Genetic variation in resistance to vascular-streak dieback in cocoa (Theobroma cacao)

Summary Genetic variation in disease resistance to vascular-streak dieback (Vsd) was studied in 54 cocoa hybrid progenies from Trinitario x Amazonian crosses. The disease was assessed by the number of Vsd infected branches per tree, total length of infected branches, average depth of disease penetration within branches and percentages of infected and dead plants. Highly significant differences due to general combining ability for both Trinitario female parents and Amazonian male parents were obtained for all characters. Percent of dead plants was the only character where specific combining ability was also important. Gene effects were predominantly additive for most characters, indicating that selection for progenies resistant to Vsd is effective in the present breeding population. There is strong evidence that resistance to Vsd is in the form of horizontal resistance. It is polygenic and largely inherited as additive genes. Therefore, breeding for resistance is an effective means of controlling Vsd in Papua New Guinea.