African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730
1996
African Crop Science Journal,Vol. 4. No.l, pp. 29-40, 1996

Yield stability in relation to Striga resistance in cowpea production in West and Central Africa

N. MULEBA, J.T. OUEDRAOGO^1 and I. DRABO^1

SAFGRAD, 01 B.P. 1783, Ouagadougou 01, Burkina Faso ^1 INERA, 01 B.P. 7192, Ouagadougou 01, Burkina Faso

(Received 21 November, 1994; accepted 20 March, 1995)


Code Number: CS96037
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ABSTRACT

Striga gesnerioides causes severe cowpea ( Vigna unguiculata ) yield losses in West and Central Africa. Fields have been abandoned by farmers due to crop failures attributed to Striga-infestations. Studies comprising three sowing dates and two regional cultivar adaptation trials were conducted in fields naturally infested by Striga. The sowing date experiments were established in the Sudan savanna zone in Burkina Faso from 1984 to 1987. Three dates, namely, early (late-June to early-July), intermediate (mid- to late-July), and late (early- to mid-August) were used. The first regional adaptation trial included some countries in West Africa in 1989 and 1990. The second trial was extended to Central Africa in 1991 and 1992. The objectives were to study Striga- resistance of genotypes, its stability (or durability) both in time and space and to devise production technologies for minimising yield losses under Striga - infestations. Cultivar Suvita-2 had high and stable resistance to Striga-infestation and a high yield that was least stable across years. Regionally, no single cultivar was immune from Striga-infestations across locations. Only B301, in contrast to two other resistant controls IT82D-849 and Suvita-2, combined high stable resistance to Striga-infestation and high stable yield under Striga-infestations across locations in West and Central Africa. Other cultivars that combined high or moderate resistance to Striga infestation and high yield, and moderate to high stability across locations included TN93-80 and TNI21-80, in West Africa, and KVx402-19-1 in West and Central Africa. Although no genotype was invulnerable to Striga-infestation in all locations of the sub-region, it appears that Striga damage could be controlled through proper choice of sowing dates and better adapted, high yielding and Striga resistant cultivars.

Key Words: Striga gesnerioides, tolerance, Vigna unguiculata

RESUME

Le Striga gesnerioides inflige d'importants degats a la culture en Afrique du niebe (Vigna unguiculata en de l'Ouest et du Centre Des champs sont abandonnes par les paysans suite aux cultures completement ratees, attibuees aux attaques de Striga. Des etudes comprenant les dates de semis et deuz essais regionaux d' adaptation de cultivars ont ete conduites en parcelles infestees naturellement par la Striga. Les essais de dates de semis etaient implantes en savane Soudanienne au Burkina Faso de 1984 a 1987. Ils consistaient en: semis precoce (fin Juin a debut Juillet), semis intermediaire (mi-a fin Juillet), et semis tardif (debut a mi-Aout). Les essais regionaux d'adaptation ont ete menes: le premier, dans quelques pays de l'Afrique de l'Ouest en 1989 et 1990 et le second, etait etendu jusqu'en Afrique du Centre en 1991 et 1992. Les objectifs de ces etudes consistaient a evaluer la resistance au Striga des genotypes ainsi qu'a determiner la stabilite de celle-ci tant dans le temps que dans l'espace eta de nouvelles technologies de production devant restreindre au minimum les pertes de rendement infligees par le Striga a la culture du niebe. Le cultivar Suvita-2 avait une forte resistance aux attaques de Striga et un haut rendement. La resistance tout comme le rendement de ce cultivar etaient stables a tres stables a travers les annees. En essais regionaux, aucun genotype n'etait exempt d'attaques de Striga a toutes les localites. Seul B301, contrairement aux deux autres temoins resistants IT82D-849 et Suvita2, combinair une forte resistance aux attaques de Striga et un haut rendement qui etaient tres stables a travers les localites tant en Afrique de l'Ouest que du Centre. Les autres cultivars combinant une resistance au Striga et un haut rendement stables a tres stables a travers les localites etaient: TN9380 et TN121-80 pour l'Afrique de l'Ouest, et KVx402-19-1 pour l'Afrique de l'Ouest et du Centre. Bien qu'aucun genotype n'etait invulnerable aux attaques de Striga a toutes les localites dans la sous region, il apparait que les degats infliges par le Striga a la culture du niebe peuvent etre restreints au minimum a travers un choix judicieux de la date optimale de semis, et des cultivars bien adaptes a haut rendement et resistants au Striga.

