Theobroma cacao, is a tropical cash crop that is of significant economic importance to developing countries like Cameroon. Despite this, cacao production is limited by many fungal diseases leading to great losses. Principal among these diseases is black pod disease caused by Phytophthora megakarya. Research in plant pathology has developed a number of ways to improve the crop’s resistance against this pathogen such as development of resistant clones and the use of chemical fungicides. However, these methods have their downsides. Biological control has been developed as an alternative effort. It involves the use of microorganisms or organic composites like chitinous sources to combat the disease. The aim of this study is therefore to evaluate the effect of oyster shell on Theobroma cacao seedling against Phytophthora megakarya in nurseries. The effect of Oyster shell on resistance was done by applying them to soil samples in the green house. The morpho-physiological and biochemical traits of the plants were evaluated. After twelve weeks of growth, the growth parameters (length of plant, number of leaves and surface area of the plant, volume of roots, dry and fresh weight of roots and shoots) were evaluated. The leaf inoculation tests were carried out on whole leaves and leaf discs to assert the capacity of oyster shell to induce resistance. Also, fungal charge in the soil was determined by inoculating cacao pods with soil suspensions collected after 12 weeks of growth. Lastly, the biochemical defense marker were determined by evaluating the quantity of total proteins, total phenols and the enzymatic assay of peroxidases, chitinases, polyphenol oxidases and β-1, 3-glucanases from inoculated leaves compared against the non-inoculated. There was an increase in all the growth parameters of plants in soil samples treated with Oyster shell powder compared to the control. Level of necrosis in pods inoculated with soil suspension containing oyster shell powder was low compared to the control, implying fungal charge in treated soil samples reduced. The disease severity index in leaves and leave discs from plants treated with oyster shell powder showed were lowest. The levels of all the biochemical markers were relatively higher in plants from soil samples treated with oyster shell powder. These results correlated with the morpho-physiological parameters. This therefore suggests that as levels of biochemical markers increase, the growth parameters of the plant are higher, thus susceptibility to black pod disease reduces. Hence, oyster shell powder could serve as a biocontrol agent against black pod disease.