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United States Department of Agriculture

Agricultural Research Service

Research Project: West African Phytophthora Disease Resistance Screening Program

Location: Subtropical Horticulture Research

Project Number: 6038-21000-023-05-T
Project Type: Trust Fund Cooperative Agreement

Start Date: Jul 8, 2013
End Date: Jul 7, 2014

The primary goal of this project is to standardize screening methodologies used in West Africa for the selection of new productive, black pod (Phytophthora megakarya) resistant cultivars of cacao. To attain that goal, screening methodologies developed by the USDA-ARS/Centro Agronomico Tropical de Investigacion y Ensenanza (CATIE) joint cacao breeding program, that efficiently facilitate the selection of genotypes with high levels of resistance to black pod, will be used in this project. These techniques are currently being implemented in Central America and have resulted in the selection of black pod resistant cultivars that have been recently released by CATIE. 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 two specific goals: 1. Provide training in artificial inoculation methods on detached pods of Theobroma cacao L. with isolates of Phytophthora megakarya to determine plant specific resistance to infection. a. Isolation of Phytophthora megakarya strains from naturally infected cacao pods and maintenance in culture. b. Selection of uniform pods and standardization of inoculation methods. c. Standardized scoring of black pod disease severity. 2. Support the use of artificial inoculation methods on detached pods of Theobroma cacao L. with various strains of Phytophthora megakarya in several West African countries to determine plant specific resistance to infection and yield loss. a. Provide training in the use of software to measure plant response to pathogen. b. Support and coordinate a network of pathology specialists.

Black Pod disease is the most widespread and destructive disease of cacao, being caused by several species of Phytophthora. Globally, the disease causes yield losses estimated in recent years at 450,000 tons, and farm losses range from 30–90 percent of the crop. In West Africa, two species are responsible for the majority of losses. Phytophthora palmivora, a pan tropical species infecting a wide variety of host plants and Phytophthora megakarya, an aggressive and extremely destructive species that is rapidly spreading through cocoa producing regions. As chemical control is expensive and not entirely effective with additional environmental concerns on the use of chemicals in agricultural production, there is an urgent need to identify plants with effective genetic resistance to the disease. The identification and use of disease-resistant plants will help to reduce future crop losses and secure cocoa production from West Africa. Thus, an effective disease resistance screening method is required for effective and durable resistance to black pod disease, which is dependent on the ability of the screening to mimic the conditions that occur in the farmers’ field. Since cacao is grown over a wide geographical range spanning several countries in West Africa, the use of standardized materials for the inoculation test is an important factor in ensuring uniform and reproducible results from biological tests. Several artificial inoculation techniques are known and practiced in laboratories worldwide. Testing of pods rather than leaves moves the assay to the economically important part of the crop since there is a good correlation between pod tests and observation of disease resistance in the field. Resistance to black pod 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, it is very important that the standardization of assays should be achieved soon since molecular markers are already being used in the cacao breeding program and assays are being developed for marker selection in current breeding populations. These molecular marker assays will be used to pre-select genotypes for field testing, allowing greater efficiency in the identification of superior individuals.

Last Modified: 07/26/2017
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