Mots Cles: Striga gesnerioides, tolerance, Vigna unguiculata

INTRODUCTION

Striga gesnerioides (Willd.) Vatke, attacks cowpea (Vigna unguiculata (L.) Walp.) in West and Central Africa, particularly in the Sahel and Sudan savanna zones (Musselman and Parker, 1982; Ramaiah et al., 1983). It is also found in the coastal savanna along the Atlantic Ocean as well as on sandy and/or shallow gravelly soils in the Guinea savannas in Benin, Ghana, Togo and Sierra Leone (RENACO, 1990). There exist, at least three locations where cowpea suffers from severe Striga damage in West Africa; in the triangle between Lokossa, Bihicon and Save in southern Benin, Congo/Bolgatanga in northern Ghana; and Cinzana, Malt. Cowpea fields in these regions are being abandoned. Elsewhere, in northern Nigeria, Striga may cause cowpea yield losses varying from few kg ha^-1 to total crop failure (Obilana, 1987): this had also led to the abandonment of Striga infested fields to cowpea production (Emechebe and Leleji, 1988). Recent reports also suggest that the productivity of new improved cowpea cultivars is being reduced under farming conditions in Striga-infested regions of Ghana, Mali and Nigeria (Sanders et al., 1994).

Cowpea cultivars resistant to Striga have been identified (Aggarwal et al., 1984; Singh and Emechebe, 1990; Aggarwal, 1991 ). The resistance of Suvita-2 and B301 to Striga was reported to be controlled by a single dominant gene (Aggarwal et al., 1984; Singh and Emechebe, 1990). However, Obilana (1987), reviewing data of Aggarwal et al. (1984), discussed the quantitative nature of the gene action controlling cowpea Striga-resistance. Suvita-2 and some of progenies exhibited high resistance to Striga only, in Burkina Faso and, to some extent, in Malt. The same genotypes were particularly vulnerable Striga infestations in Niger and Nigeria. They, therefore, contrasted with B301 and IT82D-849 which exhibited high resistance in Burkina Faso, Niger and Nigeria (Aggarwal. 1991). Therefore, genetic resistance offers one of the means controlling Striga in West and Central Africa.

Cowpea management studies related to Striga infestations conducted on day length sensitive (DS) cultivars (Muleba and Mosarwe, 1994, showed that these cultivars were particularly vulnerable to Striga attacks when sown before mid-July in the Sudan savanna zone. This, however, contrasted with the reaction of day length insensitive (DI) cultivars which gave maximum yield in early sowing (late June to early July) and suffered severe damage only in late sowing (late July to early August) due to the combinations of drought and Striga attacks. They also tended to yield equal to or higher than DS cultivars. Such results, coupled with the monogenic nature of Striga resistance, suggested that more agronomic research was needed to mitigate Striga damage on cowpea for sustainable production by the peasant farmers. The purpose of this paper is to report progress made since 1984 in the management of cowpea, particularly DI cultivars, under Striga infestations; the identification of new Striga-resistant and high yielding cultivars, and the stability of the resistance and yield of the cultivars across locations in semi-arid West and Central Africa.

MATERIALS AND METHODS

Two sets of experiments consisting of sowing date and genotypic responses under natural Striga infestations were Striga infestations were conducted.

Sowing date experiments. Five cowpea cultivars susceptible to Striga-infestations were tested against a resistant cultivar, Suvita-2, using three sowing dates in a Striga-infested plot. Five of the cultivars; TVx3236, TN88-63, IT82E-32, KN-I and Suvita-2 were day length insensitive (DI); while the sixth, Local Ouahigouya or Local Koakin, was day length sensitive (DS).

The cultivars used were described elsewhere for their adaptation to the Sudan savanna in the absence of Striga-infestations (Muleba et al., 1991). The experiment was established in the Sudan savanna at Kamboinse and Gampela, Burkina Faso in 1984-1986 and in 1987. The sowing dates were: early (20 June to 5 July), intermediate (10 to 25 July) and late (1 to 15 August). The local DS cultivar, Ouahigouya, was replaced by the local DS cultivar, Koakin, from 1986 to 1987 because the former's critical photoperiod in early to mid-September exposed it to severe drought damage in mid- to late-September during flowering, pod set and grain filling. In 1987, the experiment was discontinued at Kamboinse and re-located at Gampela (40 km from Kamboinse) because Striga intensity at Kamboinse had declined to an extent that it had little impact on cowpea productivity.

The experiment was a split-plot design, with four replications. Sowing dates were assigned to main-plots and cowpea cultivars to sub-plots. Plots were 4.5 x 5 m. After ploughing and harrowing the field, one-year-old Striga plant materials (crushed seeds and plant debris rate of 10 g m^-2 ) mixed with wet sand at and, were broadcast on the field and dug under with a hand-hoe to ensure uniform Striga- infestation. Single superphosphate fertilizer at 22 kg P ha^-1 was also broadcast on the field and incorporated before sowing. Two seeds per hole were sown on flat beds and later thinned to one plant, 2 weeks after sowing. The spacing was 75 cm between and 20 cm within rows. Rows were earthed up at 3 weeks after sowing and ridges were tied every 1.5 m to capture and retain rain water.

