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African Crop Science Journal,Vol. 6. No.3, pp. 227-240, 1998 Genetics of resistance to Sphaceloma scab of Cowpea S. Tumwegamire, P.R. Rubaihayo and E. Adipala Department of Crop Science, Makerere University, P.O Box 7062, Kampala, Uganda (Received 9 April, 1998; accepted 1 June, 1998) Code Number:CS98025 ABSTRACT Although scab caused by Sphaceloma sp. is a major disease of cowpea in tropical and sub-tropical areas, little is known about the genetics of resistance to the disease. This study investigated i) relative importance of general (G.C.A) and specific (S.C.A) combining ability effects in the inheritance of resistance to scab on foliage and pods; ii) heritability of the resistance to scab infections on leaves and pods; iii) the genetic relationship between resistance to foliar and pod infections; and iv) the direct and indirect effects of scab foliar and pod infections on seed yield and yield components of cowpea. Ten cowpea lines [39, 46, Kvu 454, Kvu 175 -resistant; Iv 1658, Iv1075, 82, Kvu 530, SLA 59 -moderate resistance; and Era7 -susceptible] were half-diallel crossed during the first season of 1996 at Kabanyolo, Uganda. The F1 and F2 seeds were field grown together with their parents during the second season of 1996 and during the first season of 1997, respectively. A randomised complete block design (RCBD) with three replications consisting of single row plots, 3 m long, spaced at 75 cm apart, were used for all trials. Scab infected cowpea residues were introduced between rows three weeks after emergence (WAE) to ensure uniform scab infection. Foliar and pod scab severities were scored independently basing on a scale of 1 (resistant) to 5 (susceptible). Diallel analysis was done according to Griffing's (1956) method 2 model 1. Both G.C.A and S.C.A effects were important for resistance to scab infection whether on the foliage or pods suggesting the importance of both additive and non-additive gene actions for scab resistance. However, high G.C.A:S.C.A ratios (37.2 and 35.8 for foliar and pod scab severities, respectively) indicated preponderance of additive genetic variance for each trait. The resistant parents were found good general combiners for resistance to scab infections. Broad-sense heritability estimates were high for both resistance to foliar (93.8%) and to pod scab infections (97.0%). Narrow-sense heritability estimates were also high for both traits (79.8% and 84.5%, respectively). Correlation analysis revealed high phenotypic (0.983) and genotypic (0.949) correlation coefficients indicating a strong relationship between the two traits. Foliar scab severity exhibited a negative direct effect on yield, but also indirectly affected it through reduced pods per plant and pod length. Similarly, pod scab severity indirectly reduced yield through pods per plant and pod length. This suggested deleterious effects of scab disease directly on yield or indirectly through reduction in the number of pods per plant and pod length. Key Words: Combining abilities, heritability, scab, Sphaceloma, Vigna unguiculata RÉSUMÉ Quoique la gale causée par Sphaceloma sp. est une maladie importante de vigna dans les zones tropicales et sous tropicales, il y a cependant peu d'information concernant la génétique de la resistance de la maladie. Cette étude a été conduite pour déterminer i) l'importance relative des effets de l'habileté à la combinaison générale (G.C.A) et spécifique (S.C.A) dans l'héritage de la résistance à la gale sur les feuilles et les gousses; ii) l'héritabilité de la résistance aux infections de la gale sur les feuilles et sur les gousses; iii) la relation génétique entre la résistance des feuilles et de gousses à l'infection; et les effets directs et indirects de la gale des feuilles et de gousses sur le rendement en graines et les composantes de rendement de vigna. Dix lign_es de vigna {39, 46, Kvu 454, Kvu 175 -résistantes; Iv 1658, Iv 1075, 82, Kvu 530, SLA 59 - modérement résistantes; et Era 7 - susceptible] ont été croisées moitié-diallèles pendant la première saison de 1996 à Kabanyolo, Uganda. Les semences de F1 et F2 ont été semées au champ ensemble avec leurs parents respectivement pendant la seconde saison de 1996 et la première saison de 1977. Un modèle de bloc randomisé complet avec trois répétitions consistant des parcelles d'une seule rangée de 3 m de longueur et espacée de 75 cm a été utilisé pour tous les essais. Les résidues des plants infectés avec la gale ont été placées entre les rangées trois semaines après l'émergence afin d'assurer une infection uniforme de la maladie. La sévérité de la gale sur les feuilles et celle sur les gousses ont été