Germplasm enhancement for resistance to black pod disease
A germplasm enhancement programme (GEP) was initiated in 1998 to accumulate resistance genes to black pod disease (Phytophthora pod rot or Ppr) by exploiting the rich genetic diversity in the International Cocoa Genebank, Trinidad (ICG,T). During the 5-year period of this programme, 960 genotypes were screened for resistance to Ppr using the detached pod inoculation method. Using data on Ppr and witches' broom (WB) resistance, and pod index, 136 genotypes were selected as base parents in the GEP. Over a 4-year period, 96 progenies were established, and 3486 seedlings were screened for resistance to Phytophthora palmivora using a leaf disc method. Among these seedlings, 0.2% (6) were found highly resistant, 11.2% (389) resistant, 52.5% (1829) moderately resistant, 35.3% (1231) susceptible and 0.9% (31) highly susceptible. No immunity was observed. The resistant and moderately resistant seedlings constitute 63.7% of the population evaluated. This may be considered unusual in a crop that is highly vulnerable to Phytophthora infection, but it demonstrates the effectiveness of the selection criteria for the base parents, and confirms that resistance to Ppr is heritable. The progeny distributions of most families showed the occurrence of useful transgressive segregates with higher levels of resistance than the parental genotypes. Narrow-sense (h2n) and broad-sense (h2b) heritabilities were estimated at 0.33 and 0.51, respectively, for a subpopulation of 36 progenies. Using the same population, expected genetic gain was estimated at 0.98. This implies that the selected seedling population would have an average genetic resistance level of 2.42 on a 0-5 resistance rating scale, which is more resistant than SCA6 (2.64), a resistant control clone in the experiment. Among the 3486 seedlings screened, 856, consisting of the resistant and moderately resistant seedlings, have been established in the field. These seedlings are being evaluated for vigour, precocity, Ppr and WB resistance. Further selection will be guided by the expression of these traits in the individual plants. A second cycle of crosses is proposed among the promising resistant genotypes arising from the first cycle. The selection criteria adopted in this programme should facilitate effective selection of promising genotypes/populations with enhanced levels of resistance to Ppr. This material will be made available to national breeding programmes and should allow breeders to combine resistance with good yield potential in new cocoa varieties.
| Main Authors: | , , , , |
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| Format: | book_section biblioteca |
| Language: | eng |
| Published: |
CFC
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| Subjects: | F30 - Génétique et amélioration des plantes, H20 - Maladies des plantes, |
| Online Access: | http://agritrop.cirad.fr/537698/ http://agritrop.cirad.fr/537698/1/ID537698.pdf |
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