The crop was uniformly sprayed with the insecticides Monocrotophos (R), at 12 g a. i. ha^-1 when aphids (Aphis craccivora Koch) were observed; Deltamethrin (R), as 12 g a.i h^-1 at flower bud formation; and a mixture of Deltamethrin (R) at g a. i.ha^-1 and Endosulfan (R), at 400 g a.i. ha^-1 10 days after flowering for protection against insect pests. Plots were kept weed free by hand-hoeing before Striga emergence and manually by hand-pulling after Striga emergence. Observations recorded included number of days to 50% flowering days to Striga emergence, and Striga shoot density assessed at cowpea ripening in the two central rows of each plot. Grain yield was assessed from the four central rows of each plot. Pods were harvested at maturity, air-dried to a constant weight for 10 days and threshed.

The data were subjected to analysis of variance on a yearly basis and across years to test the year effect (including locations), sowing date, cultivar and their interaction effects. "Student" t-tests based on appropriate error mean squares were used to establish significant levels.

Stability of Striga density and yield of tested cultivars were assessed, for the highest level of interaction (two- or three-way) that was significant, using the Finlay and Wilkinson (1963) model: Y[ij] =u + d[i] + b[i][j] + a[ij], where Y[ij] is either the Striga density or yield of the ith cultivar under the jth condition, u is the mean Striga density or yield of all cultivars in all conditions, d[i] is the Striga density or yield deviation of the ith

cultivar, b[i] is the regression coefficient for the ith

cultivar on I[j] (it measures the stability of mean

Striga density or mean yield across years/locations), I[j] is the environmental index (or mean Straga density or yield of all cultivars) in the jth condition and a[ij] is the deviation from regression of the ith cultivar on the jth conditions.

Regression analysis of mean Striga density and cultivar seed yield on condition indices were based on twelve (four years/locations, three sowing dates each) or four (four years/locations) conditions. The latter conditions were used when only the two-way interaction (years/locations x cultivars) was significant. The coefficient of determination (r^2) associated with the regression line of mean Striga density and yield on condition indices was also computed to determine the percentage of sum of squares accounted for by the regression line. Correlation coefficients (r) between Striga density and yield were also computed each year to measure the impact of Striga-infestation on cowpea productivity.

Genotypic response experiments. Two regional trials were conducted to test the Striga-resistance and stability of newly developed cultivars with known resistant and susceptible controls in West and Central Africa. The first trial conducted from 1989 to 1990 consisted mainly of new cultivars developed from a cross involving Striga-resistant cultivar Suvita-2 and Striga-susceptible cultivars, KN-I and IAR1696. In the second trial, conducted from 1991 to 1992, cultivars were progenies from KVx crosses with Striga resistant cultivars B301 and Suvita-2 including some of the descendants of the latter cultivar. In both trials, the cultivars were selected for adaptation to semi-arid zones as well as for Striga resistance as observed in the field and confirmed in pot culture under the Striga-infestation conditions at Kamboinse in Burkina Faso. Each experiment consisted of 12 entries in a 3 x 4 compact block replicated four times. The experimental plot had four rows, 4 m long and 75 cm apart. Final Striga count and cowpea harvesting followed the same procedures as in the sowing date experiments.

The data were subjected to analysis of variance on a yearly basis and across years to test the location effect (including years), genotype and location/year x genotype interaction effects as described by McIntosh (1983). A mixed model was used, with locations as random and cultivars as fixed factors. A pooled error variance was used to test the significance of location/year x genotype interaction effect. The variance of the latter was then used to test the significance of the genotypic effect. "Student" t-tests based on appropriate error variances were used to establish significant levels. Stability of Striga density and yield of genotypes were assessed, using the Finlay and Wilkinson (1963) model, when the location/year x genotype interaction was significant.

RESULTS

Sowing date experiments. Only years/locations, cultivars and year/location x cultivar interaction affected Striga density (Table 1 ). At Kamboinse, Striga density declined as the number of years of experimentation increased. Striga- infestations at this location were significantly less than at Gampela. Suvita-2 was the only cultivar highly resistant to Striga-infestation, and the resistance was highly stable (b<1) across years/locations.

Years/locations, sowing dates, cultivars and year/location x sowing date x cultivar interaction influenced cowpea yield (Table 2). Except for cultivar TVx3236, dry years (1984, 1986, and 1987), late sowing and susceptibility to Striga infestation reduced significantly cowpea yield (compare Tables 1 and 2). Among Striga susceptible cowpea cultivars, yields of local Ouahigouya or local Koakin, IT82E-32 and KN- I were highly stable across years/locations (Table 2). Suvita-2 was the highest yielding cultivar, though not significantly different from TVx3236. Both these cultivars and TN88-63 had the least yield stability across years/locations, however, the yield of Suvita-2 tended to be more stable than that of TVx3236.

GENOTYPIC RESPONSE EXPERIMENTS

The 1989-90 regional trial. Both main effects, locations/years and genotypes, and their interaction affected Striga density in West Africa (Table 3). Cinzana and Koporo in Mali and Minjibir in Nigeria were the most densely Striga-infested locations. The resistant controls and two test cultivars, TN93-80 and TNI21-80, were the only genotypes that exhibited high resistance to Striga; and the resistance was highly stable across locations/years. It should be noted, however, that Niger was not included in this trial. The Striga- susceptibility of the KVx396 test cultivars, except KVx396-8-5 and KVx3966-1, was the least stable.

