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United States Department of Agriculture

Agricultural Research Service

Research Project: Optimization of Disease Resistance Phenotyping Methodologies in Cacao, Avocado and Mango

Location: Subtropical Horticulture Research

Project Number: 6038-21000-023-08-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Sep 1, 2013
End Date: Aug 31, 2018

The primary goals of this project include the standardization of screening methodologies used in the selection of new, productive, Black Pod (BP; Phytophthora spp.), Witches’ Broom (WB; Monitiophthora perniciosa), Frosty Pod (FP; Monitiophthora roreri), Ceratocystis (Ceratocystis cacaofunesta), and Vascular Streak Dieback (VSD; Oncobasidium theobromae ) resistant cultivars of cacao. Another primary objective is to develop screening methodologies for identification of avocado genotypes (Persea americana Mill) that are tolerant to the laurel wilt disease and Phytophthora root rot (PRR). The last objective is to phenotype mango breeding populations that are segregating for anthracnose resistance. To attain these goals, several disease resistance screening methodologies developed by different research programs need to be standardized; this will efficiently facilitate the selection of genotypes with high levels of disease resistance in cacao, avocado and mango. Research performed in genetic resource evaluation, plant pathology screening techniques, and practical field selection are all part of a global strategy to develop superior planting material for farmers worldwide. The project has four specific goals: 1. Develop standardized methodologies for the evaluation of disease resistance to WB, FP, BP, Ceratocystis, and VSD of a large cacao field breeding program with field trials in eight countries and Hawaii and Puerto Rico in the U.S. 2. Utilize phytopathological screening methodology to identify genotypes with high levels of disease resistance in cacao, avocado and mango from trials with collaborators overseas and from the USDA germplasm collections of tropical/subtropical fruits. 3. Coordinate and assist in the phenotypic data collection process. 4. Work jointly on the development of molecular methods and other cutting-edge genomic technology as additional tools to develop disease diagnostic assays for disease identification.

Cacao production is plagued by very serious losses globally from diseases. BP, WB, FP, VSD and Ceratocystis have produced a significant decrease in cacao production worldwide. Five species of Phytophthora are the cause of BP disease causing severe losses around the world. In the Americas the cacao industry has undergone a severe production decline due to losses principally to BP, WB, and FP. For instance, in Brazil, the cacao industry was devastated by WB in the 1990s. This aggressive disease reduced Bahia’s cacao production from over 405,000 metric tons in 1986 to less than 130,000 metric tons in 1998. FP has caused serious losses in Ecuador, Peru, Bolivia, Colombia, and Central America. It has been estimated that up to 80% of the pods on some farms in Ecuador and Colombia are lost due to FP. WB and FP are not yet found in West Africa or South Asia. VSD is the cause of major production losses in South Asia and has not been found in the Americas yet. Currently, the most widely grown commercial cultivars in West Africa, which produce ~70% of the world’s cocoa, are susceptible to WB and FP. Thus uniform phenotypic disease data are crucial for obtaining precision in the results of marker association studies. If we do not have uniform phenotypic disease data, to use together with the sequence data from two cacao genomes, our marker association project results will lack the necessary precision to accomplish our primary goals which are: the development of cacao cultivars utilizing the new molecular genetic tools to increase the efficiency of the selection process, and to identify quantitative trait loci (QTL) for anthracnose resistance in mango, and resistance to Phytophthora root rot (PRR) and laurel wilt in avocado. Due to the non-uniformity of screening methodologies used, there are currently in the literature conflicting reports regarding the location of QTLs associated with BP. Resistance to BP has been evaluated in the field using percent pod rot and using the leaf disk inoculation method; however, only a weak correlation exists between the two methods of measuring resistance. Furthermore, different QTL were detected with each of the two methods, indicating perhaps different mechanisms of resistance or low repeatability of the assay. Therefore, a standardized, reliable and reproducible assay must be identified soon. Without such an assay, molecular markers that are used currently in the cacao breeding program may be uninormative.

Last Modified: 2/23/2016
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