Morphological and molecular identification of Phytophthora tropicalis causing black pod rot in Mexico

Theobroma cacao L. is cultivated in Mexico, primarily in Tabasco, a southern state where the climatic conditions are suitable for producing cacao, the source of chocolate. However, these conditions are also suitable for cacao pathogens such as Phytophthora spp., the causative agents of black pod rot, a disease that is difficult to eradicate once established. In this study, we collected cacao pods exhibiting the symptoms and signs of black pod rot from different locations in Tabasco. Several recovered isolates exhibited the typical colony morphology of Phytophthora, including a white mycelium, stellate with chrysanthemum shape and appressed appearance with slightly torulose hyphae. Both sporangia and chlamydospores were absent in culture media, but were observed after induction in a sterile soil solution. Sporangia were papillated, and the sporangial shapes were globose, ellipsoid, and obturbinate, while the sporangiospores were umbrella shaped and simple sympodial. Chlamydospores were spherical, terminal, and subterminal. The concatenated analysis of the internal transcribed spacer region (ITS1-5.8S-ITS2 = ITS), the cytochrome c oxidase subunit II (COXII), the translation elongation factor 1α (TEF1) and β-tubulin (BT) placed strain PtCa-14 among several P. tropicalis strains, confirming the identity of this species. The pathogenicity tests on pear and peach fruit and cacao pods showed the ability of this oomycete to induce rot. Taken together, these results indicated the presence of P. tropicalis in the Mexican state of Tabasco and demonstrated its ability to cause black pod rot.