Biopesticide Control of Some Important Okra (Abelmoschus Esculentus (L.) Moench) Insect-Pests and Viral Diseases in Three Okra Cultivars

Abstract

The production, processing and marketing of Okra (Abelmoschus esculentus L. Moench), a vegetable valued for its rich nutritional and medicinal benefits, serves as an important means of employment and income generation to many peasant farmers. The production of the crop is, however, constrained by the incidence of pests (whitefly and flea beetle) and okra yellow vein mosaic virus [OYMV] and okra mosaic virus [OMV]) diseases. To overcome the health and environmental risks associated with the excessive use of synthetic agrochemicals, the predominant means of control of these pests and viral diseases, the efficacy of crude leaf extracts as biopesticides from Neem, Jathropha and Lemon grass on the incidence and severity of OMV and OYVMV as well as crop damage due to whitefly and flea beetle, were evaluated in three okra cultivars [F1 Kirene (F1K), FI Sahari (F1S) and Asontem (AST)]. Among the three plant extracts tested, Neem extract treatment induced significantly (p<0.05) the lowest mean count of whitefly [ASTNEM (18.91), F1KNEM (22.17), F1SNEM (24.49)] compared to Jathropha extracts [ASTJAT (27.99), F1KJAT (28.73), F1SJAT (28.74)] and Lemon grass extract treatments [ASTLEM (34.22), F1KLEM (32.77), F1SLEM (32.67)]. Similar results were obtained for mean population of flea beetle. With respect to insect pests damage to the okra cultivars, Neem extract caused significantly (p<0.05) the lowest severity of damage [F1KNEM (1.53), F1SNEM (1.58), ASTNEM (1.63)] compared to Jathropha extract treatment [F1SJAT (2.74), ASTJAT (2.75), F1KJAT (2.77)] and Lemon grass extract [F1SLEM (2.97), ASTLEM (3.64), F1KLEM (3.73)]. Similarly, the application of Neem extract significantly (p<0.05) reduced the mean incidence (21.84%) of the viral diseases than Lemon grass extract (25.28%), Jathropha extract (25.44%) and the Control (28.89%). In-vitro confirmation test using Enzyme-linked immunosorbent assay (ELISA) revealed that majority (86.67%) of the treatment combinations showed single infection of OMV disease while in 13.33% of the treatment combinations mixed-infection of OMV and OYVMV diseases was observed. In terms of yield levels, chemical pesticide treatment produced significantly (p<0.05) highest yield (186.92 kg/ha) compared to Neem extract treatment (144.81 kg/ha), Jathropha extract (139.06 kg/ha), Lemon grass extract (115.75 kg/ha) and the Control (94.02 kg/ha). However, Neem extract performed best among the tested plant extracts. Therefore, in a second experimental trial to ascertain the best efficacious dose for Neem extract application, varying concentrations of 20 ml/L, 30 ml/L and 40 ml/L were used and compared with the chemical “Akape” in the three okra cultivars (F1K, F1S and AST) instead of the 50ml/L applied in the first experimental trial. Treatment with “Akape” recorded significantly (p<0.05) the lowest mean whitefly populations than treatment with plant extracts. However, 20 ml/L Neem extract treatment produced significantly (p<0.05) lowest mean count of whitefly [F1KN1 (17.88), ASTN1 (23.95), F1SN1 (29.01)] compared to 40 ml/L Neem extract [F1KN3 (37.90), ASTN3 (39.57), F1SN3 (38.22)]. Similarly, although the application of the synthetic chemical (insecticide) “Akape” resulted in the best performance in reducing flea beetle populations, it was observed among the Neem extract concentrations, 20 ml/L treatment produced significantly (p < 0.05) lowest flea beetle mean count [F1KN1 (24.04), ASTN1 (25.61), F1SN1 (29.41)] compared to 30 ml/L [F1KN2 (49.89), ASTN2 (48.73), F1SN2 (51.92)] and 40 ml/L [F1KN3 (72.68), ASTN3 (65.88), F1SN3 (80.88)] treated okra cultivars. Okra cultivars treated with 20 ml/L Neem extract had significantly (p<0.05) lowest severity of insect pest damage compared to treatment with 40 ml/L Neem extract. Of the three concentrations of Neem extract applied, 20 ml/L (N1) significantly (p<0.05) reduced severity of mixed-infection of OMV and OYVMV) in all the three okra cultivars [ASTN1 (0.64), F1KN1 (0.84), F1SN1 (1.23)]. Yield of okra cultivars treated with chemical pesticide was significantly (p>0.05) highest (234kg/ha). However, yield obtained with 20 ml/L Neem extract treatment (207kg/ha) was significantly (p>0.05) highest compared to the other Neem extract doses. It is noteworthy that cultivar F1 Kirene recorded significant (p<0.05) the highest yield (207kg/ha) followed by F1 Sahari (139kg/ha) and Asontem (127kg/ha). From the obtained results, the cultivation of the okra cultivar F1 Kirene could be combined with Neem extract at a concentration of 20 ml/L to obtain effective disease and pests control for high yields.