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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 4, Num. 1, 1996, pp. 71-77
African Crop Science Journal,Vol. 4. No.l, pp. 71-77,1996

The effect of okra mosaic virus and beetle damage on yield of four okra cultivars


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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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 Key Words: Beetle damage, Nigeria, okra, okra mosaic virus, pesticide, yield


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


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.


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

    V-P+ = pod weight of virus infected plants without pesticide application.

If there was no interaction, the yield differences between virus infected and virus free plants would be the same whether pesticide is applied or not.

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).


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:

       [(V+P- - V-P-) - (V+P+ - V-P+)/= 
    [(243.3 - 179.0) - (243.0 - 179.2)J = 0.5 

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

Treatment^a   Pod weight  Number of  Plant height  Date of  
             (g plant-^1) fruits        (cm)       first       
                          plant^-1                 harvest
V-P-             81.1      3.1          82.3        62 
V+P-             59.7      2.6          78.9        67 
V-P+             81.0      3.2          77.1        63 
V+P+             59.7      2.6          78.2        68

Std. error        4.12     0.16          3.16        0.83

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).

Treatment^a    Nitrogen content (%)
               73 DAS    79 DAS
V-P-           2.11      2.82 
V+P-           2.20      2.70 
V-P+           2.10      2.56 
V+P+           2.22      2.51
Average        2.1       2.6 
Std. error     0.11      0.14

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

Source of 
variation  df     SS         MS      F-value  Pr > F
Blocks     3    332,256    110,752 
Virus      1    17,576      17,576    0.58    0.467 
Beetle     1    259,794    259,794    8.52    0.017 
           1    75,543      75,543    2.48    O. 150 
Main plot error    
           9    274,379     30,487 
Cultivar   3    102,680     34,227    2.56    0.070 
           3    31,124      10,375    0.78    0.516 
           3    103,294     34,431    2.57    0.069 
           3    44,908      14,969    1.12    0.354

Split-plot error    
          36   481,881      13,386 
Corrected total    
          63  1,723,435

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.


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'

Cultivar       V-B-    V-B+    V+B-    V+B+   Average
Average harvested fruit weight per plant (in g) (Std. error = 14.45)

Awgu Early     61.6    14.6    16.3    34.9    31.9 
Jockoson       83.4    22.4    90.2    32.3    57.1 
NHAe47-4      101.5    31.4    51.9    36.8    55.4 
V35            63.5    34.8    49.2    44.7    50.5

Average        77.5    28.3    51.9    37.2    48.7

Average harvested number of fruits per plant (Std. error = 0.53)

Awgu Early      2.8     0.9     0.9     1.7     1.6 
Jockoson        3.6     1.1     3.6     1.6     2.4 
NHAe47-4        4.0     1.4     2.4     1.7     2.4 
V35             2.7     2.1     2.1     1.9     2.2 
Average         3.3     1.4     2.2     1.7     2.2

Average plant height (in cm) (Std. error = 10.32)

Awgu Early     47.7    12.9    17.1    26.2    26.0 
jockoson       31.1    11.7    30.9    17.9    22.9 
NHAe47-4       34.2    15.9    25.7    18.4    23.6 
V35            29.6    22.6    22.4    17.9    23.1

Average        35.7    24.0    15.8    20.1    23.9

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.


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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Longman Scientific and Technical.

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Copyright 1996 The African Crop Science Society

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