évaluées indépendamment selon une échelle de 1 (résistant) à 5 (susceptible). L'analyse diallélique a été faite suivant la méthode 2 modèle 1 de Griffing. Les effets de GCA et SCA ont été importants pour la résistance à la gale soit sur les feuilles ou sur les gousses suggérant ainsi l'importance des actions additives et non-additives des gènes pour la résistance à la gale. Cependant, les rapports élevés entre GCA:SCA (37.2 et 35.8, respectivement pour la sévérité sur les feuilles et sur les gousses) ont indiqué la prépondérance de la variance génetique additive pour chaque trait. Les parents résistants ont été trouvés de bons combinateurs généraux pour résistance aux infections de la gale. Les estimations d'héritabilité au sens large ont été élevées et pour la résistance des feuilles ( 93.8 %) et pour celle des gousses (97.0 %) aux infections de la gale. Les estimations d'héritabilité a sens étroit ont été aussi élevées (79.8 % et 84.5 %, respectivement pour tous les deux traits). L'analyse de la corrélation a montré des hauts coefficients de corrélation phénotypique (0.983) et génotypique (0.949) indiquant ainsi une forte relation entre les deux traits. La sévérité de la gale sur les feuilles a exhibé un effet négatif direct sur le rendement , mais elle a aussi affecté le rendement en réduisant le nombre de gousses par plant et la longueur des gousses. De méme, la sévérité de la gale sur les gousses a indirectement réduit le rendement en réduisant le nombre des gousses et la longueur des plants. Ceci a montré les effets déstructifs de la gale sur le rendement à travers la réduction du nombre des gousses par plant et la longueur des gousses. Mots Clés: Habiletés à la combinaison, héritabilité, gale, Sphaceloma, Vigna unguiculata INTRODUCTION Cowpeas (Vigna unguiculata L. Walp) are widely distributed throughout the tropics and sub-tropics with major producing areas being Nigeria, Niger, Burkina Faso, Togo, Senegal, Tanzania, Sudan, Uganda, Malawi, Zimbabwe, and South Africa (Purseglove, 1988). In Uganda, 90% of the crop is grown in the Northern and North Eastern regions (Rusoke and Rubaihayo, 1994; Sabiti et al., 1994; Omongo et al., 1997). While cowpea seed yields greater than 2000kg ha-1 are common in experimental stations (Rusoke and Rubaihayo, 1994; Edema, 1995; Omongo, 1996; Bua, 1996), only 200-400 kg ha-1 are achieved at the farm level (Sabiti et al., 1994; Omongo, 1996). In Nigeria, yields average 100-300 kg ha-1 (Raheja, 1986; Singh et al., 1992), whereas in Ghana, yields are even much lower, 80 kg ha-1 (Marfo, 1986). These low yields result mainly from insects and diseases attacks, but insect pests cause the greatest yield reductions (Edema and Adipala, 1996; Omongo et al., 1997). Under disease epiphytotic conditions, however, diseases can also cause significant yield losses (Rusoke and Rubaihayo, 1994; Edema and Adipala, 1994; Edema and Adipala, 1996; Edema et al., 1997). Scab, caused by a species of Sphaceloma, is a major and damaging disease in Uganda (Iceduna, 1993; Edema et al., 1997), and elsewhere in Africa (Emechebe, 1980; Allen, 1983). Sphaceloma scab is characterised by development of silvery grey, circular to oval lesions on stems, leaves and their petioles, peduncles and pods. In severe infections, such lesions coalesce, causing distortion and flower bud abortion (Singh and Allen, 1979; Singh and Rachie, 1985; Iceduna, 1993). Conditions like successive days of wet weather are ideal for scab development (Emechebe, 1980; Singh and Rachie, 1985), although Iceduna (1993) observed more disease during dry conditions in Uganda. Secondary spread of conidia is by rain splash, run-off and wind-blown moisture (Singh and Allen, 1979; Singh and Rachie, 1985). Scab has been observed to cause yield losses of up to 60% and total crop destruction under disease epiphytotic conditions have been recorded in Nigeria (Emechebe, 1980). It is especially destructive on the leaves and pods. Under severe infections, leaves become "cupped" with numerous whitish scab lesions that often give rise to "short holes" (Emechebe, 1980; Iceduna, 1993). Scab has a puckering effect on the leaf lamina. These attacks reduce the photosynthetic surface area of the leaves. Grain yields are also reduced through scab's deleterious effects on yield components like number of pods, and seeds per pod. Pods usually become deformed and are sometimes transformed into mummies (Emechebe, 1980; Iceduna, 1993). Control measures recommended against scab are sanitation, crop rotation, fungicidal spray and host resistance. Sanitation and crop rotation require community application in order to be effective. This is hardly achieved. The fungicide mancozeb (Dithane M-45) (Iceduna