TABLE 1. Striga density response to Year/location, sowing dates and cultivars; and Year/location x cultivar interaction as stability Parameters: slope (b) and coefficient of determination (r^2) under Striga infested plots at Kamboinse and Gampela, Burkina Faso (1984-87)

Location effect/ Sowing date           Cowpea       Stability
year             effect                cultivar    parameters+
                                       effect
-------------  --------------- Cultivars ---------- ----------
Locat- Striga    Sowing Striga         Striga Reaction
ion/   density   date   density        charact- +/-
year   (shoots          (shoots        eristics      b    r^2
       m^-2)             m^-2)         
--------------------------------------------------------------
Kambolinse

1984    1.34    Early    6.66   Suvita-2  0.00  HR  0.00  0.16

1985    2.14    Inter-   6.00   TVx3236   9.02  HS  1.55  0.99

                mediate
1986    0.16    Late     5.61   TN88-63   4.71   S  0.73  0.99

Gampela                         
                               Ouahigouya 
                               local or  
1987    20.73  LSD(0.05)* N.S. or Koakin  7.57  HS  1.20  1.00

                               local    
LSD(0.05)$3.09                 lT82E-32   6.77  HS  1.00  0.99

                               KN-1       8.47  HS  1.50  1.00

                               LSD(0.05)# 2.62

b = regression slope.
r^2= regression coefficient of determination.
+b=1, stable; b>1, least stable; b<1, highly stable; data based on two-way (year/location x cultivar) interaction ( or four Years/locations).
+/- HR, highly resistant; MR, moderately resistant; R, resistant; S, susceptible; NS, highly susceptible.
$ LSD for comparison of year means across locations.
* LSD for comparison of sowing date means across Year/locations.
# LSD for comparison of cowpea genotypes.

TABLE 2 Cowpea yield response to Year/location, sowing dates and cultivars; and cultivar interaction as stability Parameters: slope (b) and coefficient of determination Year/ location x sowing date x at Kambolnse and Gampela in (1984-87)

Location effect Sowing date      Cowpea cultivar  Stability
/Year           effect           effect           parameters
------------- ------------       --------------   ------------
Location/     Sowing
Year   Yield   date        Yield  Cultivar Yield    b     r^2 
     (kg ha^-1)          (kg ha^-1)      (kg ha^-1) 
--------------------------------------------------------------
Kamboinse 
1984    242   Early         662   Suvita-2  746   1.22    0.92
1985    871   Intermediate  608   TVx3236   722   1.44    0.81

1986    669   Late          386   TN88-63   661   1.20    0.67 
       
Gampela                          Ouahigouya
                                 local or
1987    425   LSD(O.05)*    115  Koakin     282   0.40    0.44
                                 local
                                 IT82E-32   376   0.77    0.80
              
LSD (0 05)+/-                    KN-1       523   0.97    0.82
        133                      LSD 0.05)#  89
        

b = regression slope.
r^2 = regression coefficient of determination.
+b = 1, stable; b > 1 least stable: b < 1, highly stable: data based on three-way (Year/location x sowing date x cultivar) interaction (12 conditions, i.e., 4 Years/locations and 3 sowing dates).
+/- LSD for comparison of year means across locations.
* LSD for comparison of sowing date means across Years/ locations.
# LSD for comparison of cowpea genotypes.

This indicated that they exhibited moderate to high resistance in some locations and susceptibility to high susceptibility in other locations. These cultivars differed, thus, from KVx396-8-5 and KVx3966-1 and the susceptible controls; the susceptibility of the latter cultivars was either highly or moderately stable across locations/years.

Both main effects and their interaction affected yield significantly (Table 4). Minjibir in Nigeria, was the highest yielding location whereas Cinzana in Mali was the lowest yielding location. Of the five Striga-resistant cowpea cultivars (Table 3), only the resistant control, B301, and the TN cultivars were the highest yielders although they did not differ significantly from the two susceptible test cultivars, KVx396-8-5 and KVx396-4-4-4 (Table 4). The yields of B301, TN93-80 and TN121-80 were also stable across locations/years (b = 1) and its regression line accounted for most of the yield variations ( r^2 > 0.65 ). The two other resistant controls, Suvita-2 and IT82D849, together with the susceptible controls, yielded the least. The yields of the former genotypes were highly stable. They contrasted with those of the latter genotypes that were stable across locations/ years. This indicated that factors other than Striga- infestation might have handicapped the performance of the two resistant controls.

