Genetic research on cocoa diseases at CATIE (1960-1990)
Disease resistance studies carried out at CATIE till 1990 are reported here. Techniques developed in Ecuador were used in the inoculation system for moniliasis. Several clones became less infected than the majority; planting of this material will show if it is also more resistant to natural infection in the field. Studies to investigate how moniliasis penetrates the pod in field trials were also carried out. Ceratocystis wilt is economically important in those countries with highly flavoured cocoa, whose genetic material is susceptible to this disease. Heritability of natural disease resistance was studied in cocoa hybrids planted in 1966 in a 6 x 6 simple square lattice to form a half diallel. The variance studies showed 38 percent additive, 31 percent dominant, and 31 percent environmental effects. Clone IMC-67 had the higuest positive general combining ability (GCA) and ICS-45 the most negative. IMC-67 and SPA9 probably have dominant genes, and ICS-45 had a heterozygous dominant gene for susceptibility. Another conclusion was that the cocoa cortex test for Ceratocystis resistance showed only part of the total resistance of the same material in the field. The inoculation response of most of the clones changed with time (20 years), and clones which were resistant in one test were found to be susceptible when inoculated with a different culture of the same pathogen at a later time. Various methods for inoculating the black pod pathogen in different organs have been developed, and most of the Costa Rican collection has been evaluated with different methodologies. Some of the clones were classified as resistant and others as having low resistance. Preliminary studies based on field observations suggested that there were few dominant genes. Genetic studies demonstrated the action of several genes with more additive than dominant action. All progeny of SCA-6 was resistant. In other inoculation studies "beginning of the injury" susceptibility was shown to be highly heritable with genes of additive action, and in some cases with modifier genes. Pound-7 had the largest GCA. An experiment with artificial inoculation, planted in 1962 at La Lola and sbmitted to a 3 x 3 diallel analysis, demonstrated that dominant genes were more important than additive ones and that some cultivars has epistatic effects on the progeny. Cultivar UF-273 was shown to have the largest GCA, demonstrating that resistance is controlled by more than one pair of genes. Any future breeding program will have to take this complexity into account. An experiment was planted in 1962 at La Lola to investigate resistance to cushion gall using different numbers of crosses of UF-resistant clones. From this experiment, a 3 x 3 diallel for genetic studies was analyzed with UF-29, UF-242 and UF-273 clones. Results showed a Va of 2.3552 and a Vd of 10.2541, demonstrating that, as in Phytophthora pod rot, dominant genes are more important that additive ones, and that some cultivars showed epistatic effects on the progeny. GCAs for the clones were 0.2253, 0.5453 and 0.7706 respectively. Cultivar UF-273 proved to be the best for transmission of resistance to its progeny. Results indicated that resistance is controlled by more than one gene
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| Format: | biblioteca |
| Published: |
Reading (RU) INGENIC
1999
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| Subjects: | THEOBROMA CACAO, RESISTENCIA A LA ENFERMEDAD, ENFERMEDADES DE LAS PLANTAS, FITOMEJORAMIENTO, MONILIOPHTHORA RORERI, CERATOCYSTIS FIMBRIATA, PHYTOPHTHORA PALMIVORA, BUBA DE PUNTOS VERDES, CALONECTRIA RIGIDIUSCULA, FUSARIUM DECEMCELLULARE, INVESTIGACION, CATIE, |
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