Crop Protection 21 (2002) 629–634 Shade trees are alternative hosts of the cocoa pathogen Phytophthora megakarya I.Y. Opoku*, A.Y. Akrofi, A.A. Appiah Cocoa Research Institute of Ghana, P.O. Box 8, Tafo, Akim, Ghana Received 31 August 2001; received in revised form 30 October 2001; accepted 18 December 2001 Abstract 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. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Phytophthora megakarya; Alternative hosts; Cocoa 1. Introduction Two species of Phytophthora, P. palmivora and P. megakarya are the main causal pathogens of black pod disease of cocoa in West Africa. P. palmivora has been reported on a large number of host plants. Ashby (1929) described P. palmivora as an omnivorous tropical fungal pathogen of world-wide distribution, existing as a number of morphologically and sometimes pathologi- cally distinguishable strains on a wide range of cultivated plants. Over 60 host plants are known to be attacked by P. palmivora and of these about 40 occur in Ghana (Turner, 1961; Macfarlane, 1968). Chee (1969) also listed 138 plant species as hosts of P. palmivora of which 78 are economically important whilst the rest are ornamental, shade or hedge plants. The host plants were grouped into their respective families by Nienhaus (1960), and these included members of the Sterculiaceae, Palmae, Euphorbiaceae, Araceae and Caricaceae. From available literature, the only known host of P. megakarya is Theobroma cacao (Brasier and Griffin, 1979b). The world distribution of P. megakarya suggests that the fungus is indigenous to West Africa, apparently confined to the West African Sub Region including Nigeria, Cameroun, Equatorial Guinea, Gabon, Togo and Ghana (Brasier and Griffin, 1979a; Djiekpor et al., 1981; Dakwa, 1987). To date, it is only in Cote d’Ivoire that P. megakarya incidence has not yet been reported. This suggests that P. megakarya had some wild or alternative host(s) plants before cocoa was introduced into the West African Sub Region. In Ghana, P. palmivora was reported as indigenous to forest soils (Dakwa, 1974), and the use of Gmelina arborea as a shade tree for cocoa was discouraged partly because its fruits supported growth of P. palmivora after falling on the soil (Asare-Nyako, 1969). *Corresponding author. Tel.: +233-27-609900; fax: +233-27- 609910. E-mail address: iopoku@crig.org (I.Y. Opoku). 0261-2194/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0261-2194(02)00013-3