TABLE 3. Striga density response to locations and genotypes, and location x genotype interaction as stability parameters: slope ( b ) and coefficient of determination( r^2 ) under seven (location/year) Striga infested conditions in West and Central Africa (1989-1990)

Location/year               Cowpea genotype  Stability         
effect                      effect           parameters+
-------------- Cultivars+/- --------------   -----------
Year/    Striga             Striga   charact-  b    r^2
Location density            density  eristics    
         (shoots            (shoots  reaction $
          m^-2)              m^-2)
--------------------------------------------------------------
1989
               1. Test cultivars

Mali            KVx396-11-6    3.15    S    2.88    0.72 
Koporo    2.31  KVx396-8-5     2.32    S    0.71    0.07 
Nigeria         KVx396-6-1     2.08    S    0.39    0.04 
Minjibir  0.84  KVx396-4-4-2   2.86    S    2.30    0.82 
Senegal.        KVx396-4-4-4   2.90    S    3.25    0.87 
Ndatt Fall    
          0.94  TN93-80        0.18    HR   0.10    0.05 
                TNl21-80       0.18    HR   0.20    0.41

1990

             2. Resistant controls

Mali            Suvita-2    0.23    HR    0.22    0.38 
Ginzana   2.99  IT82D-849   0.11    HR    0.04    0.04 
Koporo    1.97  B301        0.10    HR    0.01    0.01 
Nigeria      3. Susceptible controls 
Minjibir  2.30  IT82E-32 #  2.82     S    1.13    0.37 
Togo            IT82E-32    4.67     S    1.31    0.16 
Pissare   1.24 
                LSD(0.05)++ 1.73 
LSD (0.05)#    
          1.37

b = regression slope.
r^2 = regression coefficient of determination.
+b = 1, stable; b > 1, least stable; b < 1, highly stable.
+/- Previously classified as resistant and susceptible controls; for test cultivars, their resistance was to be confirmed.
$ HR, highly resistant; MR, moderately resistant; R, resistant; S, susceptible; HS, highly susceptible. LSD for comparison of location means across years.
# Cultivar IT82E-32 was tested twice in 1989 and replaced by lT82E-16 in 1990.
++ LSD for comparison of cowpea genotypes.

The 1991-92 regional trial. Locations/years, genotypes and location/year x genotype interaction influenced Striga- infestations in West and Central Africa (Table 5). Gabougoura in Niger was the most densely Striga-infested location in 1991. It was followed by Pissare in Togo, during both years and by Tarna in Niger, and Koporo in Mali, in 1992. Only two genotypes, B301, a resistant control, and IT81 D-994 a test cultivar. exhibited high resistance to Striga. The resistance of both genotypes was highly stable across locations/ years. Three genotypes, IT82D-849, a resistant control, KVx4O2-19-1 and KVx397-6-6. both test cultivars, also exhibited resistance to Striga that was moderately to highly stable except for IT82D-849. The resistance of the latter cultivar tended to be the least stable. TN5-78. a resistant control became susceptible to Striga-infestation in West and Central Africa. but the susceptibility tended to be least stable.

Both main effects and their interaction also influenced yield (Table 6). Koporo in Mali, and Konni of Niger were the highest yielding locations. In contrast, both locations in Benin and Minjibir in Nigeria yielded the poorest. B301, a highly resistant genotype (Table 5), yielded the highest (Table 6). Its yield was, however. most significantly different from that of test cultivars KVx402-52, KVx402-19-1, KVx402-19-5, KVxI64-65-5 and TN5-75. But unlike all these genotypes, the yield of B301 was highly stable across locations/ years. IT81 D- 994 and IT82D-849, highly resistant and resistant genotypes respectively (Table 5), yielded the lowest and not significantly different from the susceptible control cultivar IT82E-32 (Table 6). The yield of these three cowpea genotypes was highly stable across locations/years.

TABLE 4. Cowpea yield response to year/locations and genotypes and year/location x genotype interaction as stability parameters: slope (b) and coefficient of determination (r^2) under six (locations/years) Striga infested conditions in West and Central Africa (1989-1990)

Location effect   Cowpea genotype effect Stability parameters+
----------------- ---------------------- ---------------------
Location   Yield     Cultivar+    Yield 
         (kg ha^-1)             (kg ha^-1)   b      r^2
--------------------------------------------------------------
1989                 1. Test cultivars

Mali                 KVx396-11-6    953     1.03    0.89
Koporo     742       KVx396-8-5    1116     1.03    0.83
Nigeria              KVx396-6-1    1035     0.99    0.84 
Minjibir  1634       KVx396-4-4-2  1067     1.39    0:97 

1990                 KVx396-4-4-4  1195     1.41    0.99
Mali                 TN93-80       1324     0.89    0.96 
Cinzana    254       TNI21-80      1428     0.93    0.96
Koporo     968       2.Resistant controls  
Nigeria              Suvita-2       888     0.55    0.64
Minjibir  1952       IT82D-849      716     0.68    0.84 
Togo                 B301          1247     0.99    0.66
Pissare    783       3.Susceptible controls 
                     IT82D-32*      861     1.16    0.96 
LSD (0.05)$ 232      IT82D-32       835     0.95    0.90 
                     LSD(0.05)#     329

b = regression slope. = regression coefficient of determination.
+b = 1, stable; b > 1, least stable; b < 1 highly stable.
+/- Previously classified as resistant and susceptible controls; for test cultivars, their resistance confirmed.
LSD for comparison of location means across years
* Cultivar IT82E-32 was tested twice in 1989 and replaced by IT82E-16 in 1990.
# LSD for comparison of cowpea genotypes.

