Analysis of genetic diversity in Theobroma cacao with emphasis on witches' broom disease resistance

To facilitate the identification of Theobroma cacao L. that possess desirable traits to meet changing production and market conditions, there is a need to understand the genetic relationships among T. cacao germplasm. In addition, new cultivars are needed to provide more broadly based resistance to devastating diseases such as witches' broom disease [Crinipellis perniciosa (Stahel) Singer]. A subset of 270 T. cacao accessions based on (i) witches' broom disease resistance data, (ii) genetic characterization experiments, and (iii) a random sampling of recently acquired accessions was selected from the extensive germplasm collection at the Centro de Pesquisa do Cacau (CEPEC; Itabuna, Bahia, Brazil) in collaboration with Fazenda Almirante, a division of M&M Mars Incorporated in Itajuipe, Bahia, Brazil. Estimates of genetic distance based on random amplified polymorphic DNA (RAPD) markers were used to evaluate the genetic diversity among the selected accessions. Incorporation of recently acquired accessions (Accessions 181-270) did not increase the breadth of the distribution of genetic diversity already present within the "original" group sampled (Accessions 1-180) suggesting new accessions collected from already sampled geographic regions do not increase the existing genetic diversity in the germplasm collection. In addition, differences in RAPD marker frequencies were associated with accessions that had a high threshold of tolerance to witches' broom disease. Most accessions exhibiting tolerance to witches' broom disease were from the Upper Amazon region, with the exception of SGU 26, a hybrid from Guatemala. This suggests that the Upper Amazon is not the only region to have genes for resistance to witches' broom disease and stresses the need for further collection and examination of germplasm from other regions.