Usefulness of the Detached Pod Test for Assessment of Cacao Resistance to Phytophthora Pod Rot

Abstract

The detached pod test by spray method (DPT-SM) was developed to facilitate the screening of cacao genotypes for resistance to Phytophthora pod rot (PPR) caused by Phytophthora palmivora. The test has been adopted in many cacao research institutes, and it is imperative that its validity be assessed. In this study, 40 genotypes from various cacao groups were selected and screened for resistance to PPR by DPT-SM and field observations. Significant variation was observed in the reactions of the selected accessions based on the disease rating scale for DPT-SM and the percentage pod rot from field observations. A correlation coefficient of 0.68 (P<0.001) was observed between the results of year-1 and year-2 field observations. However, relatively lower correlation values were obtained between year-3 and year-1 (r=0.32; P=0.041) and year-3 and year-2 (r=0.35; P=0.025) field observations. 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 suggests that the predisposing factors for PPR were unstable between the years of field observations. Data obtained from the Trinidad and Tobago Meteorological Services showed that the rainfall in November of the third year was higher than the amounts of rainfall in November of the first 2 years of field observations. This month marks the beginning of the main pod harvest season (November–February) and the high rainfall in November of the third year, and the presence of large number of mature pods may account in part for the increase in PPR in the third year of field observations than in the first 2 years. This shows that absolute reliance could not be placed on a single year of field observations in determining clonal resistance to PPR. A correlation coefficient of 0.59 (P<0.001) was obtained between the result of year-2 field observations and DPT-SM. Relatively lower correlation values were observed between DPT-SM and year-1 (r=0.55; P<0.001) and year-3 (r=0.44; P=0.005) field observations. The result of DPT-SM, however, shows a higher correlation (r=0.66; P<0.001) with the average of years 1–3 field observations. This suggests that a stronger association may exist between the result of DPT-SM and the cumulative data on field observations for a period longer than 3 years. The correlation (r=0.66; P<0.001) observed in this study confirms the usefulness of DPT-SM as an effective method of assessing clonal resistance to PPR and predicting field reaction in the long term. Since field observations are labour intensive and expensive to conduct on a yearly basis, the DPT-SM offers a cheaper and effective means of assessing clonal resistance to PPR. Being a non-destructive inoculation method, the DPT-SM provides a suitable option for cacao collections in genebanks to be assessed. It is also a cost-effective method for use in cacao breeding programmes. Based on its reliability, the DPT-SM has been adopted in the CFC/ICCO/IPGRI cocoa project ‘Cocoa Germplasm Utilization and Conservation: A Global Approach’ for the assessment of cacao resistance to PPR in several cocoa producing countries.