Potentiel bactéricide des extraits des graines de Thevetia peruviana (pers.) K. Schum sur Ralstonia solanacearum, agent causal du flétrissement bactérien chez la tomate (Solanum lycopersicum L.)

Abstract

The tomato (Solanum Lycopersicum L.) is widely cultivated all over the world. But this crop is prone to disease, particularly bacterial wilt caused by Ralstonia solanacearum, resulting in huge yield losses (90,62 %). The general objective of the present study was to inhibit the development of R. solanacearum on tomatoes by the use of extracts from the seeds of Thevetia peruviana. To this end, thirty (30) samples of diseased tomato plants were taken in the field in 03 agro-ecological zones in order to characterize the bacterial population. To this end, thirty (30) samples of diseased tomato plants were taken in the field in 03 agro-ecological zones in order to characterize the bacterial population. Three extracts of T. peruviana seeds (aqueous extract, extracted with ethanol and oil) and five treatments: T0 (control without treatment); T1 (12,5 μl/ml); T2 (25 μl/ml); T3 (50 μl/ml); T4 (100 μl/ml) were used and tested in vitro on the most virulent isolates. Three fractions (hexane, ethyl acetate and 1-butanol) of each extract were also performed and tested in vitro on an isolate at different concentrations. GC-MS of the most effective fractions against R. solancearum and molecular docking were performed to determine the main secondary metabolites responsible for the antibacterial activity of the extracts. The effective fractions were used in greenhouses and fields to inhibit disease progression. Field tests were carried out at balamba 1 in the Centre region during two seasons, following the split-plot experimental setup consisting of two factors, including a main factor consisting of two varieties (Cobra and Roma) and a secondary factor consisting of 05 treatments (T-, T+, Fae.EA, Fae.EOH, Fhe.HL). All biochemical test results demonstrated that tomato plant isolates were the pathogen R. solanacearum. Two biovars have been identified as responsible for tomato infection in Cameroon: biovar 2 and biovar 3 (majority) and two breeds have also been determined: breed 1 and breed 3 (majority). Isolates Fm6 (race 3 bv2) and Bfo (race 3 bv 3) were more virulent. All extracts inhibited 100 % in vitro growth of the isolates tested at the dose of 100 μl/ml. The lowest MIC values were obtained with the ethanol extract, which justifies its greater efficiency compared to the other two extracts. The ethyl acetate fraction of the aqueous extract was more effective in vitro with a 100 % inhibition at a dose of 25 μl/ml and a CMB of 30 μl/ml. GC-MS analysis of these three fractions revealed the presence of antibacterial compounds and molecular docking showed that 1,2-15,16-diepoxylhexadecane; eicosanoic acid; hexadecanoic acid butyl ester; diisooctyl phthalate; bis(2-ethylhexyl)phthalate; azulene; Eicosanoic acid, methyl and (Z)-7-hexadecenoic acid, methyl ester interact with the active sites of the bacteria's proteins, while blocking their activity. These three fractions reduced the incidence of bacterial wilt by 88 100 % in greenhouses and 70-85 % in the field, and can be exploited as a biological bactericide in integrated pest management of R. solanacearum.