Preliminary evaluation of double cross hybrids for yield and vascular streak dieback (VSD) disease resistance in cocoa (Theobroma cacao L.)

Abstract

Cocoa is highly influenced by the climate change and growing environment, necessitating a long term and dynamic breeding programme. Even though the breeding programmes primarily focus on the development of high yielding varieties, outbreak of new pests and pathogens shift the priority to the development of resistant varieties. Vascular Streak Dieback disease (VSD) caused by Ceratobasidium theobromae (Samuels et al., 2012), pose a great threat to cocoa crop, causing complete defoliation and eventual death (Abraham et al., 2002). Even the high volume spray of chemicals was ineffective in disease control (Prior, 2007), and the only way to tackle it is to breed resistant varieties. Resistance breeding may result in yield reduction (Xu et al., 2017) however, breeding for double cross hybrids can overcome this situation (Gallais and Guy, 1971). Average yield superiority of the double cross hybrids over the F1 hybrids has been shown by many scientists (Sriani et al., 2003; Ghanwat et al., 2016). Twenty double cross hybrids, bred for vascular streak dieback disease and planted during 2017, were used for the present study. Morphological characterization of the hybrids was carried out based on the quantitative and qualitative characters. Thirteen pod characters, twelve floral characters, six bean characters and flush colour of the leaves were studied. Except colour of the petal and number of the ridges and furrows, all other characters have expressed high variability among the double cross hybrids. The double cross hybrids have exhibited significant difference for fat and polyphenol content. All the twenty double cross hybrids were screened and scored for the VSD resistance in the field condition, using the score chart (Abraham et al., 2000). Based on the disease intensity, they were classified into eleven resistant and nine partially resistant.