Comparison of Mycelial Growth of Phytophthora palmivora and Phytophthora megakarya in Potato Glucose Broth

Abstract

Cacao (Theobroma cacao) is the most economically important species in the genus Theobroma and is primarily cultivated in tropical regions of West Africa, Central and South America, and Southeast Asia. Black pod rot of cacao, caused mainly by the oomycetes Phytophthora palmivora and Phytophthora megakarya, poses a serious threat to cacao production systems. Infected pods initially develop dark-brown or black, sometimes water-soaked lesions and may eventually rot, resulting in complete fruit loss. Although the growth of P. palmivora in potato dextrose broth has been documented, information on the growth of P. megakarya in potato-based liquid media remains limited. This study aimed to isolate and compare the mycelial growth of P. palmivora and P. megakarya in Potato Glucose Broth. Under the tested conditions, P. palmivora produced visible mycelial growth, whereas visible mycelia of P. megakarya were not observed. The spent broth of P. palmivora cultures became clearer and had a higher mean residual glucose concentration (32.7 mmol/L) than the P. megakarya cultures, which retained the initial colouration and had a lower mean residual glucose concentration (26.7 mmol/L). These observations indicate that the two-species exhibited different culture responses in Potato Glucose Broth. The findings may support preliminary laboratory comparison of the species, although further studies incorporating biomass measurement, appropriate controls, and confirmatory morphological or molecular methods are needed.