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The effect of okra mosaic virus and beetle damage on yield of four okra cultivars
S. VANLOMMEL, L. DUCHATEAU^1 and J. COOSEMANS Department of Phytopathology, K.U.Leuven, De Croylaan 42, 3001 Leuven, Belgium. ^1 Department of Statistics, Faculty of Agriculture, K.U.Leuven, Kardinaal Mercierlaan 92, 3001 Leuven, Belgium. (Received 3 March 1995; accepted 21 March 1996)
Code Number:CS96042
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ABSTRACT
The effect of beetle damage and infection by okra mosaic virus
(OMV) on pod yield, number of fruits, plant height and time of
harvest for okra (Abelmoschus esculentus) was
investigated. A preliminary greenhouse experiment was set-up
in order to examine the effect of OMV in combination with the
application of the pesticide Nuvacron 40 SCW on the cultivar
Jockoson. The greenhouse experiment revealed that there was no
interaction between the applied pesticide and the infection by
OMV for the measured parameters (P=0.98). Virus infection in
this greenhouse experiment significantly reduced yield by 26 %
(P
RESUME
Nous examinons ici l'effet sur le gombo du rongement par
les coleopteres et de l'infection par le virus de la mosaique
du gombo (VMG). Les parametres examines sont: le rendement
(poids), le nombre des fruits, le moment de la recolte et la
taille des plants du gombo (Abelmoschus esculentus).
Une experience en serre a permis de mettre en evidence
l'effet conjugue du VMG avec application de l'insecticide
Nuvacron 40 SCW sur la variete Jockoson. Cette experience en
serre a montre que l'insecticide n'a d'effet ni sur le VMG ni
sur les plantes (P=0.98). L'infection du virus en serre a
cause une diminution significative de l'importance de la
recolte de 26% (P<0.0001) et a retarde le moment de la recolte
de 5 jours (P<0.0001). La mesure de la teneur en azote des
fruits pour les quatre traitement (absence de l'insecticide
et du virus; absence de l'insecticide et presence du virus,
presence de l'insecticide et absence du virus; absence e
l'insecticide et presence du virus) n'est pas
significativement differente. Le rongement par les coleopteres
a cause une reduction moyenne de 49 % du rendement a ciel
ouvert pour les quatre varietes de gombo (Awgu Early,
Jockoson, NHAe47-4 et V35) (P=O.017). Au cours de cette meme
experience, l'effet du VMG (reduction de 16% de la recolte)
n'a pas pu etre mis en evidence de maniere significative
(P=0.467). L'effet sur les quatre varigres n'a pas ete le
meme. Pour la variete Awgu Early, le rendement a ete faible.
Les varietes Jockoson et NHAe47-4 ont genere une recolte plus
importante mais ont fait preuve d'une resistance plus faible
au rongement par les coleopteres que la variete V35. La
variete V35 enfin, a donne des resultats moyenne.
Mots Cles: Gombo, Nigeria, pesticide, recolte,
rongement par les coleopteres, virus de la gombo mosaique du
gombo
INTRODUCTION
Okra (Abelmoschus esculentus (L.) Moench) is a
vegetable grown commonly in West-Africa and can serve as a
protein source (Karakoltsides and Constandinides, 1975). In
Nigeria, if grown without controlling insect pests, okra is
heavily infected by okra mosaic virus (OMV), atymovirus, and
attacked by two beetle species, Podagrica unifortna and
P. sjostedti (Chrysomelidae) which are the important
vectors of OMV (Lana and Taylor, 1976). Although OMV is
estimated to cause as much as 18% loss (Lana, 1976), little
scientific evidence exists to support such estimated losses
caused by the virus. An experiment with a natural spread of
OMV through beetles as vectors indicated yield losses from 7
to 39% (Atiri and Ibidapo, 1989). In their study, however, the
possible interferences between beetle feeding damage, the
susceptibility of the plant to virus attack, the plant's
physiological state and yield were neglected although they
exist (Gibbs and Harrison, 1976). The damage caused by feeding
beetles has never been directly investigated, but was
estimated by simulating damage through manual defoliation and
debudding of plants (Iremiren, 1987; Olasantan, 1986; 1988).
