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 disease s of cacao occurring worldwide. Losses due to black pod caused by P. palmiv ora are still moderate in Cote d'Ivoire but P. megakarya causes high losses in Ghana and other Central African countries. Variation in field attack ha s been observed between cacao genotypes, but evaluation of pod losses is un suitable for obtaining rapid progress in breeding. Results of inoculation t ests using young detached leaves, twigs and roots, obtained from field and nursery plants, are presented here and compared to field resistance of simi lar genotypes observed over a 10-year period. Nine different Upper Amazon F orastero genotypes were tested together with progenies obtained by crossing these with the susceptible check IFC5 (Amelonado genotype). Rank correlati ons between the early screening tests and the level of field attack were po sitive and mostly significant (r = 0.58-0.95). The coefficient of correlati on 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 correl ated (r = 0.6-0.9). Resistance of the Upper Amazon parents was well correla ted with the resistance of their cross progenies (r=0.7-0.9), suggesting th at resistance is highly heritable. Resistance of leaves and twigs from the nursery was better correlated with field resistance than resistance of leav es and twigs from the field, which might result from more uniform growing c onditions in the nursery. Inoculation of leaves appears the most suitable e arly screening method for black pod resistance. Application of this test in breeding more resistant cacao cultivars is discussed.