Evaluation de l’effet de l’amendement des sols avec la poudre des coquilles d’huitres et ses dérivés sur la qualité des semences plants de cacaoyer (Theobroma cacao L.) vis-à-vis de Phytophthora megakarya

Abstract

Seedling production is a key step in the process of creating new cocoa plantations. Cocoa seedlings are generally obtained from unsterilized soil or soil already contaminated with Phytophthora megakarya. The presence of P. megakarya in the soil is a real problem in cocoa seedling production. In order to overcome these attacks, the use of biodegradable organic substances such as oyster shells is an alternative to chemical control methods. The aim of this work was to evaluate the effect of soil amendment with an oyster shell biofungicide on the stimulation of the cocoa plant defense systems against P. megakarya in the nursery. For this purpose, the activating potential of oyster shells and oyster shell biofungicide to stimulate cocoa plant defense mechanisms was evaluated by soil amendment at different concentrations under controlled and farmers’ conditions. After soil amendment, the agronomic parameters of growth, growth physiology, soil parameters and biochemical markers of resistance were analyzed. The degree of expression of phytoalexins and defense genes was carried out by RP-HPLC-DAD-ESI (+) -MS and RT-PCR analyses respectively. The results obtained show that soil treatment with these composites improves the sanitary quality of cocoa seedlings in controlled and farming conditions through the stimulation of agronomic and physiological growth parameters such as chlorophyll and stomatal resistance. The latter induces the reduction of the load of P.megakarya in the soil and leads to the increase of the soil pH by about 32 % as well as the improvement of the nutritional status of the soil by about 90 %. These composites stimulate the production of proteins PR2, PR3, PR9, total phenolic compounds as well as the enzymatic activities of PAL and PPO. Moreover, RP-HPLC-DAD-ESI (+) -MS analyses show that the '' Bio-Oyster Shell biofungicide '' induces the early synthesis of flavonols and a phenolic acid derivative (brevifolin). RT-PCR analyses show that the biofungicide induces the early overexpression of the TcPer-1, TcGlu1, TcChiB and TcMYBPA genes before and after infection. This stimulation based on biological substances makes it possible to pre-immunize cocoa plants against possible attacks in the field and to improve seed quality and productivity of organic cocoa in Cameroon.