Effect of cacao black pod rot screening method on disease resistance determination

Black pod rot, caused by several species of Phytophthora, is responsible for greater losses than any other disease affecting cacao. Due to the widespread distribution of this disease, the development of resistant germplasm would benefit farmers around the globe. Breeders use various approaches to screen material for resistance to Phytophthora spp. These include calculating the incidence of infected pods in the field, and evaluating lesion development on pods and leaves following artificial inoculation. Due to significant variability, the method utilized for assessing disease resistance can influence conclusions. To determine how screening methods affect final disease resistance ratings, four cacao clones (BE 10, HY 271419, RIM 15 [MEX], and EET 236 [ECU]) were assessed using disease incidence calculated from field evaluations and lesion area seven days after inoculation (dai) of detached pod. Disease incidence differed significantly among clones (p<0.0007), ranging from 6.1% for BE 10 to 24.0% for HY 271419. RIM 15 [MEX] and EET 236 [ECU] were moderately susceptible with 10.3% and 21.9%, respectively. Differences among clones were also detected based on lesion area (p< 0.032), however, their relative ranking differed: BE 10 (53.9 cm²), HY 271419 (80.64cm²), RIM 15 [MEX] (95.7cm²), and EET 236 [ECU] (102.4cm²). This variation may be due to differences in the resistance component assessed by each method (pre- vs. post-penetration) or the inability to distinguish between diseases causing identical lesions in the field. The improved understanding of the how screening methods used can affect the disease resistance determination in cacao germplasm will benefit breeders and farmers.