Identification des champignons responsables des pourritures de Dioscorea spp. dans le Mbam et Kim et activité bioprotectrice des combinaisons d’huiles essentielles de Cymbopogon citratus et Syzygium aromaticum

Abstract

Yams (Dioscorea spp.) contribute to the daily diet and the incomes improvement of farmers all over the world. However, constraints during storage not only limit yam’s shelf life, but also their availability on the markets through out the year. Improving post-harvest handling of yams becomes absolutely necessary. This study aimed to provide informations on the post-harvest losses in the main farming area in Cameroon, Mbam and Kim, identify the responsible microorganisms and, to assess the bioprotective potential of biopesticide formulation with essential oils (EO) of Cymbopogon citratus and Syzygium aromaticum. A survey was carried out using a questionnaire, to collect data on farmer’s perception of the post-harvest conservation of yam tubers along with yams samples collected from 81 producers from three districts in the Mbam et Kim (Ntui, Mbangassina and Ngoro). The fungi were isolated from tubers with rotting symptoms, and the virulent isolates were identified by sequencing the ITS1-5.8S rDNA-ITS2 nucleotide sequence. The EO were extracted by hydrodistillation and their chemical composition, analyzed by GC/MS. The EO were evaluated for antimicrobial activity using agar dilution and microdillution methods and their combination by the checkerboard microdilution method. After an in sitiu pretest on the yam slices, the combination of interest (40/40 mg/mL) was formulated using both tween 80 and kaolin and tested in vivo, in a semi-controlled storage environment. In addition, the effect of the treatments on the nutritional parameters of the tubers was evaluated using the standard protocols.Out of the 81 farmers surveyed, 71 blamed yam rot as the main cause of postharvest loss during the storage of tubers. The losses due to dry rot and soft rot with the incidence of 33.33 % and 41.66 % respectively, were recorded in the three districts. Seven species of fungi were identified: Aspergillus niger, Bionectria ochreuleca, Fusarium solani, Fusarium oxysporum, Penicillium mononematosum, Rhizopus oryzea and Talaromyces flavovirens. P. mononematosum was reported here for the first time as being the most virulent with a severity index of 58.33 %. These pathogens produce amylase, cellulase, proteases and lipase enzymes generaly involved in the mechanism of pathogenesis. The yields of extraction were 0.136 % (C. citratus) and 6.34 % (S. aromaticum). The EO of C. citratus contain cis-ocimène (55.79 %) and S. aromaticum contain limonène (42.1 %), as the major compounds. The Minimum Inhibitory Concentrations (MIC) of these EO on mycelial growth and spore germination varied from 0.156 to 2.5 mg/mL. The combination of EO gave synergistic interactions with the values of Fractional Inhibitory Concentration Index (FICI) of 0.49 on R. oryzea and A. niger, additive on P. mononematosum (FICI = 0.56) and indifference (FICI = 1.5) on F. solani and F. oxysporum. In situ, the combination of EO (40/40 mg /mL) completely inhibited the infection of the tuber fragments by P. mononematosum, the Reduction Index Decay (RID) of 1 and 0.25 for the preventif and curative test, respectively. After 2 months of storage the formulation based on EO+Tween was the best (RID = 0.41) protector of yams. Moreover, the biopesticide did not significantly influence the nutritionnal parameters of yams tubers. The results obtained demonstrate that a biopesticide based on the combination (40/40 mg/mL) could be alternatively used for the post-harvest storage of Dioscorea spp.