Rapid screening of cacao genotypes for field resistance to Phytophthora palmivora using leaves, twigs and roots

Black pod, caused by Phytophthora spp. is one of the most important diseases of cacao occurring worldwide. Losses due to black pod caused by P. palmivora are still moderate in Cote d'Ivoire but P. megakarya causes high losses in Ghana and other Central African countries. Variation in field attack has been observed between cacao genotypes, but evaluation of pod losses is unsuitable for obtaining rapid progress in breeding. Results of inoculation tests using young detached leaves, twigs and roots, obtained from field and nursery plants, are presented here and compared to field resistance of similar genotypes observed over a 10-year period. Nine different Upper Amazon Forastero genotypes were tested together with progenies obtained by crossing these with the susceptible check IFC5 (Amelonado genotype). Rank correlations between the early screening tests and the level of field attack were positive and mostly significant (r = 0.58-0.95). The coefficient of correlation was slightly higher for leaves (r = 0.88) and roots (r = 0.89) than for twigs (r = 0.76). Also, resistance of the different plant organs was correlated (r = 0.6-0.9). Resistance of the Upper Amazon parents was well correlated with the resistance of their cross progenies (r = 0.7-0.9), suggesting that resistance is highly heritable. Resistance of leaves and twigs from the nursery was better correlated with field resistance than resistance of leaves and twigs from the field, which might result from more uniform growing conditions in the nursery. Inoculation of leaves appears the most suitable early screening method for black pod resistance. Application of this test in breeding more resistant cacao cultivars is discussed.