TABLE 5. Striga density response to years/locations and genotypes, and year/location x genotype interaction as stability parameters: slope (b) and coefficient of determination(r^2) under twenty five (year/location) Striga infested conditions in West and Central Africa (1991-1992)

Location/year effect   
---------------------------
Location    1991      1992  
            (shoots m^-2)
---------------------------
Benin 
Tindji       0.29    0.46 
Zakpota      0.32    0.49 
Burkina Faso 
Kamboinse    0.61    2.46 
Kouare       0.78    0.72 
Cameroon 
Dourom       nd      1.20 
Maroua       nd      1.72 
Ghana 
Manga        0.85    0.55 
Mali 
Koporo       0.68    5.99 
Niger 
Gabougoura    
            15.85     nd 
Konni        1.19    2.37 
Taana        0.71    6.04 
Nigeria 
Bakura       nd      0.30 
Mafara       0.18     nd 
Minjibir     0.73    1.28 
Togo 
Pissare      4.11    4.62 
LSD(0.05)#       1.70

Table 5 contd./                               
                 Cowpea genotype effect        Stability
                 ----------------------------  parameters      
                              Striga            b    r^2       
                              character-      ------------ 
                              istics
                             Density Reaction*
Location         Cultivars$   Shoots (m^-2)    
---------------------------------------------------------- 
                 1. Test cultivars    
Benin            KVxI64-65-5    1.53    MR    1.02    0.85
Tindji           KVx291-47-222  1.75    MR    0.45    0.35
Zakpota          KVx397-6-6     1.31    R     0.95    0.87
Burkina Faso     KVx402-5-2     4.08    S     1.72    0.81
Kamboinse        KVx402-19-1    1.21    R     0.61    0.85
Kouare           KVx402-19-5    1.85    MR    1.71    0.75
Cameroon         KVx305-118-31  3.10    S     1.32    0.86
Dourom           IT81 D-994     0.71    HR    0.40    0.76
Maroua           2. Resistant controls 
Ghana            TN5-78 + +     2.60    S     1.25    0.81
Manga            IT82D-849      1.16    R     1.26    0.73
Mali             B301           0.80    HR    0.78    0.77
Koporo           3. Susceptible controls 
Niger            IT82E-32       6.07    HS    0.53    0.09
Gabougour        
Konni            LSD (0.05) ++  1.67                      
Taana    
Nigeria 
Bakura   
Mafara   
Minjibir 
Togo 
Pissare  
LSD(0.05)#

b = regression slope.
r^2 = regression coefficient of determination.
+b = 1, stable; b > 1, least stable; b < 1, highly stable.
nd, no data.
$ Previously classified as resistant and susceptible controls; for test cultivars, their resistance was to be confirmed.
HR, highly resistant; MR, moderately resistant; R, resistant; S, susceptible; HS, highly susceptible. LSD for comparisons of location means across years.
+ + TN 5-78 is a sister variety of Suvita-2
+ + LSD for comparisons of cowpea genotypes.

DISCUSSION

Sowing dates did not influence Striga density of DI cowpea cultivars. Also the combination of Striga-infestation and drought, whether due to dry years or late sowing, decreased the productivity of DI cultivars. It appeared that the optimum sowing date for DI cowpea cultivars, under natural Striga-inlestation in the Sudan savanna in Burkina Faso, was in mid-July or earlier. These results, especially the effects of early and late sowing, contrast with the findings of DS cowpea cultivar study (Muleba and Mosarwe, 1994). Late relative to early sowing, in that study, significantly reduced Striga density and increased yield of DS cultivars. The optimum sowing date for DS cultivars was, therefore, mid- July or later.

TABLE 6. Cowpea yield response to years/locations and genotypes and year/location x genotype interaction as stability parameters: slope (b) and coefficient of determination (r^2) under twenty three (year/location) Striga infested conditions in West and Central Africa (1991-1992)

Location/year effect   Cowpea genotype effect  Stability,      
                                               parameters +
---------------------   ---------------------  ------------ 
Location  1991   1992    Cultivars $    Grain    b    r^2 
                                        yield 
           (kg ha^-1)                 (kg ha^-1)
------------------------------------------------------------
                          1. Test cultivars 
Benin                     KVxI64-65-5    772    1.02   
0.75 
Tindji     431     569    KVx291-47-222  728    1.09    0.82 
Zakpota    223      nd    KVx397-6-6     761    1.08    0.82 
Burkina Faso              KVx402-5-2     880    1.13    0.76 
Kamboinse  680     845    KVx402-19-1    846    1.19    0.90 
Kouare    1459     582    KVx402-19-5    843    1.27    0.77 
Cameroon                  KVx305-118-31  763    1.17    0.79 
Dourom     nd      687    IT81D-994      514    0.75    0.43 
Maroua     nd      679    2. Resistant controls 
Ghana                     TN5-78 #       782    1.28   
0.68 
Manga      804     556    IT82D-849      690    0.73    0.64 
Mall                      B301           925    0.73    0.47 
Koporo    1515    1151    3. Susceptible controls 
Niger                     IT82E-32       749    0.56    0.29 
Gabougoura 741     nd 
Konni     1162    1200    LSD (0.05)+ +  157 
Tarna     1044     127 
Nigeria 
Bakura     nd      616 
Minjibir   559     315 
Togo 
Pissare   1000     793 
LSD (0.05)*  11     337

+b = 1, stable; b > 1, least stable; b < 1, highly stable.
nd, no data.
$ Previously classified as resistant and susceptible controls; for test cultivars, their resistance was to be confirmed.
* LSD for comparison of location means across years.
# TN5-78 is a sister variety of Suvita-2.
+ + LSD for comparison of cowpea genotypes.

