Foliar resistance of cacao (Theobroma cacao) to Phytophthora palmivora as an indicator of pod resistance in the field: the effect of light intensity and time of day of leaf collection

Resistance of cacao leaves to Phytophthora palmivora was studied with regard to the time of leaf collection (morning, afternoon) and the degree of exposure of the leaves to light in the field (low, medium and high). The efficiency of leaf disc inoculations in predicting field resistance of nine clones was compared with that of detached and attached pod inoculations. Significant effects were observed, with leaves exposed to high light intensity and collected early in the afternoon showing highest susceptibility. The effect of time of leaf collection was reduced when leaves were stored overnight and leaf discs prepared and inoculated the following day, as compared to inoculations on the day of collection. Interactions between the main factors were significant, though less substantial than the clone effects. The most significant correlations with pod resistance (r = 0.70 to 0.97) were obtained for leaves collected early in the morning and exposed to intermediate shade conditions in the canopy. For other treatments, the correlations with pod resistance were still positive (r = 0.23 to 0.83) but often not significant. Pod inoculations in the laboratory were better correlated with field resistance (r = 0.92) than pod inoculations in the field (r = 0.72). Detached pod inoculations were also better correlated with leaf disc inoculations than those of attached pods. The results confirm the validity of laboratory inoculations of leaves and pods to assess field resistance to Phytophthora. Standardization of the leaf disc test is essential to obtain reliable results. (Resume d'auteur)