Testing for the resistance of newly generated hybrid cacao germplasm in the gene pool of Cocoa Research Institute of Nigeria (CRIN) against Phytophthora megakarya pathogen causing black pod disease of cocoa.

Abstract

Black pod disease caused by P. Palmivora and P. megakarya is an important disease affecting cacao in West Africa which produces 70% of the world output. Resistance to the pathogen is therefore, an important factor to be considered when breeding for high yielding and earliness in fruiting. Resistance to the pathogen using Leaf Disc Test (LDT) was conducted at the Plant Pathology Laboratory, Ibadan, on detached 15mm diameter discs of two-month-old leaves obtained from nineteen newly generated cacao hybrids in the gene pool of the Institute. Attached Whole Leaf Test (AWLT) was also conducted on few selected hybrids in the screen house at the same institute. Ten l zoospores suspension of P. megakarya pathogen adjusted to inoculum strength of 3x105 zoospores/ml using haemocytometer was used for the inoculation of the cacao LD. Using 0-5 disease rating scale, data was taken on the 5th and 7th day after inoculation for LDT and on the 6th day after inoculation for AWLT. Data obtained were subjected to Analysis of Variance and significant means were separated using Student Newman Kuels Test at p<0.05. The LDT classified the hybrids into five groups namely: Resistant (hybrid 001, 003 and 005); Moderately Resistant (hybrid 006, 007, 008,013,015,018 and 019) Moderately Susceptible (hybrid 004, 014 and 017); Susceptible (hybrid 002, 010, 011 and 016) and Highly Susceptible (hybrid 009 and 012). Scores from LDT significantly correlated (r= 0.92**) with that of AWLT. It was observed from the study that cacao hybrid 001, 003 and 005 showed traits of resistant and therefore, could be incorporated into the national breeding programs for the development of high-yielding and resistant cacao cultivars. The moderately resistant hybrids could be conserved for future germplasm enhancement program.