Cowpea cultivars performed differently under Striga-infestation in sowing date experiments. Susceptible cultivars, Local Ouahigouya or Local Koakil IT82E-32 and KN-I had stable to highly stable low yields across years/locations. This indicated that these cultivars did not take advantage of rainy years or early sowing to give high yields as did TVx3236 and TN88-63 that were also Striga-susceptible. Thus, the former cultivars suffered severe Striga damage compared to the latter. TVx3236 and TN88-63 (especially the former cultivar that supported a higher Striga density than the latter) were, therefore, Striga-tolerant. They too suffered Striga damage as they yielded equal to or less than Suvita-2. Muleba et al. (1991) reported TN88-63 and TVx3236 to be better adapted, and better yielding than Suvita-2 in the Sudan savanna zone, in the absence of Striga-- infestation. Suvita-2 was not adapted to this agro-ecological zone because of its high susceptibility to cowpea mosaic virus transmitted by aphids (A. craccivora).

Continuous cultivation of Striga-susceptible cultivars in the same field appeared to cause a decline in Striga density as shown at the Kamboinse location from 1984 to 1986 (Table 1). This is also supported by the impact of Striga-infestation on cowpea yield as the coefficients of correlation (r) between Striga density and yield decreased over the years. They were: r = -0.36** in 1984, r = -0.26* in 1985 and r = -0.12 in 1986 at Kamboinse; and r = - 0.56** in 1987 at Gampela. These results suggested that cowpea yield losses due to Striga-infestation diminished as the number of years of cultivation of Striga-susceptible cultivars increased. It should be noted that 1984 was the third year of cultivation of Striga-susceptible cultivars in the field where the experiment was established at Kamboinse.

The Striga control, provided by a continuous cultivation of susceptible cowpea cultivars, has also been observed in Mali; a pathogenic fungus was believed to infect Striga plants

resistance that was highly stable only in West Africa (Niger was not included in the trial (Table 3)). When Niger was included and the trial extended to Central Africa, TN5-78 became Striga-susceptible, and the susceptibility was least stable indicating its tendency to be heavily attacked by Striga in densely infested locations.

Test cultivars that exhibited stable to highly stable Striga-resistance include TN93-90, TNI21-80 (Table 3), IT81D-994, KVx402-191 and KVx397-6-6 (Table 5). Apart from IT8 ID994 and, to some extent, KVx397-6-6, the above listed cultivars, together with B301, were high yielders and their yields were stable to highly stable across locations/years. Other cultivars of interest with high yield and stability across locations/years were KVx396-8-5, KVx396-4-4-4, KVx402-19-5, KVx402-5-2 and KVx16465-5. They were either moderately resistant to parasitising cowpea (B. Dembele and A. Konate, 1988, Cowpea Pathology, IER, Sotuba/Bamako, Mali; personal communication). S.A. Shoyinka (1985, Cowpea Pathology, University of Ire, Ire, Nigeria; personal communication), reported such a fungus to be Macrophomina phaseolina. The pathogenicity of M. phaseolina on S. gesnerioides parasitising cowpea was recently demonstrated in England (J. A. Irvine. 1993, Botany Department, University College, London, England; unpublished data).

In regional trials, some locations were more densely infested than others. At Gabougoura in Niger, the most densely infested location, all cowpea genotypes were heavily infested by Striga. Only the susceptible control, IT82E-32, and two test cultivars, KVx291-47-222, IT81D994 were less densely infested than others. Since the field at this location was an old orchard infested with a soil borne disease (charcoal rot or ashy stem blight caused by Rhizoctonia bataticola Copr.), and these three cultivars were less affected than others by the disease, it can be inferred that the sanitary conditions of a field may influence the Striga-resistance of a genotype.

Of the three resistant controls, only B301 exhibited a high resistance to Striga that was highly stable across locations/years. IT82D-849 was moderately to highly resistant, and the resistance varied from highly to least stable. In contrast to the aforementioned controls, Suvita2 or its sister variety TN5-78 expressed high resistant or tolerant to Striga-infestation.