et al., 1994) or a mixture of benomyl and mancozeb (Emechebe, 1980) effectively control the disease. However, the resource poor peasant farmers, who grow > 90% of the crop, can not afford the costs involved in the use of chemical sprays (Edema, 1995). Therefore, the use of host resistance is perhaps the most practical control measure available to the farmers, and is environment friendly (Rusoke and Rubaihayo, 1994). In Uganda, some field studies indicated possible existence of scab resistant lines within Kabanyolo germplasm collection (Takan, 1989; Iceduna et al., 1994; Nakawuka and Adipala, 1997). These studies identified lines 46, 39, Kvu 145, and Kvu 454 as resistant (Iceduna et al., 1994). Nakawuka and Adipala (1997), suggested greater role of general combining ability (G.C.A) effects than specific combining ability (S.C.A) effects in the inheritance of resistance to scab but they also recommended further studies into the genetics of the disease. Therefore, nature and mechanism of inheritance of resistance to the scab fungus needs to be established to facilitate planning and implementation of a breeding programme for resistance to the disease. The present study was intended to gain insight into the genetics of inheritance of resistance to Sphaceloma scab, particularly to foliar and pod infections. In the study of Iceduna (1993), some lines which were resistant to foliar infection succumbed to pod infection or vice-versa. This suggested the possibility that resistance to foliar and pod infection might be under the control of different genes. Therefore, knowledge of how foliar scab infection relates to pod scab infection is needed. This is important because more often than not, evaluation and screening trials have been based on assessment of the whole plant disease situation (Iceduna et al., 1994). This may not always be reliable; indeed Nakawuka (1995) found no relationship between the two stages of infection in some of the lines studied. It is therefore important to gain insight into the nature of resistance to the two stages of the disease. Such information would facilitate proper disease assessment and identification of resistance sources. Also lacking is information on how each stage of scab infection relates to yield and yield components of cowpea. This study therefore, attempted to establish the relationship, if any, between resistances to foliar and to pod scab infections as well as determining the effect of the two stages of infection on seed yield and seed yield components of cowpea. MATERIALS AND METHODS The materials studied are listed in Table 1 and included 10 cowpea parents selected among 80 cowpea accessions screened for scab resistance in previous studies (Iceduna et al., 1994; Nakawuka, 1995). Four of the parents (39, 46, Kvu454, Kvu175) were reported to be resistant (R), five parents (iv 1658, iv 1075, 82, Kvu530, SLA59) were moderate resistant (MR) and one (Era7) was susceptible (S) (Iceduna et al., 1994; Nakawuka, 1995). TABLE 1. Cowpea parents used for genetic studies of resistance to Sphaceloma scab
These parents were considered as a fixed sample so that inferences from analysis could be limited to parents themselves and the resultant set of crosses. Crosses were made in the mesh house among the 10 parents according to Griffing's (1956) diallel experimental method 2 model 1. The F1 and F2 generation evaluations were conducted in the field, at Makerere University Agricultural Research Institute, Kabanyolo (MUARIK) during the second rains (October-December) of 1996 and first rains of 1997 (March-June). Flowers used as females were emasculated in the evening using a pair of scissors regularly sterilised using alcohol (95%), and hand-pollinated in the morning hours of the next day (IITA, 1982). Flowers at all plant growth stages were used to optimise the number of successful crosses. F1 mature seeds from each successful cross were harvested separately, and were field grown during the second rainy season of 1996 at MUARIK. A field previously under Solanum potato was used to reduce inoculum levels of diseases of cowpeas. A randomised complete block design (RCBD) with three replications and 3-m single-row plots of ten plants each was used. The rows were spaced at 75cm apart with within row spacing of 30cm. Insect pests like aphids (Aphis craccivora Koch), thrips (Megalurothrips sjostedti Tryborn), sucking bugs (Clavigralla spp.) and pod borers (Maruca testularis Geyer) were controlled by spraying the plants with dimethioate (200 g ai ha-1) equivalent to 30ml per every 18 litres of water. This also enabled the control of viral diseases transmitted by some of these insects. The crop was not protected from other