The results from simulation studies are not reliable since
they assume that mechanical damage done by man is similar to
the feeding damage by beetles which may not be true. Therefore
there is a lack of information on the extent of damage and
yield loss caused by the beetles. Before developing rational
and economical control measures, the magnitude of the loss
caused by pests and diseases must be evaluated so that it can
be related to gain obtained (James, 1974).
The objective of this study was to evaluate the extent of
okra yield losses caused by OMV and/or beetle damage in
Nigeria. It was essential for the interpretation of the field
experiments results to ensure that there was no interaction
between virus infection and pesticide application. A
greenhouse experiment was therefore conducted. Nitrogen
contents in pods were also evaluated.
MATERIALS AND METHODS
Both experiments were conducted at the University of
Nigeria, Nsukka.
Greenhouse experiment. The effect of pesticide
application and virus infection was investigated in the
greenhouse. The three treatments, application of the
pesticide, inoculation with OMV and a combination of both and
the control without any treatment were each replicated 10
times and randomly assigned to an experimental unit. An
experimental unit consisted of a 10 litre pot.
The pots were filled with poultry manure and sterilised
soil mixed in a ratio of one volume of manure to four volumes
of soil. The okra cultivar used was Jockoson. Seeds were sown
in three holes pot^-1 and two seeds hole^-1, and later thinned
to three plants pot^-1. The pots were arranged in rows at a
spacing of 70 cm between rows and 35 cm within the rows.
Weekly application of the insecticide Nuvacron 40 SCW (Ciba
Geigy) at a dose of 2 ml l^-1 water was started 24 days after
sowing (DAS) and continued until 60 DAS.
Inoculum of the virus was prepared by homogenising the
leaves of OMV infected okra plants with mortar and pestle in a
O.01M neutral phosphate buffer. The okra plants that were used
for the propagation of the virus were grown in a greenhouse
and were tested for the presence of OMV by the ELISA
technique. Immediately after preparation, the crude sap was
applied with a gauze pad on carborundum dusted leaves. Plants
were inoculated 26 DAS at the 3-4 leaf stage, considered to be
an effective time in order to achieve a high yield reduction
(Atiri and Varma, 1983).
Fruits were picked three times per week, counted and
weighed. Date of first harvest was recorded and plant heights
were measured at the last harvest (96 DAS). Five pods from
different plants and different experimental units per
treatment were picked at 73 and 79 DAS, sliced, dried in an
oven at 60 C and ground in a blender. Two months later, the
nitrogen content of the pods was determined by the Kjeldahl
assay method and repeated twice for each pod.
The interaction between the two factors, pesticide and
virus, for the response variable weight was calculated as:
[(V+P - V-P-) - (V+P+ - V-P+)] where:
V-P- = pod weight of virus infected plants without pesticide
application.
V+P+ = pod weight of virus infected plants without pesticide
application
V-P+ = pod weight of virus infected plants without pesticide
application.
Field experiments. These experiments were conducted in
a field which laid fallow for 4 years. Four cultivars were
tested: Awgu Early, Jockoson, NHAe47-4 and V35.
The yield reduction of the different okra cultivars due to
beetle damage and virus infection was investigated in a
balanced split-plot experiment were four blocks. The main plot
treatments were virus and/or beetle attack. Each main plot was
divided into four split plots and the four cultivars were
randomly assigned to one of the split-plots. Each split-plot
contained four plants. Plants were spaced 45 cm apart in the
row with 85 cm between the rows. Three seeds were sown in each
hole and later thinned to one plant hole ^-1. Two rows of
border area were sown on each side of the field. A
NPK mixture(15:15:15) was applied at a rate of 200 kg ha^-1 4
weeks after sowing, by line dressing. Plots were weeded at
three-week intervals.
The young seedlings were covered by cages which were built
with a cylindrical base of PVC encircled with insect gauze
with a mesh 01'0.75 x 0.50 min. The mesh reached up to a
height of 30 cm. Two beetles of the species Podagrica
uniforma were added to each plant by means of an aspirator
15 DAS and were left there for 10 days. From preceding beetle
counts in the field, it was believed that these populations
best resembled the natural conditions at this time of the
year. Mechanical inoculation of the plants with OMV
accomplished in accordance with the procedure outlined for the
greenhouse experiment.
