Influence of vermichar and Trichoderma harzianum in controlling Phytophthora palmivora infection and promoting cocoa seedling growth in soil and cocopeat

BACKGROUND: Seedling blight, caused by Phytophthora palmivora, is one of the main diseases affecting cocoa seedling production. The pathogen is mainly controlled using chemical fungicides, contributing to the adverse cocoa environmental impact. Organic amendments and beneficial microorganisms may suppress pathogens activities in soil. Therefore, it is hypothesized that the application of biochar-mediated vermicompost (vermichar) and Trichoderma harzianum may have a suppressive effect against P. palmivora. Treatments containing vermichar, T. harzianum and their combination were applied at 5% rate to topsoil and cocopeat substrates. Leaf number, plant height, stem girth, chlorophyll content, lesion length and rate of survival were monitored during the growth period. RESULTS: Plants grown on vermichar and T. harzianum-treated substrates had a higher leaf chlorophyll content (between 1.03- and 2.71-fold), plant height (between 1.10- and 1.28-fold) and leaf number (between 1.10- and 1.64-fold) than plants grown only in soil or cocopeat. The combination of the two treatments with P. palmivora also favoured greater growth of cocoa seedlings compared to those grown on untreated media. Vermichar suppressed seedling blight when added to cocopeat medium, whereas the combination of T. harzianum and vermichar suppressed the disease in soil. A 100% seedling survival rate was observed in soil with vermichar + P. palmivora + T. harzianum and in cocopeat with vermichar+ P. palmivora. CONCLUSION: Results show that the vermichar/T. harzianum combination is a viable sustainable management strategy for breeding of disease-free (plants with no symptoms) cocoa seedlings. However, the applicability of the best treatment rate during the transplanting phase of cocoa seedlings needs further research. © 2025 Society of Chemical Industry.