Cacao resistance to Phytophthora: Effect of pathogen species, inoculation depths and pod maturity

Abstract

Two species of Phytophthora (P. palmivora and P. capsici) and inoculations at two depths (3 mm and 9 mm) were tested each on 10 clones of Theobroma cacao to determine their effects on pod resistance. Ripe and unripe pods were also assessed to determine the influence of physiological status of the pod on the expression of resistance. The two pathogens tested (P. palmivora and P. capsici) differed significantly in their reactions on pods, with P. palmivora being more aggressive than P. capsici. However, the lack of interaction between clones and pathogen species and the similarity in the ranking of clones based on lesion size suggested that selection for resistant clones can be based on one of the two pathogens, preferably the more aggressive one. Pod reactions differed between inoculation depths (3 mm and 9 mm), and between pod maturity stages (ripe and unripe pods) with relatively larger lesions being recorded at 9 mm depth and on unripe pods as compared to those observed at 3 mm depth and on unripe pods, respectively. The magnitude of increase in lesion sizes, however, varied with genotypes, indicating that inoculation depth and pod maturity stage should be standardized in screening cacao germplasm for resistance to Phytophthora.