Although beetles are present during the whole growing
season in non-sprayed fields, beetles were not left longer on
the plants because of the limited size of cages. Once the
beetles were removed, plots were sprayed weekly with
Nuvacron (2 ml Nuvacron 1^-1 water) for insect control until
60 DAS. In spite of these precautions, a few plants not
inoculated with the virus still became infected (7%). The
effect of the virus infection on these plants was ignored
since symptoms appeared much later (more than 40 DAS) when the
consequences of virus infection were negligible (Atiri and
Varma, 1983). All plants in the field were tested for the
presence of OMV by the ELISA technique 50 DAS.
The fruits were picked and weighed at optimal marketing
quality which was when they were still tender and the tip
could be broken with a characteristic crispness. Harvesting
was done times a week. The date of first harvest was recorded
and the final height of each plant was measured at the end of
harvest.
Data obtained were subjected to analysis of variance
using the GLM procedure of the SAS statistical software
package (SAS, 1989).
RESULTS
The dependent variables which are weight, number of fruits,
date of first harvest and height, were normally distributed
according to the Shapiro Wilk statistic, which justified the
use of the GLM procedure for analysis of the data.
Greenhouse experiment. The interaction between the
factors pesticide and virus for the response variable weight
was calculated as:
There was no interaction (P=O 98) between pesticide
application and virus infection for pod weight. Pesticide
application did not influence the pod weight per plant (P=O
99) while virus infection significantly reduced yields by 26%
(P<0.0001). Plant height did not differ significantly
for the different treatments. There were significant
differences in number of fruits (P<0.0007) and was first
date of harvest (P<0.0001) between the virus infested and
the healthy plants. The pesticide application did not affect
the number of fruits or the day of harvest when tested at a 5
% significance level. Nitrogen content (Table 2) seemed to be
nearly identical for fruits from the different treatments and
was close to Purseglove's (1968) value of 2.2%. There seemed
to be a change in nitrogen content with time (P=0.04).
Field experiments. The reduction in pod weight caused
by beetle attack was 49% (p=0.017)(Table 3). The effect of the
virus infection which resulted in a weight reduction of 16%,
was not statistically significant (P=0.467).
TABLE 1. Average weight (g), number of fruits, plant height
(cm) and date of first harvest (days after sowing or DAS) and
standard error of okra for the four different treatments
V-P-: virus free, no pesticide; V+P-: infected by virus, no
pesticide; V-P+: virus free, pesticide treatment; V+P+:
infected by virus, pesticide treatment
Comparison of the cultivars indicated that there were varietal
differences in yield performance (P=0.070) and resistance to
feeding damage by beetles (P=0.069). Yield of the cultivar
Awgu Early was always low whether attacked by beetle or not
(Fig. 1). V35 appeared to be less sensitive to beetle attack
but also had, in the absence of beetles, a moderate yield. The
cultivars Jockoson and NHAe47-4 were less resistant to beetle
attack; this was more pronounced for Jockoson. Differences in
sensitivity to the effects of virus infection for the
different cultivars could not be demonstrated (PzO.516).
The effect of the treatments on number of fruits plant^-1
and plant height showed the same tendency as on harvested
weights (Table 4). The presence of beetles influenced the
number of fruits (P=0.03), TABLE 2. Average nitrogen contents
(%) and standard error of okra pods measured at two different
dates for the four treatments, but this could not be shown for
virus infection (P=0.52).
V-P-: virus free, no pesticide; V+P-: infected by virus, no
pesticide; V-P+: virus free, pesticide treatment; V+P+:
infected by virus, pesticide treatment
TABLE 3. Analysis of variance for the harvested pod weight
plant^-1 for four okra cultivars infested with Podagrica
uniforma (Jacoby) or inoculated with Okra mosaic virus in
the field experiment
There was no statistical evidence for the existence of
interaction between cultivar and virus (P=0.18). The height of
the plants was also strongly influenced by the presence of
beetles (P=0.01), while the virus infection again had no
significant effect (P=0.34). Awgu Early, which in general is
known to be a tall cultivar, was indeed taller than the other
three cultivars when not attacked by beetles. This difference
vanished if the plants were attacked by beetles. Differences
in first harvest dates were not significant.
