37, Mugande and Tanzania (AUDPC = 16.9, 16.1, 15.6, 13.9 t ha-1, respectively). The lowest yielding clones were 93-316, 93-663, 93-523 and 23/60. Clones 93-29, 93-1096 and 23/60 were adaptable to all sites.
Key Words: Adaptability, AMMI analysis, Ipomoea batatas, yield stability
Le virus de la maladie de patate douce (SPVD) causé par une infection duale de virus darrêt de croissance de patate douce chlorotique (SPCSV) et le virus plumeux de taches de patate douce est une contrainte majeure a la production de la patate douce en Ouganda, dont linfestation nécessite souvent létablissement des mesures de contrôle. Bien que parmi les mesures de contrôle disponibles lusage des variétés résistantes et de haut rendement est le moins coûteux et effectif moyen de contrôle de maladie, la réaction de plusieurs clones de patates douce à la maladie est largement inconnue. Une étude était conduite dans trois régions géographiques (Namulonge, Kachwekano, Bulegeni, Mbarara et Serere) en Ouganda pour évaluer la réaction de patate douce (SPVD) et le rendement de 15 clones. Létude était menée dans des blocks complétement choisis au hasard avec 4 reproductions par clone. Mbarara et Namulonge avaient une incidence élévée de SPVD (surface sous une courbe de maladie en progrès, AUDPC = 0,77; 0.47; respectivement), pendant que Serere avait la plus basse incidence (AVDPC = 0.13). Les clones obtenus du Centre International de Patates (CIP) étaient hautement susceptibles au SPVD, pendant que les races Ougandaises étaient tolérantes au SPVD même sous haute pression de la maladie. Les clones Zapallo, Kemb 37, Mugande et Araka rouge étaient les plus infectés par la maladie (AVDPC = 1,17; 1,04; 0,96; 0,77; respectivement), pendant que les clones 93-523, 93-1927, 93-493 et 93-319 chacun avait de manière conséquente un bas AVDPC aux travers tous les emplacements (AVDPC = 0, 086; 0,112; 0,114; 0,118). Le clone 93-52 était de haut rendement à travers tous les emplacements (17,7 t/ha) suivi par 93-1927, Kemb 37, Mugande et Tanzania (AVDPC = 16,9; 16,1; 15,6; 13,9 t/ha; respectivement). Les clones de plus bas rendement étaient 93-316, 93-663; 93-523 et 23/60. Les clones 93-29; 93-1096 et 23/60 étaient adaptables a tous les sites.
Mots Clés: Adaptabilité, analyse dAMMI, Ipomea batatas, stabilité de rendement
The need for resistant varieties of sweetpotato (Ipomoea batatas Lam.) to overcome sweetpotato virus disease (SPVD) has been evident in East Africa since a half-century ago, when the disease was first discovered in Uganda (Gibson et al., 2000). Use of resistant varieties is a cheap and effective means of controlling SPVD, and is compatible with subsistence agriculture in Uganda (Mwanga et al., 2001). The objective of sweetpotato variety improvement research undertakings is the development of varieties with high yielding potential, resistance to diseases and minimum sensitivity to seasonal fluctuations over a wide range of environments (Turyamureeba et al., 1997).
In Uganda, development of resistant varieties has been done through exchange of materials between National Agricultural Research Organisation (NARO), International Potato Centre (CIP), International Institute of Tropical Agriculture (IITA) and local germplasm collections. These materials were evaluated for yield and resistance to SPVD and weevils (Cylas spp). Varieties bred by IITA in West Africa for resistance to SPVD caused by sweetpotato chlorotic stunt virus (SPCSV WA) (Schaefers and Terry, 1976) and Nigerian isolates of Sweetpotato featherly mottle virus (SPFMV) succumb to SPVD when grown in Uganda (Mwanga et al., 1991). Also many CIP materials are wiped out by SPVD under natural field conditions (G. Turyamureba, Abii Agricultural Research and Development Centre, Arua, pers. comm.). In some cases, resistance in released varieties has broken down in locations where high SPVD pressure is experienced, resulting in variable performance across the test locations (Turyamureeba et al., 1997). Thus, the need to evaluate the reaction of several sweetpotato clones to SPVD infestation and yield performance under multiple locations in Uganda.
Recently, the introduction of pathogen-tested elite clones and varieties collected world wide for evaluation in sub-Saharan Africa, was initiated by CIP and these are being assessed. Parallel efforts by the Ugandan National Potato Programme has resulted into identification of promising clones through a series of evaluation and selection.
However, selection of stable clones is often difficult because of genotype and environment (G x E) interactions. The G x E interactions make it difficult for breeders to compare the performance of varieties across environments (Annicchiarico and Perenzin, 1994). An ideal clone should be one that does not vary in performance across environments. Fortunately, however, the use of the Additive Main Effects and Multiplicative Interaction (AMMI) model provides a clear basis for comparison of G x E interraction effects. The model utilises the standard two analysis of variance (ANOVA) and Principal Component Analysis (PCA) to identify patterns in the data (Gauch, 1992; Gauch and Zobel, 1996). The model is reported to be more effective in partitioning effects of genotypes, environment and their interaction than other models (Zobel and Wallace, 1995; Ntawuruhunga et al., 2001).
