Shade trees are alternative hosts of the cocoa pathogen Phytophthora megakarya

Two methods of isolation, direct plating on selective agar medium and baiting with cocoa pod husks, were used to isolate Phytophthora megakarya from root pieces of some shade trees. Isolates were identified on the basis of their growth rates, colony morphology and sporangium characteristics. Pathogenicity tests were conducted on detached green mature cocoa pods and stems of the relevant host trees. After 36 months of sampling and baiting, P. megakarya was isolated from the roots of four out of 34 shade tree species examined. The host trees were Funtumia elastica (Apocynaceae), Sterculia tragacantha (Sterculiaceae), Dracaena mannii (Agavaceae) and Ricinodendron heudelotii (Euphorbiaceae). P. megakarya isolations were made in both the dry and wet seasons. The rate of recoveries were very low in both seasons ranging from 0.6% to 1.2%. The highest recoveries were in October and the lowest in December and February. In general, plating onto medium was slightly superior to cocoa pod husk baiting for the recovery of P. megakarya. Colonies of P. megakarya isolates from the trees were morphologically indistinguishable from a reference isolate, but were less virulent on cocoa pods than the reference isolate from cocoa. The epidemiological significance of these findings are not clear, but roots of the host trees were likely to be sites for survival and not for multiplication of P. megakarya. Field observation indicated that levels of black pod incidence on cocoa trees around the affected shade trees were not greater than those in other parts of cocoa plantation. This is the first reported isolations of P. megakarya from roots of plants other than cocoa. © 2002 Elsevier Science Ltd. All rights reserved.