Foliar resistance of cacao (Theobroma cacao) to Phytophthora palmivora as an indicator of pod resistance in the field: interaction of cacao genotype, leaf age and duration of incubation

• Published in: Plant pathology Vol. 55; no. 6; pp. 776 - 782
• Main Authors: Tahi, G.M, Kébé, B.I, Sangare, A, Mondeil, F, Cilas, C, Eskes, A.B
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01.12.2006; Blackwell Publishing Ltd; Blackwell
• Subjects: Agronomy. Soil science and plant productions; bioassays; Biological and medical sciences; black pod; clones; cocoa; correlations; disease resistance; disease severity; early screening; field resistance; Fundamental and applied biological sciences. Psychology; fungal diseases of plants; Fungal plant pathogens; general combining ability; genetic variation; Genetics and breeding of economic plants; genotype; germplasm screening; incubation period; leaf age; leaf‐disc inoculations; leaves; Pathology, epidemiology, host-fungus relationships. Damages, economic importance; Pest resistance; Phytopathology. Animal pests. Plant and forest protection; Phytophthora; Phytophthora palmivora; phytophthora pod rot; plant pathogenic fungi; Plant pathogens; pods; Theobroma cacao; Varietal selection. Specialized plant breeding, plant breeding aims; Correlation; Plant pathogen; Phycomycetes; Artificial selection; Stimulant plant; Phytophthora palmivora; Fungi; Dicotyledones; Angiospermae; Plant tissue disc; Early selection; early screening; Age; Theobroma cacao; Thallophyta; Fruit; Incubation; Interaction; Tropical crop; phytophthora pod rot; Genotype; field resistance; Duration; Plant leaf; correlations; Indicator; Fungus resistance; cocoa; Screening; leaf-disc inoculations; black pod; Sterculiaceae; Vegetative apparatus; Spermatophyta; Nonspecific resistance
• ISSN: 0032-0862; 1365-3059
• DOI: 10.1111/j.1365-3059.2006.01453.x

The effects were studied of four leaf development stages (LDS) and three durations of incubation (DI) on the accuracy of leaf-disc tests on eight cacao (cocoa) clones (C) for predicting field resistance to phytophthora pod rot caused by Phytophthora palmivora. The clones were known to possess different general combining abilities (GCA) for pod resistance in the field, evaluated monthly at harvest over a 9-year period. Disease severity (DS) was affected strongly by C, DI and LDS, with increasing levels of significance. Two- and three-way interaction effects were smaller than the clone effect, but still significant. Clone effects were most significant for LDS3 (i.e. leaves 50-60 days old) and for DI5 and DI7 (observations made 5 and 7 days after inoculation, respectively). Coefficients of rank correlation between DS and field results were significant for seven of the 12 treatments, with highest values obtained again for treatments LDS3/DI5 (r= 0·87) and LSD3/DI7 (r= 0·93). Pooling of data for different LDS and DI treatments did not further improve the correlation with field results. However, these correlations were improved (from an average of 0·74 to 0·88) when the GCA values for field resistance were based on weekly observations, carried out in one year, including losses of pods and cherelles. It was concluded that, when carried out in a standardized manner and under optimal conditions, the leaf-disc test may explain 75-90% of the genetic variation for field resistance of cacao genotypes to P. palmivora.