Évaluation de l’efficacité des extraits des graines de Moringa oleifera (Lam) (Moringaceae) contre Sclerotinia sclerotiorum (Lib) de Bary, et les insectes ravageurs du haricot commun (Phaseolus vulgaris L.).

Abstract

The common bean (Phaseolus vulgaris L.) is the second most cultivated legume in Cameroon after groundnut because of its high starch and protein. However, its production remains limited because of diseases, the most important of which is white rot (Sclerotinia diseases) and insect pests causing losses that can range from 45 to 100% in the field and in storage. Faced with pest’s threat, the most commonly used control method is chemical control. Although this method is the most effective, its use remains have an impact on the environment and human health. The adoption of alternative solutions that are friendlier to human health and the environment is essential. The use of plant extracts as an alternative to chemical pesticides in the fight against pathogens and insect pests of crops is imperative and topical. The objective of this study is to control the attacks of S. sclerotiorum and insect pests on common bean by Moringa oleifera seed extracts (EGM). Phytochemical screening, analysis by HPLC-MS and GC-MS of the different extracts were carried out. In vitro, four extracts (acetone, ethanol, methanol and aqueous) at concentrations (C1: 12.5; C2: 25 and C3: 50 μl/ml), a control (C0: 0 μl/ml) and a dose of (3.33 g/l) synthetic fungicide having 80% of Manebe as active ingredient were tested on two isolates of Sclerotinia sclerotiorum. The percentages of inhibition, concentrations reducing mycelial growth by 50 and 90%, were determined. In vivo, the pesticidal potency of M. oleifera seed extracts (acetone, ethanol, methanol) at concentrations (C1: 12.5; C2: 25 and C3: 50 μl/ml), a control (C0: 1 ml of ethanol) and a synthetic insecticide having 2% of Pirimiphos-methyl as active ingredient was evaluated on adult Acanthoscelides obtectus insects in storage using the MEX variety common bean. The field experiment was carried out using a split-plot design with four repetitions. Growth parameters, disease incidence and severity, insect damage and yield were assessed on two improved common bean varieties (V1: GLP 190 S and V2: NITU G16187) and five treatments (T0: control, T1: insecticide having 50% of Cypermethrin as active ingredient, T2: M. oleifera aqueous extracts, T3: M.oleifera ethanol extracts, T4: fungicide having 80% of Manebe as active ingredient). The results of the phytochemical analysis by HPLC-MS and GC-MS revealed the presence of several compounds such as Hesperidin, quinic acid, gallic acid, Protocatechic acid, 4-OH Benzoic acid, Cyranoside, Isoquercitrin, Cosmosiin, Quercitrin, Luteolin, Naringenin, Salicylic acid, Fumaric acid, Aconitic acid, n-Hexadecanoic acid, tridecanoic acid, Cis-Vaccenic acid and Apigenin etc. belonging to the main families of secondary metabolites with known biopesticidal activity. The in vitro results show that acetone and ethanolic extracts of M. oleifera showed at the highest concentration (C3: 50 μl/ml) a percentage inhibition of 97.18 and 100% respectively for isolate 1 and 56.02 and 97.44% respectively for isolate 2. The lowest minimum inhibitory concentrations (MIC50) were obtained with the Ethanol extract (0.87 and 1.70 μl.ml-1 for isolate 1 and 2 respectively). Lower MIC90 (2.30 and 2.59 μl.ml-1) were obtained with the Ethanol extract for isolates 1 and 2 respectively. In vivo, the insects mortality rate treated with EGM at the dose (C3: 50 μl/ml) is not significantly different from the treatment with the chemical insecticide. The number of eggs laid was higher in the negative control (236.67 eggs) compared to the Methanol extract (2.66 eggs) and Acetone (0.00 egg) at dose (C3: 50 μl/ml). In the field, the results show that the extracts inhibited the development of the disease in both varieties. At 8 SAS of the campaign (2016-2017) the plots treated with acqous extract (17.86% and 8.64%) and ethanolic extract (11.7% and 7.91%) showed lower disease severity compared to the T0 control treatment (47.32% and 56.14%) in the two varieties V1 (NITU G16187) and V2(GLP 190S) respectively. Concerning damage caused by insect pests, at 8 SAS of the campaign (2016-2017) a high severity of attack was recorded in the control plots (28.89% and 33.45%) compared to plots treated with the Aqueous extract (14.44% and 13.55%) and with the Ethanolic extract (12.82% and 13.33%) respectively for the V1 and V2 varieties. Plots treated with acqousous extract (1222.9 and 1277.8 kg/ha) and ethanol extract (1501.6 and 1485.9 kg/ha) of variety V1 (GLP 190S) respectively obtained significantly higher yields in both seasons (2016-2017; 2017-2018) than the control plots (774.25 and 737.75 kg/ha). The aqueous and organic extracts of Moringa oleifera seeds contain phytochemicals molecules which alone or in interaction with each other have antifungal and insecticidal activity against Sclerotinia diseases and bean pests. These extracts significantly reduced the damage caused by insect pests of common bean in the field and in storage and the impact of S. sclerotiorum on bean plants and therefore they can be used as an alternative to chemicals pesticides.