Comparison of resistance to Phytophthora pod rot in a ring test using leaf disc inoculation at ten sites

Phytophthora pod rot (Ppr) caused by different species of Phytophthora is the most widespread disease of cocoa. All producing countries are affected by this disease, albeit at varying intensities. Selecting resistant planting materials is therefore one of the priorities of genetic improvement. In order to select cocoa clones displaying resistance to Phytophthora, a ring test was organized in all the countries involved in the CFC/ICCO/IPGRI project. The different countries involved carried out a test on leaf discs for which the protocol had been drafted during earlier research. The test was used to classify different clones tested in each country according to their relative susceptibility to the Phytophthora species occurring in that country. An overview of the results is presented, focusing particularly on the interaction between the clones studied and the laboratory where the assessments were carried out. The interaction was statistically significant, indicating that clone classification was not identical for the different countries. Some clones, such as PA107 had stable reactions; others such as the Guianan clone GU255V had very unstable reactions: it was found to be very resistant in some laboratories and very susceptible in others. These interactions were due partly to the different isolates or species of Phytophthora used for the tests, or to different testing procedures used in different countries. Better standardization of the assessment protocols would nonetheless improve the reproducibility of the test. Mistakes in the identity of the clones in the various countries can also be a major factor for the significant clone x laboratory interaction. (Résumé d'auteur