Sources of Resistance to Emphasis Type="Italic"P

Authors: A D Iwaro D R Butler A B Eskes

Publish Date: 2005/11/05

Volume: 53, Issue: 1, Pages: 99-109

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Abstract

The International Cocoa Genebank Trinidad ICGT contains about 2000 cacao Theobroma cacao L accessions The purpose of the collection is to provide a source of genes for the genetic improvement of cacao through breeding In fulfilling this objective it is imperative that this collection be evaluated to identify major sources of genes for the genetic improvement of important agronomic–economic traits In this study 816 cacao accessions were evaluated for resistance to Phytophthora pod rot black pod disease which causes enormous loss of yield in cocoa production throughout the worldThe study identifies promising resistant genotypes that could be exploited in cacao breeding programmes It further compares the levels of resistance to Phytophthora pod rot among two cacao types wild and cultivated types three major groups Forastero Trinitario and Refractario and 11 accession groups B AM CL ICS IMC JA LP MOQ NA PA and TRD The distribution of scores for the 816 accessions showed skewness towards the susceptible end of the disease rating scale indicating that a large proportion 689 of the sample was susceptible disease rating 6–8 to Phytophthora pod rot However 129 of the sample population was found to be resistant disease rating 1–3 and 182 moderately resistant disease rating 4–5 Significant differences were observed between the wild and cultivated accessions and among the Forastero Refractario and Trinitario groups Marked differences were also observed among 11 accession groups each of which was represented by at least 20 genotypes Higher proportions of resistant 177 and moderately resistant 226 genotypes were observed in the wild accessions than in the cultivated varieties 94 resistant and 144 moderately resistant The Forastero group consisting of many wild accessions was found to contain more resistant 180 and moderately resistant 231 genotypes than either the Trinitario 48 resistant and 136 moderately resistant or Refractario 113 resistant and 154 moderately resistant Among 11 accession groups the largest percentage of resistant 242 and moderately resistant 288 genotypes were found in the PA Forastero Different accession groups had varying proportions of resistant and moderately resistant genotypes This is not unexpected in an outbreeding crop such as cacao with a high level of heterogeneity This finding reinforces the idea of a prebreeding programme germplasm enhancement to accumulate resistance genes over several populations as a strategy for improving the genetic base of resistance in national cacao breeding programmes One hundred and five promising resistant genotypes were identified among the 816 accessions evaluated These accessions are potential sources of resistance genes for the development of highyielding resistant varieties in cacao

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