Wide ranging differences in tolerance to Phytopthora palmivora in four genetic groups of cacao

Authors: FISTER, ANDREW - Pennsylvania State University; LEAMDRO, MARIELA - Catie Tropical Agricultural Research; Zhang, Dapeng; MARDEN, JAMES - Pennsylvania State University; TIFFIN, PETER - University Of Minnesota; DEPAMPHILIS, CLAUDE - Pennsylvania State University; MAXIMOVA, SIELA - Pennsylvania State University; GUILTINAN, MARK - Pennsylvania State University

Submitted to: Tree Genetics and Genomes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2019
Publication Date: 12/5/2019
Citation: Fister, A., Leamdro, M., Zhang, D., Marden, J., Tiffin, P., Depamphilis, C., Maximova, S., Guiltinan, M. 2019. Wide ranging differences in tolerance to Phytopthora palmivora in four genetic groups of cacao. Tree Genetics and Genomes. 16:1. https://doi.org/10.1007/s11295-019-1396-8.
DOI: https://doi.org/10.1007/s11295-019-1396-8

Interpretive Summary: Chocolate is produced from the seeds of the cacao tree. Black pod rot is a plant disease causing serious reductions in yield worldwide. Black pod rot is caused by the plant pathogen Phytophthora. Every year, 20-30% of pre-harvest yield is lost due to the damage of black pod disease. Host resistance is the most efficient and environment friendly approach for management of black pod disease. To introduce novel sources of disease tolerance into breeding programs, host resistance was screened in four genetic groups of cacao maintained in the international cacao collection in Costa Rica. A wide range of tolerance and susceptibility were found within and among four genetic groups. Genetic identity of all tested trees was verified by DNA fingerprinting. These results provide a foundation for future genomic and transcriptomic analysis of disease tolerance and susceptibility and provide guidelines for breeders searching for novel sources of tolerance. The new source of tolerance would limit cacao disease losses, improving the lives of cacao farmers, and contribute to stabilization of cacao supplies available to the chocolate industry. These findings will be used by cacao breeders, extension staff and growers to develop new varieties and crop management practice for cacao production.

Technical Abstract: The tropical tree Theobroma cacao is the source of chocolate and its seeds are a major export from many producing countries in Central and South America, Africa, and Asia. Every year, 30-40% of pre-harvest yield is lost due to disease damage. Host plant resistance is the most efficient and environment friendly approach for disease management [1]. Historically, cacao germplasm resources have been underutilized in efforts to introduce novel sources of disease tolerance into breeding programs. Maintenance of cacao germplasm also relies on clonally propagated live collections, as cacao seeds do not exhibit dormancy and cannot be stored for more than a few weeks. In this study, we use a 90 SNP array to verify genetic identity of a set of clones in the International Cocoa Collection at CATIE, Costa Rica and assign the clones into known genetic groups. We also used a detached leaf inoculation strategy to measure the susceptibility of 60 genotypes to Phytophthora palmivora, a major cacao pathogen with global importance. We identified 24 genotypes with disease tolerance statistically like a standard tolerant variety (SCA6) and another 24 which performed similarly to a standard susceptible variety (ICS1). Our results indicate that each of the four-included genetic show variability for quantitative resistance to P. palmivora. These results provide a foundation for future genomic and transcriptomic analysis of disease tolerance and susceptibility in the field at CATIE and provide guidelines for breeders searching for novel sources of tolerance that can be introduced into breeding programs.