The low and highly stable yield exhibited by genotypes IT82D-849, IT81D-994 and Suvita2 with highly stable to stable resistance or high resistance to Striga (Tables 4, 6) suggest that other factors than Striga-infestation impaired these genotypes from fully expressing their yield potential. This can be explained by the high susceptibility of IIT82D-849 to bacterial blight (Xanthomonas campestris pv. vignicola). The long growth duration of IT81 D-994, exceeding 85 days versus 70 to 75 days for other cultivars, exposed this cultivar to drought damage more than the others. Likewise, the high susceptibility of Suvita-2 or TN5-78, as compared to others, exposed it to cowpea mosaic virus and physical damage by the virus vector, A. craccivora.

The results of this study partly corroborate the findings of Aggarwal (1991). They also demonstrated that none of the cowpea genotypes, including B301 and its KVx progenies and IT82D-849, were unsusceptible to Striga infestations in all locations in West and Central Africa. The response of genotypes in this study appeared to relate more to the quantitative gene action controlling Striga-resistance in cowpea as proposed by Obilana (1987) than to the monogenic dominant inheritance reported by Aggarwal et al. (1984) and Singh and Emechebe (1990). It is also possible that there exists genetic variability in the Striga population at each location. This Striga resistance in cowpea production could, therefore, explain Striga-infestation of stable resistant cultivars such as B301 and IT82D-849. However, the resistance breakdown at Gabougoura in Niger of all cultivars purported to be Striga resistant, and the moderate resistance compared to others expressed by the Striga-susceptible control, IT82E-32, at this location are still to be explained.

CONCLUSIONS

Cowpea genotypes responded differently to Striga- infestation spatially. Some genotypes had moderate to high resistance to Striga-infestation that was either least stable, stable or highly stable across locations. Others had tolerance to Striga as evidenced by high yields in spite of. being heavily infested by Striga. This implies that, there might be host specificity in cowpea Striga-resistance. Therefore, least stable Striga resistant genotypes had resistance only to one or few Striga strains, especially those whose dispersion was limited to a small geographical area. In contrast, highly stable Striga-resistant genotypes were resistant to a wide range of Striga stratus distributed in a large geographical area.

The present study and that of Muleba and Mosarwe (1994) have demonstrated the possibility of cultivating the Striga-resistant Suvita-2, for over ten years without its resistance breaking down at Kamboinse and other locations in Burkina Faso. This cultivar had no consistently high stability of Striga-resistance across locations in West and Central Africa, thus indicating that in a specific location the Striga genetic variability might be changing insignificantly over time. As such, the release of a resistant cultivar in a specified recommendation domain for commercial purposes should be based mainly on its better adaptation and other specific agronomic and economic attributes assessed in that domain.

Stable resistance cultivars of high yielding abilities across locations can be ,conveniently used in combination with optimum sowing dates to mitigate Striga-infestation in peasant cowpea production in West and Central Africa. Continuous cowpea cropping facilitated the demise of Striga populations.

ACKNOWLEDGEMENT

The contribution of national scientists belonging to the SAFGRAD cowpea network from Benin, Burkina Faso, Cameroon, Ghana, Mali, Niger, Nigeria, Senegal and Togo is highly acknowledged.

REFERENCES

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Aggarwal, V.D., Muleba, N, Drabo, I., Souma, J.and Nbewe, M. 1984. Inheritance of Striga gesnerioides resistance in cowpea. In: Proceedings of the Third International Symposium on Parasitic Weeds. 7-9 May 1984, ICARDA. ICARDA (Ed.) pp. 143-146. Ateppo, Syria.

Emechebe, A.M. and Leleji, 0. I. 1988. Striga infestation and screening for resistance to Striga in Northern Nigeria. In: State of Cowpea Research in Semi-arid Zones of West and Central Africa. Proceedings of the first seminar for cowpea lead centre scientists, November 14-25, 1988;IITA, Ibadan, Nigeria. Muleba, N. and Emechebe, A.M.(Eds.), pp. 40-42. SAFGRAD/IITA: Ouagadougou.

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Musselman. L.J. and Parker. C. 1982. Biosystematic studies on the genus Striga (Scrophulariaceae). In: Proceedings of the 2nd International Striga Workshop. Ramaiah, K.V. and Vasudeva Rao, M.J. (Eds.), pp. 1924. ICRISAT, Pantacheru, A.P., India.

Obilana, A.T. 1987. Breeding cowpea for Striga resistance. In: Parasitic Weeds in Agriculture. Mussehnan. L.J. (Ed.). pp. 243-253. CRC Press.

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and Musselman, L.J. 1983. Striga identification and Control Handbook. ICRISAT Infonnation Bulletin No. 15. RENACO (Reseau Niche d'Afrique Centrale et Occidentale) 1990. Report of the 1989-90. Regional Trials: Preliminary Results. IITA/SAFGRAD; OAU-STRC-SAFGRAD: Ouagadougou (Limited distribution); 47pp.

Sanders, JH., Bezunch, T. and Schroeder, A.C.1994. Impact Assessment of the SAFGRAD Commodity Networks. USAID/AFR; OAU/STRC-SAFGRAD; 109pp. Singh, B.B. and Emechcbe. A.M. 1990. Inheritance of Striga resistance in cowpea genotype B301. Crop Science 30:879-881.

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