Resistance to Black Pod Disease in a Segregating Cacao Tree Population

Black pod disease caused by Phytophthora spp. is the most important cacao tree disease worldwide. Genetic resistance has received much attention for the reduction of economic and environmental costs. In the present work, leaf disks from 282 individuals [F1(TSH-1188xCCN-51)] were inoculated using suspensions of 3 x 10(5) zoospores of Phytophthora palmivora and Phytophthora citrophthora. Twenty disks of each genotype were distributed in four blocks with five disks per plot. After 5 days, each inoculated leaf disk was assessed for the infection level. It was verified statistically that the progeny's parents were intermediate between the control clones Catongo (susceptible) and Sca-6 (resistant). A highly positive linear relationship was observed between infection levels caused by both Phytophthora species in the F-1 progeny (r = 0.9935**). However, the interaction was highly significant. It was also observed that the curve distribution is skewed towards higher resistance. This suggests that few genes are involved in controlling the resistance to both species, and that they maybe linked and species-specific. Although the parents did not differ statistically, the F-1 progeny segregated for resistance to each Phytophthora species and it is therefore an adequate population for breeding for resistance to black pod.