Anti-Phytophthora Activity of Carica Papaya Linn Extracts: A Case of the Two Major Cocoa Black Pod Pathogens in Ghana

Abstract

Background: Theobroma cacao (Cocoa) is a major foreign exchange earner for Ghana. Over the recent decades, there has been a consistent decline in cocoa production in the country. A major contributing factor is the cocoa black pod disease caused by Phytophthora sp. Synthetic fungicides have been used to control the pathogens, but recent calls for organic cocoa devoid of residues from synthetic fungicides, which are said to pose several health challenges as well as a negative impact on the environment, have gained attention, thus inviting investigations into more environmentally friendly and sustainable alternatives. Objective: This study explored the inhibitory potential of crude extract from Carica papaya Linn. against cocoa black pod disease caused by Phytophthora sp. Material and Methods: An initial in silico assessment was done by using compounds previously isolated and characterized from Carica papaya in the literature. These compounds were used as ligands against a phytophthora effector protein. Crude extracts were obtained from the plant materials using 70% ethanol as the extraction solvent. The crude Green Leaf Extract (GLE), Aging Induced Chlorophyll Deficient Leaf Extract (AICDLE), and Matured Black Seed Extract (SDE) were tested in vitro against precultured P. palmivora and P. megakarya using the poison food technique. GLE was modulated with Delco, a synthetic fungicide, at different ratios, which were tested in vitro and in vivo. Results: 16 of the ligands showed binding affinity higher or equal to the standard ligand alliin, suggesting Carica papaya is likely a repository of anti-Phytophthora agents. Assessment of crude extracts at concentrations 5–20 mg/ml showed fungistatic activity against P. palmivora and P. megakarya in vitro. Modulation of GLE with Delco produced some combinations that made GLE fungicidal in vitro against P. palmivora and P. megakarya with fractional inhibition concentration indices between 0.51 and 0.65, interpreted as partially synergistic. In vivo assessment of combination D (comprising Delco and GLE in a ratio of 0.3mg:3mg per ml), demonstrated effectiveness comparable to Delco alone at the recommended dosage of 5mg/ml. Conclusion: This study shows Carica papaya in a new light as a potential material that could be formulated into a natural product-based fungicide for combating Phytophthora-induced black pod disease.