|Schnell Ii, Raymond|
Submitted to: Proceedings of the International Cocoa Producer's Conference
Publication Type: Proceedings
Publication Acceptance Date: 10/20/2003
Publication Date: 10/20/2003
Citation: Johnson, E.S., O'Connor, C.B., Sreenivasan, T.N., Schnell Ii, R.J. 2003. Population structure of the witches' broom pathogen of cacao in trinidad and tobago.. Proceedings of the International Cocoa Producer's Conference. Interpretive Summary: Variability in the witches' broom disease reaction on similar cacao cultivars has been observed in Trinidad and Tobago. Only a single pathotype of the fungus was known to occur on the islands, containing very limited genetic variability. The newly observed variation in the reaction to the pathogen was a cause of concern and lead to the current investigation. The objectives of this research were to asses the genetic diversity present in markers called amplified fragment length polymorphism (AFLP). A total of 73 isolates of the fungus collected throughout the islnads were analyzed using 173 polymorphic fragments. The analysis revealed the existence of two closely related clonal lineages, one large group and a smaller group of isolates. It is clear from the analysis that the large group maintains a certain level of genetic diversity providing the opportunity for strains virulent on the current cultivars to arise.
Technical Abstract: A study on pathogeneic variability of Crinipellis perniciosa in Trinidad conducted in 1987 reported one biotype, B, of the pathogen. However, variation in disease reaction response has been frequently reported by researchers at CRU in cacao accessions used as controls in different witches' broom greenhouse screening trials. In this study amplified fragment length polymorphism (AFLP) analyses were used to reevaluate the genetic diversity of the witches' broom cacao pahtogen in Trinidad and Tobago. Isolates of C. perniciosa were collected from farmers' estates on both islands, from both Trinidad Selected Hybrid (TSH) clones and the original Trinitario cacao. At the 0.9 similarity level, two clusters were revealed from the polymorphism of 173 fragments in 73 isolates using UPGMA cluster analysis with Simple Matching coefficient. The majority or 90% of the isolates were genetically similar while the other cluster was comprised of two closely related groups with 99.8% and 90.2% P values, respectively. The relatively rare groups were not localized to any particular region or estate, but were only presnt in Trinidad on the Trinitario genotypes on older estates. These results support previous findings, in that the majority of isolates are genetically similar. The data also suggests that the C. perniciosa population in Trinidad and Tobago is composed of at least two closely related clonal lineages.