pathogens. To ensure uniform scab infection the trials were artificially inoculated with scab infested cowpea debris from previous season. The infected residues were put along the rows, three weeks after seedling emergence. Plots were clean weeded by hand hoeing two and four weeks after seedling emergence. F2 progeny evaluation was carried out during the first rainy season of 1997 also at MUARIK. Similar design, spacing, plot size, source of inoculum, and inoculation technique were used as for the F1 evaluation. Scab severity data were collected for both F1 and F2 generations. Six to seven weeks after emergence, five plants were randomly selected in each plot, tagged and severity ratings done visually on leaves and pods basing on a scale of 1 (resistant) to 5 (susceptible) (Nakawuka and Adipala, 1997). Disease ratings were done weekly for at least five weeks in each of the two experiments. Data on yield and yield components were also recorded. The yield components considered were number of branches per plant, number of pods per plant, seeds per pod, pod length (measured in cm), number of peduncles per plant, and 100 seed weight. Grain yield (kg ha-1) was estimated from yield per replicate plot. Data met the assumptions for analysis of variance, and as such, plot means were subjected to analysis of variance. F1 generation severity mean values were analysed to determine general (G.C.A) and specific (S.C.A) combining ability effects for foliar and pod severity ratings using method 2 model 1 (Griffing, 1956). The Least significant difference (LSD) was calculated and used to compare severity means, G.C.A effects and S.C.A effects. Variance and covariance analyses were performed for yield, yield components and scab severities and values were used to estimate phenotypic (rp) and genotypic (rg) correlation coefficients as described by Miller et al. (1957). Principal component analysis was also performed to establish the contributions of scab disease on yield and its components. To determine the direct and indirect effects of foliar and pod infections on yield components and grain yield, path analysis was performed as described by Dewey and Lu, 1958. Broad sense heritability estimates were obtained using variance components method (Johnson et al., 1955), for resistance to scab infection on foliage and pods. Narrow sense heritability estimates for both traits were obtained by regressing F2 progeny means on to F1 parental means evaluated in the same environment but different years (Vogel et al., 1980; Casler, 1982). This method takes into account and eliminates biases due to genotype x environmental interaction effects and error covariances between parents and progenies (Casler, 1982). The regression coefficient values were converted directly to heritability estimates, i.e., h2=b since cowpea is a self-pollinating crop (Vogel et al., 1980). RESULTS AND DISCUSSION Significant genotypic effects (P<0.001) were observed in both generations on foliage and pod scab infection (Table 2) suggesting great variability of resistance in the plants to scab. Disease reactions on leaves and pods of the F1, and F2 generations together with parents are presented in Table 3. Mean severities ranged from 1.00 to 4.00 on the foliage and 1.067 to 4.20 on the pods. Parents 39, 46 Kvu454, Kvu175, showed resistance reaction (<2.00) for both foliar and pod scab infections as expected from the work of Nakawuka (1995). Parents SLA59 and Era7 succumbed to the disease (>3.0) on both parts of the plant although SLA59 was expected to express moderate resistance (Iceduna et al., 1994). The rest showed moderate resistance (>2.00 to <3.0) as expected. The majority of the F1 hybrids showed intermediate reaction to the disease infection in comparison to their parents. However, there were some deviants, like 39 * 46 and 39 * Kvu454 which expressed more resistance than the better parent and Iv1658 * Kvu530 which expressed resistance below the poor parent's reaction. Hybrids 46 * Kvu530, Kvu454 * Kvu530, and 82 * Kvu530 also expressed resistance above, whereas Iv1075 * Kvu530 showed resistance below parents' values, only on the foliage. The F1 hybrids between the most susceptible parents, SLA59 and Era7, were very susceptible to both infections. TABLE 2. Mean squares of scab severity ratings of foliage and pods of F1 and parents and F2 and parents, grown in 1996 and 1997, respectively
b Genotypes are F, hybrids and their parents** are significant at P0.01 level *** are significant at P.001 level TABLE 3. Means of cowpea scab seventies on foliage and pods of F1 and F2 generations with the parents grown at Kabanyolo, during the second season of 1996 and first season of 1997, respectively
