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Control of Phytophthora megakarya diseases of cocoa with potassium phosphonate


Department for International Development (DFID)


Cocoa Research Institute of Ghana (CRIG)/ Natural Resources Institute (NRI) / University of Reading.

Project Description

Application of phosphonic acid by stem injection to reduce P. megakarya infection

Problem/Issue. CABI Bioscience, in partnership with the Cocoa Research Institute of Ghana and the Natural Resources Institute, are undertaking a study to develop alternative and more environmentally-acceptable measures for the management of Phytophthora megakarya black pod disease of cocoa. This devastating disease, which can cause near-total pod loss in wet seasons, continues to spread west through Ghana and is now a threat to production in neighbouring Côte d’Ivoire. Control measures are required that are suitable, economic and safe for farmers and the environment. One possibility is the use of potassium phosphite, a base commodity chemical that has been shown elsewhere to have effects against Phytophthora diseases and offers the prospect of treatment by single application in a season, rather than the repeated sprays required for conventional fungicides. Phosphite differs from conventional fungicides in that its activity is due, at least in part, to a stimulation of plant resistance responses to combat the infection. The prospects for use of induced systemic resistance in cocoa thus form an important element of the study.

Measures to Date. Potassium phosphonate injection has been shown to be at least as effective as existing recommendations for the use of metalaxyl and copper sprays. However, the problem of localised phytotoxicity remains and studies are now continuing to investigate the prospects for use of reduced doses alongside compounds and organisms capable of stimulating host defences through induced systemic resistance. The project has successfully refined the analytical method for phosphite residues, allowing more consistent detection at low levels than has previously been the case. Studies of relative operator and environmental contamination by conventional sprays and phosphonate injection are continuing.

Project Activities. CABI staff provide advisory and technical input in relation to the layout and design of experimental and survey work, the overall integration of field studies and specific technical inputs on aspects concerned with induced systemic resistance.

Achievements So Far

Large-scale field experiments have been established at two sites to evaluate formulations and application methods. Application of metalaxyl + copper was effective in controlling P. megakarya, but this requires 5 applications and is too expensive for most farmers at present. Application of phosphonic acid by stem injection is effective in reducing P.megakarya infection and requires less frequent labour inputs by the farmers, but resulted in scorching of internal tissues of the cocoa trees. Application of potassium phosphonate to the pods as a spray or paint application led to speckling (spray) and scorching (pod painting). Smaller-scale experimentation has been established to understand efficacy and mechanisms of action of potential agents of induced systemic resistance in cocoa and effects of fungicides on environmental parameters. A socio-economic survey is now underway to determine farmer perceptions of disease significance and constraints to use of disease control measures. The analytical method for phosphite residues has been extensively refined. Two Ghanaian scientists have been trained in these analytical methods and relevant equipment and materials obtained and installed to enable analysis in Ghana.

What Next

The field trials continue and results will be linked to actual residues determined in tissues. The aim is to find minimum effective doses for control of the disease without problems caused by the injection process. As induced resistance is thought to contribute to the control of fungal pathogens by potassium phosphonate, the use of an elicitor of induced resistance in combination with potassium phosphonate may prove effective in controlling the disease, whilst reducing the physical damage caused by potassium phosphonate. This approach will form the basis of the next phase of the project.

Dr. Mark Holderness m.holderness@cabi.org
Dr.Keith Holmes k.holmes@cabi.org

Start date: February 1999

End date: January 2002


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Contact CABI Commodities:
CABI Commodities Coffee Co-ordinator - Dr. Peter Baker - p.baker@cabi.org
CABI Commodities Cocoa Co-ordinator - Dr. Keith Holmes - k.holmes@cabi.org
CABI Commodities Project Administrator - cabi-commodities@cabi.org