Validation studies on the detached pod test and leaf disc inoculation method for the assessment of cocoa resistance to Phytophthora infection

Iwaro A.D., Thévenin Jean-Marc, Butler David R., Eskes Albertus. 2006. Validation studies on the detached pod test and leaf disc inoculation method for the assessment of cocoa resistance to Phytophthora infection. In : Global approaches to cocoa germplasm utilization and conservation : Final report of the CDC/CCO/IPGRI project on "Cocoa germplasm utilization and conservation: A global approach" (1998-2004). Eskes Albertus (ed.), Efron Yoel (ed.). CFC, ICCO, IPGRI. Amsterdam : CFC, 108-115. (CFC Technical paper, 50) ISBN 978-92-9043-734-5

Résumé : During the 5-year period of the CFC/ICCO/IPGRI project "Cocoa Germplasm Utilization and Conservation: a Global Approach", a series of experiments was conducted using the detached pod test - spray method (DPT-SM) and the leaf disc test (LDT) to assess cocoa for resistance to Phytophthora infections. Data obtained in the project at the Cocoa Research Unit (CRU), The University of the West Indies, Trinidad and Tobago, provide an opportunity to assess the validity of DPT-SM and LDT. Between 1998 and 2003, 742 accessions were assessed in two trials for resistance to Phytophthora pod rot (Ppr) using the DPT-SM. Significant variation was observed in the reactions of the 742 accessions based on the disease rating scale, showing that the DPT-SM could effectively discriminate between various levels of resistance to Ppr. A chi-square test did not show any significant difference between the two trials of inoculation conducted. Furthermore, a high correlation coefficient (r=0.80, P<0.001) was obtained between the two trials, confirming that the results of DPT-SM are repeatable. Among the 742 accessions tested using the DPT-SM, 40 were assessed for resistance to Ppr in field conditions at the International Cocoa Genebank, Trinidad (ICG,T). Significant variation was observed among accessions and a higher level of susceptibility was observed in the third year of field observations (63%) than in the first (15%) and second (25%) years. This shows that absolute reliance could not be placed on a single year of field observations in determining clonal resistance to Ppr. The result of DPT-SM shows a higher correlation (r=0.66, P<0.001) with the average of years 1 to 3 field observations than with the result of each year of field observations, which varied from 0.44 to 0.59. It is possible that a stronger association may exist between the result of DPT-SM and the cumulative data on field observations for a period longer than three years. The overall correlation confirms the usefulness of DPT-SM as an effective method of assessing clonal resistance to Ppr and predicting field reaction in the long term. Significant differences were observed among 33 accessions screened for leaf resistance to P. palmivora using the LDT. This shows that the LDT could effectively discriminate between different levels of resistance among cocoa accessions. Correlations among the three inoculation trials conducted varied between 0.49 and 0.77. This shows that the results of LDT are repeatable. A poor correlation (r=0.14) was observed between the results of the LDT and DPT-SM, suggesting that the forms of resistance assessed by LDT and DPT-SM might be different. Since Phytophthora infects both pod and leaf, resistance in the two organs as assessed by LDT and DPT-SM may complement each other in breeding for a higher level of resistance to Phytophthora infections in cocoa.

Classification Agris : F30 - Génétique et amélioration des plantes
H20 - Maladies des plantes
U30 - Méthodes de recherche

Champ stratégique Cirad : Axe 1 (2005-2013) - Intensification écologique

Auteurs et affiliations

• Iwaro A.D., UWI (TTO)
• Thévenin Jean-Marc, CIRAD-CP-UPR Bioagresseurs de pérennes (FRA)
• Butler David R., UWI (TTO)
• Eskes Albertus, CIRAD-CP-UPR Bioagresseurs de pérennes (FRA)

Source : Cirad - Agritrop (https://agritrop.cirad.fr/537703/)

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