A new method for the inoculation of Phytophthora palmivora (Butler) into cacao seedlings under greenhouse conditions

BACKGROUND: The black pod disease affects cacao plantations worldwide; it is caused by the oomycete species of the genus Phytophthora. The resistance of cacao plants to the black pod is commonly evaluated by artificial inoculation of the pathogen and the monitoring of the disease symptoms. However, it is difficult to identify resistant plants because the commonly used methods for the inoculation of the pathogens produce inconsistent results. Therefore, this study aimed to develop an efficient and reliable method to evaluate the resistance of Theobroma cacao seedlings to the infection by Phytophthora palmivora. RESULTS: Seedlings of different cacao genotypes were inoculated with P. palmivora under greenhouse conditions using the previously reported inoculation methods and a newly proposed method, the agar–water solution method. While none of the previously reported methods was effective, the agar–water solution method ensured a 100% seedling infection under greenhouse conditions. The proposed agar–water methodology is fast, simple and reproducible. Furthermore, the evaluation of this method in susceptible (CCN-51) and tolerant (SCA-6) T. cacao genotypes produced the expected contrasting results. CONCLUSIONS: The agar–water solution method presented in this study is an efficient alternative inoculation protocol for the identification of cacao genotypes that are resistant to black pod under greenhouse conditions.