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Cacao Pathology

The basidiomycete fungus Crinipellis perniciosa (Stahel) Singer. is the causal agent of Witches' Broom Disease (WBD) of Cacao (Theobroma cacao L.) which is the main factor limiting cocoa production in the Americas. Pod losses of up to 90% are experienced in affected areas as evidenced by the 50% drop in production in Bahia province, Brazil following the arrival of WBD in the area in 1989. The disease has proven particularly difficult to control and many farmers in affected areas have given up cacao cultivation. At UW Aberystwyth, two projects funded by Cocoa Research UK Ltd. are currently underway:- 

  1. Control of hemibiotrophy in Crinipellis perniciosa
  2. Genetic diversity and population biology of Crinipellis perniciosa

Control of hemibiotrophy in Crinipellis perniciosa

Richard Birch
E-Mail: rnb94@aber.ac.uk

C. perniciosa has separate two morphologically distinct stages in its lifecycle: Basidiospores (the only infective stage of the pathogen) germinate on susceptible host meristems and after penetration of the host tissue produce a swollen, biotrophic mycelium which ramifies through host tissue and is responsible for the characteristic broom formation. When the broom dies, the mycelium reverts to a saprotrophic state and utilises the dead broom as would any other degrading fungus, eventually forming basidiocarps and completing the lifecycle. The biotrophic phase is difficult to study because it is not possible to maintain it in artificial culture. Furthermore the factors which lead to the morphological 'switch' at the end of the biotrophic stage of infection are not understood. 

This doctoral research programme funded by Cocoa Research UK Ltd. and supervised by Drs. Gareth Griffith and Ian Scott is concentrating on the pathogen in situ using whole infected broom tissue. Whole protein extraction and expression on 2-Dimensional PAGE (polyacrlyamide gel electrophoresis)  elucidates the protein production in response to infection and something of the nature of host/parasite interaction is uncovered. 

Genetic diversity and population biology of C. perniciosa

Jean Nicholson
E-Mail: jnn@aber.ac.uk

Any useful control strategy for WBD must be effective against in a range of cocoa-growing regions. It is already known that pathogenic variation exists among isolates of Crinipellis perniciosa obtained from different areas. Genetic studies have demonstrated that pathogen populations are also genetically variable though populations of the non-outcrossing C-biotype are more homogeneous than that those of their non-pathogenic relatives (i.e. L-biotype from lianas). 

To date, however, no comprehensive analysis of the genetic structure of C-biotype populations has been conducted, including isolates from areas covering the whole geographical distribution of C. perniciosa. This project aims to answer a number of questions:-

a. What level of genetic variation exists between C. perniciosa isolates from different countries (e.g. Brazil vs. Ecuador) and within countries (Bahia vs. Rondonia)? 

b. How many times has the pathogenic C-biotype evolved and where is the epicentre of C-biotype diversity? 

c. Is the genetic structure of pathogen populations changing over time?


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Diagnostic probes for rapid identification of the black pod pathogens Phytophthora megakarya and P. palmivora in West Africa

This project was conducted in 1998 funded by the Biscuit, Cake, Chocolate and Confectionery Alliance with the aim of devising a rapid diagnostic method for differentiating between the two related pathogen which cause 'black pod' disease (Phytophthora pod rot) in cacao. P.palmivora is worldwide in its distribution but P. megakarya which is a more aggressive pathogen has been westwards during recent decades and has caused increased levels of pod loss in some parts of Ghana.  Differentiation of the two pathogens in the field is possible but requires an experience eye. The diagnostic test devised at UW Aberystwyth uses PCR to amplify specific regions of the mitochondrial genome and restriction digestion of the resultant PCR product gives rise to different banding patterns for the two pathogens.  The primers can be used on DNA extracted from dried pod samples.  

For further details of the primers used in this project click here.

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