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United States Department of Agriculture

Agricultural Research Service


Location: Tropical Crops and Germplasm Research

2009 Annual Report

1a. Objectives (from AD-416)
Acquire, propagate, and conserve tropical/subtropical crop genetic resources and associated information. Apply new or improved horticultural characterization procedures and genetic marker-based approaches for genetic diversity assessment. Apply new or improved procedures for evaluating tropical/subtropical genetic resources for host-plant resistance to high-priority diseases. Distribute tropical/subtropical crop genetic resources and transfer technologies through GRIN database, internet pages, and scientific publications.

1b. Approach (from AD-416)
Acquisition of new germplasm determined by results from the characterization and evaluation research and from consultations with other crop curators, industry representatives, and members of the Tropical Fruit and Nut Crop Germplasm Committee so as to fill genetic gaps in the collections. Priorities for new germplasm include disease resistant and/or productive accessions. Characterization efforts focus on passport, phenotypic and genotypic descriptors, and evaluation data as well as molecular approaches to determine genetic diversity baselines and horticultural identity. Development of in vitro, medium-term storage techniques for Musa sp. to increase efficiency of propagation and conservation, facilitate distribution and reduce risk of pathogen dissemination. Clonal materials in collections are propagated by grafting or from rhizomes and conserved in field collections. Collections are backed up at other sites to prevent germplasm loss from natural disasters. Field evaluations of potentially disease resistant germplasm are carried out for mango, papaya and Musa sp.

3. Progress Report
This is a service oriented project. Distributions during 2008 amounted to a total of 134 distributions of 268 accessions of tropical germplasm in the form of budwood, cuttings, rhizomes, corms, seed and fruit, which were made available to researchers and cooperators at the local, national and international level. The first cropping cycle and evaluation of Musa sp. hybrids for disease resistance to Sigatoka leaf spots has been completed and the evaluation of the second cropping cycle has begun. A disease outbreak of Fusarium wilt also known as Panama disease, which is caused by the soilborne fungus, Fusarium oxysporum f. sp. cubense, has been identified affecting our Musa spp. germplasm field accessions. At the same time we have been re-initiating some of our Musa spp. accessions in tissue culture. Currently, about 90% of the Musa spp. collection is backed up in tissue culture as well as in a greenhouse planting. Plans are to establish a new field collection propagated entirely from tissue culture plantlets before the end of this FY. Cacao research has continued to focus on the field characterization of the current collection. Traits of agronomic importance as well as clearly identifiable, inherited phenotypic traits (such as pod color) that help distinguish accessions continue to be collected. Data collection is midway through a third year of evaluation. The continued refinement of a micro-fermentation process and subsequent organoleptic evaluation (chocolate quality) continues to be carried out in collaboration with Mars, Inc. Approximately 40 cacao accessions, including hybrids from breeding programs as well as South American selections have been successfully propagated (grafted) in our greenhouses. Of these 40 accessions, 90% of the plants have been established in a replicated manner in our germplasm collection. Data for mamey sapote (Pouteria sapota) is being currently collected for the third year/cropping cycle. Traits of agronomic importance such as production and pulp sweetness (Brix) are being measured. Fruit production during this cropping cycle was significantly higher than that from last year due to the trees coming into fruit bearing age. Approximately 1000 seedlings of Spanish lime (Meliccocus bijugatus) have been planted for a second year for the establishment of a germplasm collection that will become part of USDA NPGS in order to complement the Sapindaceae NPGS collection. Seedlings will be utilized as rootstocks for clonal propagation of Spanish lime selections. To date, approximately 20 local selections have been identified and successfully grafted in our greenhouses with at least 6 replicates per selection.

4. Accomplishments
1. Microsatellite Fingerprinting of the USDA-ARS Tropical Agriculture Research Station Cacao (Theobroma cacao L.) Germplasm Collection. Pods from cacao trees are the main source of raw material for the production of chocolate. Most cacao is produced in developing countries; however most of the chocolate industry is in developed countries. A germplasm collection consisting of close to 1000 cacao trees has been maintained at the USDA-ARS Tropical Agriculture Research Station, in Mayaguez, Puerto Rico, for many years. This collection is maintained, characterized and distributed as a source of genetic material for cacao breeding programs and/or for farmers worldwide. A high degree of genetic diversity in the collection is desirable to satisfy the needs of cacao improvement programs. An experiment was conducted with molecular markers on DNA extracted from all of the trees in the collection. The research was carried out in an effort to estimate genetic diversity in the germplasm collection. Results from the study showed that the collection consists of trees with a diverse genetic background that represents the geographic origins of cacao. Nevertheless, gaps in genetic diversity were also identified and new germplasm will be introduced to include underrepresented germplasm.

Review Publications
Irish, B.M., Crespo, A., Goenaga, R.J., Ayala Silva, T., Niedz, R.P. 2009. Ploidy level and genomic composition of the USDA-ARS Tropical Agriculture Research Station Musa sp. Germplasm Collection. Journal of Agriculture of the University of Puerto Rico, 93(1-2):1-22.

Last Modified: 05/24/2017
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