The AMMI model therefore was considered appropriate to data obtained in this study and was used to assess the reaction of several sweetpotato clones yield performance of sweetpotato under multiple locations in Uganda.
Materials and methods
There are five major sweetpotato producing agro-ecological zones in Uganda; the high altitude eastern zone, the southwestern highland zone, the northern and eastern short grassland zone (characterised by bimodal rainfall and prolonged dry spells) and southern tall grassland zone (occupies large areas along Lake Victoria) (Sweetpotato Survey, 2001 unpublished data) (Fig. 1). Study sites included Bulegeni District Farm Institute (Mbale), Kachwekano Agricultural Research and Development Centre (Kabale), Serere Agricultural and Animal Research Institute (SAARI), Namulonge Agricultural and Animal production Research Institute (NAARI) and Mbarara Stock Farm. These sites represent the major agroecological zones outlined above, respectively. Climatic conditions and soil characteristics of these sites are presented in Table 1.
A total of 15 clones were evaluated at each site and these included 93-1096, 1927, 93-663, 93-523, 93-493, 93-316, 93-319, 93-29, 23/60, obtained from the Uganda National Potato Programme advanced yield trials (AYT). The CIP sub-Saharan Africa office Nairobi, supplied Zapallo, Kemb37, and Mugande. Variety 52 (Naspot 1) released in 1995 (Mwanga et al., 2001), was used as the standard check variety. Clones Tanzania and Araka Red, widely grown locally, were the controls. There were two plantings for each site, second season, 2000; and first season 2001, (2000B and 2001A), respectively.
At all locations, the trials were established in a randomised complete block design with four replicates. Plots measured 5.4 m x 4 m. For each clone, slips of 30 cm were planted 30 cm apart, on ridges, on a net plot area of 21.6 m 2, for a total of 18 vines per row.
Disease (SPVD) incidence was assessed visually at monthly intervals. All plants in a plot were scored for the incidence. This was done for a period of five months starting from one month after planting. Virus disease incidence was recorded as number of plants with SPVD symptoms per plot. The severity of SPVD on infected (visible SPVD symptoms) plants was scored, according to Hahn (1979), using score scale of 1-5 (Table 2). In addition to SPVD, incidence infestation of Alternaria leaf spot was noted. At harvest, the number of marketable (i.e., measuring > 4 cm diameter) and non-marketable tubers (i.e., measuring < 4 cm diameter) were recorded. In addition, total fresh tuber weight was taken. Weevil damage was also recorded using a scale of 1 to 5 where 1=clean tuber; 2 = <20; 3 = 21 - 50; 4 = 51 - 80 and 5= >80% of surface area of tuber bored.
Data for the incidence of SPVD were used to calculate the area under the disease progress curve (AUDPC) as described by Campbell and Madden (1990). These AUDPC values and yields obtained were then subjected to ANOVA using SAS computer package (SAS, 2000). Separation of significant treatment means was done using the Least Significance Difference (LSD) (Gomez and Gomez, 1984). Yield data were also subjected to Additive Main effects and Multiplicative Interaction (AMMI) analysis to determine G x E interaction effects. The outputs were used to identify stable yielding clones. The AMMI statistical model (Gauch and Zobel 1996) is:
Y ger = µ + a g + b e + S n l gn d en + r ge + Eger
where: Y ger is the yield of genotype g in environment e for replicate r; µ is the grand mean, a g is the genotype g main effect; b e is the environment e main effect; n is the number of PCA axes retained in the model; l n is the singular value for PCA axis n; and y gn is the genotype eigenvector value for PCA axis n. Also, d en is the environment eigenvector value for PCA axis n; r ge is the residual G x E interaction, and Eger is the random experimental error.
Results and discussion
Incidence of SPVD in different environments. The AUDPC of SPVD of the sweetpotato clones in the different sites is shown in Table 3. The AUDPCs varied among clones at each site and across sites.
Clones Zapallo, Mugande, Araka red and Kemb37 were the most infected by SPVD at all sites, while clones 93-523, 93-319, 93-1927 and 93-493 had consistently lower AUDPC across all locations. Mbarara had the highest disease pressure followed by Namulonge, Kachwekano and Mbale, in that order. There was very low SPVD incidence in Serere compared to other sites but still, Kemb37 and Zapallo had significantly higher AUDPC than other clones.
