Inter- and intraspecific morphometric variation and characterization of Phytophthora isolates from cocoa

Abstract

Difficulties in the accurate identification of the Phytophthora species responsible for black pod disease of cocoa continue to hamper effective disease control. A re-evaluation of morphological characters (Brasier & Griffin, 1979) and a detailed morphometric analysis of 161 Phytophthora isolates largely associated with black pod disease of cocoa from 17 countries worldwide have shown considerable inter- and intraspecific variation. Stable and more reliable parameters for the identification of the species responsible for the disease have been determined. Colony characteristics such as pattern and growth rate on V8 agar are reasonably characteristic for the cocoa Phytophthora species, and can be used to make preliminary identification to species level. Significant sporangial character variation was found within isolates of species from the same and different sources, highlighting the difficulties in making accurate identification on the basis of raw morphological data. Pedicel length was found to be the most consistent species-linked sporangial characteristic. Cluster plots of length/breadth ratios of sporangia versus reciprocals of sporangial pedicel length clearly separated all isolates into distinct species groups (P. capsici, P. citrophthora, P. palmivora and P. megakarya) and can be used reliably to identify accurately those pathogens involved in black pod disease outbreaks.