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Phosphonic acid: an alternative approach to the control of black pod disease of cocoa caused by Phytophthora megakarya.

Published by:
Publication date
Type of Publication:
Articles & Journals
Focus Region:
Sub-Saharan Africa
Focus Topic:
Agricultural Value Chains / Agri-Businesses
Opoku, I.Y.; Akrofi, A.Y.; Holderness, M.; Holmes, K.A.; Ackonor, J.B.; Ollenu, L.A.A.

Cocoa is the main commodity crop of Ghana and is financially important for the livelihood of many rural communities. The relatively recent arrival of the more aggressive black pod pathogen Phytophthora megakarya and its continued progress westwards threatens the cocoa farming communities. In a wider context its recent arrival in Ivory Coast, currently the worlds largest producer of cocoa, has implications for West Africa and global cocoa production. In Ghana it has already led to some farmers abandoning cocoa in Ashanti and to a lesser extent in the Western regions.

Studies undertaken by scientists at CRIG have shown that although cultural practices are effective for the control of P.palmivora they are inadequate for the control of P.megakarya. The supplementation of cultural practices with chemical application is essential to reduce the impact of P.megakarya. Recently CRIG scientists reported that fungicide adoption is on the increase in P.megakarya infected districts. Current recommendation is for the application of copper/metalaxyl formulations. The Ghanaian government has initiated a national spraying programme, centrally funded. However, in the long-term an alternative to the current copper based products is required. The application of these chemicals is expensive and often incorrectly applied by farmers, in addition the accumulation of these compounds in the environment could have long-term effects on soil biology and productivity of the cocoa agro-ecosystem. Phosphonic acid has been shown to be an effective, economically viable and environmentally safer alternative control agent of Phytophthora spp. in cocoa and other crop systems around the world.

Results are presented of field trials carried out over 3 years to assess the potential of phosphonic acid applied as stem injection, pod spray and paint in comparison to spray application of copper compounds. Six spray applications of the copper compounds and phosphonic acid were made over the season, in accordance with current recommendations for copper fungicides. Stem injection of phosphonic acid was applied once in a growing season. Comparative studies were implemented in Bechem, where P.megakarya is present and at Tafo where only P.palmivora is present.

Application of phosphonic acid as a one-off stem injection was shown to reduce the incidence of P.megakarya. Control achieved was similar to that achieved with the current recommended control regime. Implication of these results for the implementation of stem injection of Phosphonic acid as an effective, economically viable and environmentally safe alternative to copper fungicides is discussed.