These results indicate that the susceptibility of sweetpotato genotypes to SPVD is largely accounted for by the group of material, namely; (i) the clones of International Potato Center (CIP) origin, (ii) the indigenously developed clones and (iii) the control varieties, irrespective of the location. Introduced clones were highly susceptible to SPVD; with Kemb 37 and Zapallo recording the highest susceptability. Where there was high disease pressure, introduced clones had highest incidence. Even where the disease pressure was low, infection was observed on these clones. This shows that clones selected within high pressure areas of the disease will probably be more resistant compared to introduced clones. This is in agreement with the findings of Kanua and Floyd (1988) that the established varieties have been selected in response to major production constraint of sweetpotato in the area, in this case SPVD.
Clone Araka red had high infection in all sites except for Serere, where it is commonly grown, possibly because this area is characterised by low SPVD (Aritua et al., 1998). Thus, this clone may not be inherently resistant to SPVD. Clones 93-52 and Tanzania had considerable resistance even where there was high disease pressure. These materials had low SPVD incidence in the four locations and may be tolerant to SPVD. Other clones, i.e., 93-1096, 93-316, 23/60, 93-663, 93-29, 93-316 and 93-523 provided by the Uganda National Sweetpotato Programme also exhibited low disease incidence.
Alternaria leaf spot and stem blight caused by Alternaria spp., though a less prevalant disease in Uganda, is reportedly threatening the sweetpotato crop in some parts of the country. In this study it was observed on clones 93-52 and 93-29. The disease occurred mainly in two locations, Kachwekano and Bulegeni, these being highland areas. As reported elsewhere, the disease is severe in highland areas because it is favoured by low temperatures in these areas (Woolfe, 1992; Skoglund and Smit, 1994).
Yield performance and stability analysis. The results for analysis of variance of AMMI are shown in Table 4. Fresh tuber yield varied among clones (P<0.001) and environments (P<0.001), and significant (P<0.001) G x E interactions were observed (Table 4).
The first and second interaction principal component analysis (IPCA 1 and IPCA 2) were highly significant (P<0.001) in explaining the interaction effects between environments and genotypes.
Clones 93-52 gave the highest average tuber yield (17.7 kg ha -1) across the environments (Table 5). Clones 93-1927, Kemb 37 and Mugande followed in that order, yielding well above average. Least yielding clones in all environments were 93-316, 93-663 and 93-523. The highest yields were observed at Serere with average yields of 15.6 kg ha -1; while the Kachwekano site had the lowest yield average (8.1kg ha -1).
Considering IPCA 1, which had the highest interaction percentage, Namulonge had the highest eigen vector score in the second season and a score near the origin in the first season (Table 6, Fig. 2), showing a marked inconsistency for the location factor. Clone 93-316 was most interactive, while genotype 93-29 was the most stable clone, according to their scores on IPCA 1 (Fig. 2)
The biplots accounted for 90.1% of the total sums of squares. High yielding clones had positive interactions with Namulonge second season, Serere first season and Mbale second season. Namulonge first season was the most stable environment. Genotypes that were observed to interact positively with these environments were 93-52, 93-1927 and Mugande. They are well adapted to the above environments. Clones 92-3-29, 93-1096, 93-493, Tanzania, Araka red and Kemb37 were stable across the 10 environments and may be considered well adapted for all the sites.
The model (AMMI) revealed that the environment factor contributed the highest to the treatment sums of squares (43%) (Table 4). This is clearly demonstrated by the biplot graph, where performance of clones varied with seasons even in the same site. Yield of clones, therefore, was probably limited by rainfall (Table 1). Although sweetpotato is drought tolerant, tuber yields will depend on the amount of rainfall during the growth period (Purseglove, 1968; Woolfe, 1992). The other factor that influenced yield was SPVD infection. Where incidence of the disease was high, even high yielding clones did not perform well. Yield losses of up 90% due to SPVD have been reported (Gibson et al., 1998). The poor performance in Mbarara may be due to high incidence of the disease as recorded in this location (Table 3). Low yields were observed at Kabale location for both seasons probably due to low temperatures (Table 6), which affect root tuber development (Purseglove, 1968).
An ideal variety for selection should be one that is high yielding even where disease pressure is high (Kanua and Floyd, 1988) and has considerable resistance to pests and diseases. This study has shown that clones 93-1927, 93-493, 93-1096 and 93-29 are high yielding, and are tolerant to SPVD. These should be further evaluated on-farm with full participation of farmers in order to identify those with desired attributes. Clones Kemb 37, Zapallo and Mugande (CIP origin) are high yielding but highly susceptible to SPVD. They should be further bred to incorporate resistance to SPVD.
This study indicates that SPVD varies with sites and genotypes, with introduced clones being most affected. The G x E interaction effects for sweetpotato clones to virus disease and their yield performance exist among sweetpotato clones. Mbale and Kachwekano sites interacted similary and one site may be used for on station trials.
We are grateful to the Rockefeller Foundation and the Department for International Development (DFID) for providing the funds; the International Potato Center (CIP) and Natioal Agricultural Research Organisation (NARO) for providing germplasm.
©2002, African Crop Science Society
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