DISCUSSION
Unlike earlier studies, an attempt was made to
simultaneously control virus infection and attack of beetles
on okra. These two stress parameters are frequently stated to
be important constraints on okra (Lana and Taylor, 1976; Atiri
and Varma, 1983; Olasantan, 1986; 1988; Iremiren, 1987; Atiri
and Ibidapo, 1989).
The greenhouse experiment revealed that pesticide
application did not have an effect on yield per plant, number
of fruits per plant, plant height and date of first harvest,
and that there was no evidence of an interaction between
pesticide application and virus infection.
Figure 1. Average fruit weight per plant for the four okra
cultivars of the field experiments for the factor attack by beetles.
Control of beetle damage was done by spraying weekly with the pesticide
Nuvacron 40 SCW (2ml l^-1 water).
TABLE 4. Effect of Okra mosaic virus and/or beetle attack on harvested
fruit weight, harvested number of fruits plant" and plant height for the
four okra varieties, and four treatments'
V-B-: virus free, no beetle; V-B+: virus free, attacked by beetles; V+B-:
infected by virus, no beetle; V+B+: infected virus, attacked by beetle
This result was essential for the interpretation of the field experiments:
differences in yield between a treatment combination that included beetle
attack (and therefore no pesticide application), and a treatment
combination where beetle attack was absent (which implies pesticide
application), were presumed to be entirely due to the beetle attack, and
not to the pesticide application.
A 26% reduction in yield and a delay of five days in first harvest date
were observed due to infection by OMV. Field and glasshouse studies from
Lana (1976) indicated a reduction in pod yield by 12 to 18%, but no
description of the experiments was given. Since plants in our greenhouse
experiment were grown densily, the effect of OMV on plant growth will have
probably been more pronounced due to Competition with healthy plants for
sunlight. Also, as these data were obtained from a pot experiment in a
greenhouse under glass, they may not be representative of yield in the
field under natural conditions (Bos, 1982). Nevertheless, it gives an
indication of yield losses that can be expected in the field. Nitrogen
contents in pods were nearly equal for the different treatment
combinations, but seemed to increase in time toward the end of harvest
period. A satisfactory explanation could not be given considering, among
other things, the limited sampling.
The results of the field experiments show that yield losses occur and
can be tremendous if pest management is not applied. Beetle damage resulted
in significant yield losses (up to 50% reduction in yield) while the
reduction in yield induced by infection from OMV was not significant.
Yields of plants from a common treatment showed a high degree of variance,
particularly for the virus infection. The negative effect of OMV infection
as proven in the greenhouse experiment, but indicated in the field
experiments, may be masked by the numerous factors inherent in field
experiments. The lowest yield was obtained for the cultivar Awgu Early
while the others had comparable yields. Awgu Early is, however, still a
popular cultivar in the South-East of Nigeria in commercial plantings. A
variable resistance to beetle attack for the different cultivars was
observed as Jockoson and NHAe47-4 were more sensitive to beetle damage.
Higher yields were obtained in the absence of beetles, but if attacked by
beetles, lower or similar yields were obtained in comparison with Awgu
Early and V35. The reduction in yield for NHAe47-4 caused by beetle damage
confirms the results of a field experiment at NIHORT in Ibadan, Nigeria
where non-sprayed fields had a reduction in yield of 30 to 50% compared
with fields sprayed once in 2 weeks or more frequently (Agunloye, 1986).
The number of fruits plant^-1 and plant height were also clearly affected
by attack of beetles.
Considering the fact that the beetles Podagrica uniforma and
P. sjostedti are important vectors of OMV (Atiri, 1984), the
application of an insecticide could reasonably reduce the infestation of
beetles and OMV at the same time. Knowing that in 1992 the price of
Nuvacron in Nigeria was United States $6 litre^-1, and that the price for 1
kg of okra was approximately $0.3, a treatment scheme of five applications
with this pesticide at a total dose of 51 ha^'1, can be cost effectively
implemented.
ACKNOWLEDGEMENTS
Research was supported by funds of the Belgian Administration for
Development and Cooperation, and conducted at the Department of Crop
Science, University of Nigeria, Nsukka, Enugu State, Nigeria. The authors
thank Prof. E .C.K. Igwegbe and K. Ugwuoke, Department of Crop Science,
University of Nigeria, Nsukka, for their assistance and provision of okra
seeds.
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Copyright 1996 The African Crop Science Society
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