Black pod disease on cacao (Theobroma cacao, L) in Ghana: spread of Phytophthora megakarya and role of economic plants in the disease epidemiology.
To implement management strategies of Phytophthora pod rot (black pod) diseases successfully, knowledge of the role of economic plants, and the diversity and distribution of Phytophthora species on these plants in the cacao ecosystem is essential. In this study, the spread of Phytophthora megakarya in the cocoa growing districts of Ghana was monitored through annual surveys from 1985 to 2012. In addition, from July to October, 2008, Phytophthora was isolated from randomly collected non-symptomatic rootlets of economic plants on cacao farms and from symptomatic pods and stems of cacao from 139 locations in 22 districts in the cacao growing regions. A total of 1459 Phytophthora isolates from 22 plant species belonging to 19 families were characterized. The Phytophthora isolates showed six different morphological patterns on Campbell Vegetable Juice Agar medium. The patterns were variable stellate striate, cotton wool-like, deep cotton wool-like only at colony margin, light cotton wool-like pattern only at colony margin, and dense aerial, fluffy colonies without pattern. However, microscopic examination of the sporangia of the isolates revealed only two species, Phytophthora palmivora and P. megakarya. P. megakarya was found to have spread from Akomadan and Bechem in 1985 to 50 more administrative districts in 2012. This study also represents the first report of isolation of P. megakarya from Xanthosoma saggitifolium, Musa paradisiaca, Elaeis guinnensis, Persea americana, Carica papaya, Mangifera indica, Colocasia esculenta, Athyrium nipponicum and Ananas comosus on cacao farms in Ghana. The fact that economic plants on cacao farms harbour Phytophthora species may explain the failure to obtain appreciable control of Phytophthora diseases as the current management of these diseases is mainly targeted at the host crop, Theobroma cacao.