Leaf disc method for screening Ceratocystis wilt resistance in cacao

Aiming to evaluate an indirect screening approach for Ceratocystis wilt resistance, the leaf disc (LD) method, two assays were conducted to assess its efficiency in discriminating resistant/susceptible genotypes. The first one used five and the second 16 cacao clones. For each assay, 40 discs of 2-month-old leaves were cut, wounded at the midrib, laid down in wet sterilized foam and inoculated (20 μL - 3 × 104 CFU/mL suspension of Ceratocystis cacaofunesta spores). The number of perithecia formed per leaf disc was assessed 4 days after inoculation (DAI). To assess the correlation between the LD method and field resistance, 16 branches of the same 16 clones were randomly selected and inoculated (30 μL/3 × 104 CFU/mL). The lesion length, width and area were measured 60 DAI. In both assays it was possible to distinguish resistant from susceptible clones. The clonal heritabilities were high (h2clo ≥ 77.19) for the assays in the lab and in the field, the highest estimate being obtained for perithecia number (h2clo = 95.82). The correlation of perithecia number with lesion width was positive and highly significant (r = 0.65, p-value = 0.009). Therefore, high genetic gains on field resistance can be achieved by indirectly selecting through the LD method.