a Genotypes are F1 and F2 hybrids and their parentsbScab severities on foliage and pods were scored using a scale of 1-5. where 1 is resistant and 5 is very susceptible The significant genotypic effects recorded among the F1 hybrids and parents (Table 2) paved way for combining ability analysis as required (Griffing, 1956). Mean squares for both general (G.C.A) and specific (S.C.A) combining ability effects for resistance to scab infections on the foliage and pods of the F1 and the parents are presented in Table 4. TABLE 4. Mean squares from combining ability analysis for scab severities on both the foliage and pods of cowpea hybrids and parents grown during the second season of 1996
**significant at P<0.01 probability level TABLE 5. Estimates of general combining ability (GCA) effects for scab severities scored on the foliage and pods of cowpea F1 hybrids and parents during the second season of 1996
Gi and Gi-Gj variances for foliar scab severity are 0.0015 and 0.0033, respectively; for pod scab infections are 0.0009 and 0.0019 The G.C.A means tested significant (P<0.001) for resistance to both types of infections. The mean squares for S.C.A effects also tested significant in both cases. This indicated existence of both additive and non-additive gene effects for the inheritance of resistance to scab on the foliage and pods (Baker, 1978). However, comparing the relative magnitudes of G.C.A and S.C.A, all the ratios of GCA:SCA (Table 4) for all traits measured were greater than unity indicating preponderance of additive gene effects for the inheritance of resistance to scab infections. Similar observation was made on yield components by Mak and Yap (1977) in long beans and Nakawuka (1995) on whole plant disease assessment in cowpeas. Additive gene effects suggests involvement of minor genes and hence quantitative inheritance of resistance to scab (Falconer, 1989). However, the number and the identity of such genes were not established by the present study, and further study to this effect would be necessary. The predominant component of genetic variation determines the choice of an efficient breeding method for incorporation of concerned genes into new materials (Dabholkal et al., 1989). The results also suggest that breeding methods like recurrent selection that exploit additive genetic effects may be useful in incorporating resistance (assumed to be polygenic) of scab to susceptible cowpea varieties (Dabholkal et al., 1989). Estimates of G.C.A effects associated with each of ten inbreeds for both foliar and pod scab infections are shown in Table 5. Parents 39, 46, Kvu454, and Kvu175 showed negative G.C.A effects whereas parents SLA59 and Era7 showed the highest positive G.C.A effects. The negative estimates indicated a high gene frequency for scab resistance in the former parents while positive ones indicated a low gene frequency for scab resistance (Gevers and Lake, 1994). The former parents are thus good sources and best combiners for resistance in a hybridisation programme. G.C.A estimate of inbred 39 was the highest for foliar scab resistance while that of inbred 46 was the highest for pod scab resistance (Table 5) suggesting that both lines combine best for the inheritance of resistance to scab infection on foliage and pods (Gevers and Lake, 1994). The same parents were recommended by Nakawuka (1995) as best sources of resistance on the basis of the whole plant infection. The G.C.A effects of parents for the two infection stages showed a significant (P<0.001) correlation (r=0.978) probably reflecting similarity in the genetic mechanisms conditioning resistance to the two stages of scab infections (Wallace and El-Zik, 1989). William et al. (1989) suggested similar genes conferring resistance to fall armyworm and to southwestern borer in maize, as a result of a high correlation coefficient of G.C.A effects between the two. TABLE 6. Estimates of specific combining ability (SCA) effects for scab severities scored on foliage and pods on hybrids and parents during the second season of 1996
