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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 4, Num. 1, 1996, pp. 29-40
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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
Sizes of Files:
Text: 47K
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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.
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Copyright 1996 The African Crop Science Society
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