Evaluation of methods for assessing resistance of cacao Theobroma cacao L. cultivars and Hybrids to Phytophthora palmivora (Butler) Butler. Boletim técnico N. 62

Abstract

This study was conducted to select or develop a rapid and easy but reliable standardized inoculation method for preliminary screening of cacao for resistance to P. palmivora. Previously published methods and various modifications of some of them were examined using the same cacao cultivars and the same P. palmivora isolate throughout. Results from these methods were compared with each other and with data of natural infection in the field. Screening methods examined were: 1) Inoculation of attached pods using zoospore suspension without wounding or mixed mycelial-sporangial suspension with and without wounding, 2) Inoculation of detached pods using zoospore suspension with and without wounding, 3) Inoculation of blocks of pod pericarp tis sue with zoospore suspension, 4) Inoculation with mycelium-agar discs of seedling-stems and branches on the tree, 5) Inoculation of attached leaves with mycelium-agar discs or zoospore suspension, 6) Inoculation of seedling roots with zoospore suspension, 7) Inoculation of germinated seeds with zoospore suspension or mycelial sporangial suspension. Methods varied in the degree with which they differentiated between resistant and susceptible cultivar reactions, and in consistency over replicated tests. However, the rnost resistant cultivars (Pound 7, Scavina 6, Scavina 12 and Catongo) were graded as resistant in most methods. The most reliable screening method for use with fruiting trees was point-inoculation of unwounded attached pods using zoospore suspension and recording percentage infection and lesion diameters. Inoculation of pre-germinated peeled seeds with a drop of zoospore suspension and, following planting, recording percentage emergence of healthy seedlings was the most satisfactory method for screening hybrid progenies.