Genetic analysis of cocoa (Theobroma cacao L.) hybrids and screening superior hybrids for major biotic stress

Abstract

Cocoa is a crop highly influenced by climate change and growing environment, which make it necessary to have long term and dynamic breeding programme. Yield improvement was the prime objective of most of the earlier breeding programmes. However, with the emergence and spread of many diseases and pests, more emphasis is given for evolving disease and pest tolerant cocoa varieties, without sacrificing yield. At present, one of the main challenges faced by cocoa growers is Phytophthora pod rot caused by Phytophthora palmivora. Since this disease is prevalent during rainy season it is very difficult to control using fungicides. Cultivation of resistant varieties is the most effective and ecofriendly method of control. Tea mosquito bug (TMB) (Helopeltis theivora) is a major sucking pest of cocoa causing damage to young shoots, cherelles and pods. The development and use of mirid resistant cocoa varieties is the only effective alternatives to chemical control against TMB.

Twenty cocoa hybrids evaluated in the comparative yield trail (CYT) were considered for the present study. Morphological characterization of the hybrids were carried out based on quantitative and qualitative characters. Thirteen pod characters, twelve floral characters, six bean characters and flush colour of leaves were studied. Except colour of petal and number of ridges and furrows, all other characters expressed high variability among the hybrids. Hybrids were also evaluated based on biochemical properties of beans including fat content, total polyphenol content and total antioxidant activity. Hybrids exhibited significant difference for biochemical characters. More over phenol content showed significant correlation with antioxidant activity.

Based on biochemical and economic characters hybrids were scored and ranked. Six hybrids having top rank were selected as superior genotypes and they are PII 12.11, PIV 19.9, VSDI 33.4, VSDI 11.11, VSDI 29.9 and PIV 59.8.

II The twenty cocoa hybrids included in the study were screened for Phytophthora resistance by artificially inoculating Phytophthora culture on detached cocoa pods. Based on the disease resistance reaction, the hybrids were classified using the score chart. Hybrids exhibited differential response towards Phytophthora pod rot resistance screening. Hybrids PIV 59.8, VSDI 10.13, VSDI 11.11 and PIV 31.9 were found to be highly resistant whereas, hybrids PII 12.11, SIV 5.15, SIV 1.6, PIV 26.8 and VSDI 29.9 exhibited resistance towards Phytophthora. Correlation studies between Phytophthora pod rot resistance and pod husk biochemical properties revealed that pod husk phenol and pod wax content are positively correlated to Phytophthora resistance.

Artificial screening for TMB resistance was carried out on budded plant and detached pods to study the reaction of different hybrids against Helopeltis theivora. Budded plants of all the hybrids and medium matured freshly collected pods of selected six hybrids were screened. Based on the number of feeding punctures, hybrids were grouped into different classes following score chart. Hybrids PIV 59.8, PIV 60.9, PII 12.11, VSDI 33.4 and PIV 56.9 with average number of feeding punctures less than three on plants were included in highly resistant class. When cocoa pods of six hybrids were screened for TMB resistance, PII 12.11 and VSDI 33.4 had the least number of feeding punctures after 72 hours of feeding by TMB.

Among the six hybrids selected based on economic and biochemical performance, five hybrids i.e. PII 12.11, VSDI 33.4, VSDI 29.9, VSDI 11.11 and PIV 59.8 exhibited highly resistant to slightly resistant reaction towards both the biotic stress considered.