S.C.A effects for each of the 45 individual crosses are presented in Table 6. Significant positive S.C.A effects were observed in crosses between parents 46 and Kvu530 for both foliar and pod scab infections. Significant positive S.C.A effects were also observed for crosses 39 * 82, 39 * Kvu530 and 39 * SLA59 for pod scab resistance and Kvu 454 * Kvu 530 for foliar scab resistance. The positive S.C.A effects suggested that those respective specific combinations would be useful to breeders in improvement of scab resistance in cowpea (Lal et al., 1975; Jatasra, 1980). Hybrid 39 * Kvu454 was the only one with significant negative S.C.A effects. Such a hybrid may not be helpful in improvement of scab resistance as the negative values are indicative of increase in susceptibility compared to the parents (Jatasra, 1980; Kang et al., 1995). The second objective of the present study was to estimate heritability for scab resistance on cowpea; an aspect which was lacking in literature. This was done using scab scores for the parents and F1's grown in 1996 and F2's grown in 1997 (Table 7). The estimates varied with the type of the estimate as indicated by Fehr (1987) and Falconer (1989). Broad sense heritability (h2B) estimates, obtained by variance component method, were found to be high for resistance to both types of infections. This was an indication of a high proportionate genetic variance value attributable to the observable phenotypic variance (Falconer, 1989). Direct selection based on phenotypic variance for scab resistance would therefore suffice for improvement of resistance against both types of infections (Falconer, 1989; Diz and Schank, 1995). TABLE 7. Heritability estimates for cowpea scab resistance to foliar and pod infection
h2B =broad-sense heritability TABLE 8. Phenotypic and genotypic correlation coefficients of yield, yield components and scab severities on foliage and pods of cowpea
P and G are phenotypic and genotypic correlation coefficients, respectively The correlation coefficient must exceed 0.15 and 0.30 to be significant at the 0.05 and 0.01 probability levels, respectively Narrow-sense heritability (h2B) estimates were also high, though lower than broad-sense ones suggesting a high proportionate of additive genetic variance value attributable to the genetic variance observed (Falconer, 1989). This suggested additive nature of inheritance of scab resistance as suggested by the results of combining ability (Table 5). Uguru (1995), similarly concluded additive inheritance for pod length following high narrow-sense heritability estimates obtained. Heritability has a major impact on the choice of the methods of breeding (Fehr, 1987). For example, Uguru (1995) recommended application of selection pressure to improve pod length of cowpea given high heritability estimates obtained. Recurrent phenotypic selection was recommended to improve resistance to leaf spot disease (Septoria spraguei) in Russian Wildrye (Berdahl and Krupinsky, 1995) and for improvement of seed related characters in hexaploid hybrids between pearl millet and elephant grass (Diz and Schank, 1995) given the higher narrow-sense heritability estimates obtained for the respective characters. Pedigree breeding approach with selection beginning in F2 generation was suggested (Marfo and Hall, 1992) to incorporate genes conferring heat tolerance during early floral bud development in cowpea. The present study recommends phenotypic recurrent selection to improve scab resistance in cowpea varieties. The phenotypic and genotypic correlation coefficients for all possible comparisons among the nine traits studied, are presented in Table 8. Both types of coefficients were comparable in magnitude for most comparisons, signifying a fairly small error effect in the estimates (Miller et al., 1957; Dewey and Lu, 1958). Number of peduncles exhibited the highest coefficient (r= 0.915) with yield followed by pods per plant, seed weight, number of branches and pod length in that order. This suggested that number of peduncles and the latter yield components to be the major yield factors in cowpea (Patil and Bhapkar, 1987; Uguru, 1995). The genotypic foliar and pod scab severity correlation (rg =0.949) was significant (P<0.001) probably indicating that similar genes or genetic mechanisms were involved in the inheritance of resistance to the two types of scab infection (Wallace and El-Zik, 1989). These results differ from the report of Nakawuka (1995) which suggested that the two traits were inherited differently in some of the cultivars studied. Both foliar and pod scab infections exhibited significant (P<0.05) negative correlations (r = -0.223; r= -0.202, respectively) with yield of cowpea. Their corresponding phenotypic coefficients were also significant (0.05) though lower in magnitude (r= -0.186 and r= -0.159, respectively). High genotypic variance of the materials studied with respect to scab resistance may be responsible for the low coefficients observed (Miller et al., 1957; Dewey and Lu, 1958). However, the results suggested deleterious effects on grain yield of cowpea. Considering the yield components, foliar scab infection showed negative correlations with all of them but significantly (P<0.05) with number of seeds per pod (r= -0.213) and number of branches (r= -0.194). Pod scab severity similarly showed negative correlations