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Variability of Phytophthora species causing black pod disease of cocoa and implications for assessment of host resistance.


Association of Commonewealth Universities


Cocoa Research Institute of Ghana (CRIG)

Project Description

Phytophthera megakarya infected pod

Problem/Issue. Phytophthora pod rot, commonly known as "black pod" disease is found in all cocoa regions of the world and causes an estimated 44% of the total global crop loss in cocoa. A number of Phytophthora species are known to cause black pod disease including P.palmivora, P.megakarya, P capsici, P citrophthora . Of these, P. megakarya is only found in West Africa and is the most aggressive species - causing 100% losses in some areas of Ghana. The pathogen has been isolated close to the border with Ivory Coast, the world's largest producer of cocoa (currently 30%).

Sporangia of Phytophthera megakarya


The project has 3 objectives:

  1. To assess the variation between and within isolates of Phytophthora species from cocoa using cultural and morphological characters and biochemical and molecular techniques
  2. To relate the variation observed to levels of aggressiveness (ability to kill cocoa) and
  3. To screen cocoa germplasm for resistance particulary to P.megakarya

Measures to Date. Control of black pod depends on several factors such as climatic conditions, which species of the pathogen are present in the locality, and are the affected trees are growing on plantations or on small holder blocks. The various methods can be categorized as cultural, chemical, biological and the use of resistance /tolerant varieties. Cultural control aims to reduce the humidity thus discouraging infection and reducing source of inoculum such as diseased pods. Chemical control involves the use of fungicides - copper compounds have been used for number of years but their use has environmental and health implications. Latterly, systemic compound have been used and have been shown to be more effective. Biological control- use of antagonists is being studied in West Africa and in Central America whilst developing genetic resistance against the pathogens that cause "black pod" is practiced in all cocoa growing areas of the world. However, these studies are often constrained by lack of understanding of the variation within the pathogen population and hence, which isolates to use to screen the germplasm. The project aims to address these issues.

Project Activities. The Organisation has over 70 years of experience of the taxonomy of Phytophthora, based on morphological and cultural characteristics which resulted in a number of internationally used and recognised keys for classification. However, increasingly , molecular approaches and techniques are being applied to this group at CABI Commodities. These approaches include MtDNA analysis, ALFPs and analysis of the ITS regions. These molecular techniques are used to confirm identities of unknowns and help identify wrongly typed isolates. See Phytophthora Identification Service.

Achievements So Far

170 isolates received as Phytophthora have been characterised based on morphological assessments of colony patterns, sporangial dimensions and characteristics as well as their mating types (sexual reproduction). Useful morphological characters have been identified for effective identification of isolates. Molecular techniques used have also confirmed identities and help identify wrongly typed isolates as well as elucidated the genetic linkages between species and isolates from different geographical origins.

What Next

Further studies with regard to the mechanisms of resistance and screening will be conducted in light of the results determined in the current project.

Dr. Julie Flood (Co-ordinator) j.flood@cabi.org
Dr. Alex Appiah

Start date: Oct 1997

End date: March 2001

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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