with all the yield components but significantly (P<0.05) with number of branches (r= -0.213), number of pods per plant (r= -0.198), pod length (r= -0.177), seeds per pod (r= -0.225) and number of peduncles per plant (r= -0.187). These results suggest deleterious effects of scab disease on yield components as earlier on suggested by Emechebe (1980). Similar association of disease level with yield and plant height was reported for septoria leaf blotch in wheat (Camacho-Casas et al., 1995). Principal component analysis results (Table 9) were consistent with results from correlation analysis. Pods per plant and number of peduncles per plant were the major yield contributors, followed by number of branches, seed weight, and pod length. Results also indicated negative contributions of both the foliar and pod scab infection on grain yield and yield components of cowpea. Both stages of infection were found to have significant (P<0.05) negative effects on the number of seeds per pod, the same component correlations were negative with both types of infection. TABLE 9. Latent vectors of principal component analysis based on correlation matrix for scab severities, yield and yield components of cowpea
The coefficients must exceed 0.15 and 0.30 to be significant at the 0.05 and 0.01 probability levels. respectively The correlation coefficients were partitioned into direct and indirect causes or effects and the results are presented in Table 10. Residual effects (0.462) indicated that, 53.78% of the variation observed in grain yield of cowpea was directly and indirectly influenced by the yield components and scab infections on the foliage and pods. TABLE 10. Direct and indirect effects of scab severities on foliage and pods, and yield components on grain yield of cowpea
Residual factors 0.462 Number of pods per plant (1.669), followed by pod length (1.375) showed the largest positive direct effects on the grain yield suggesting that pods per plant and pod length are the major yield components for grain yield of cowpea. Similar findings were reported by Nakawuka (1995) and Chauhan et al. (1980) also working on cowpea. Number of pods per plant is also a major contributor to grain yield of pigeonpeas (Patel et al., 1988). However, more important for the purpose of this study were effects of scab on yield and its components. Negative effects of scab were evident on yield and yield components (Table 10). Foliar scab severity exhibited negative direct effects (0.588) suggesting its reducing effect on yield. This agrees with Emechebe (1980) who reported that scab causes leaf "cupping", leaf lamina puckering, and "short holes" on the affected leaves hence reducing their photosynthetic surface area and capacity. According to Hay and Walker (1989) reductions in photosynthetic surface area and capacity of leaves affects grain filling and growth; this may account for the reducing effects of foliar scab severity on grain yield. Foliar scab severity was also found to affect cowpea grain yields indirectly through its reducing effects on the number of pods per plant (-0.195) and pod length (-0.243). Despite the unexpected direct positive effects exhibited, pod scab severity was similarly found to indirectly affect grain yield through the same yield components (-0.330 and -0.175, respectively) in addition to foliar scab severity (-0.558). These findings suggested further deleterious effects of scab on grain yield of cowpea through reducing the number of pods per plant and pod length. Similar conclusions were made by Camacho-Cacas et al. (1995) based on the negative direct effects of septoria leaf blotch severity on yields of wheat. In addition, Emechebe (1980) and Iceduna (1993) observed that pods become deformed and transformed into mummies thus reducing their length. These observations explain the yield reducing effects of scab shown through reducing number of pods per plant and pod length. Our findings indicate that additive gene action perhaps play a greater role than non-additive gene effects on inheritance of resistance to scab whether on foliage or pods. The resistance is also highly heritable and phenotypic recurrent selection may serve as the appropriate breeding method. Findings also indicate that foliar and pod scab infection stages may be under similar gene mechanisms. The deleterious effects of scab were found to be through its reduction in pod numbers and pod length. Acknowledgement This paper is part of M.Sc. thesis submitted by the first author to Makerere University, Kampala, Uganda. Funding was provided by the Rockefeller Foundation Forum on Agricultural Resource Husbundry Grant RF95007# 77. 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