The "Guiana" genetic group: A new source of resistance to cacao (Theobroma cacao L.) black pod rot caused by Phytophthora capsici

Abstract Black pod rot, caused by Stramenopiles of the genus Phytophthora , leads to serious production losses in all cocoa growing zones. In order to reduce the impact of these pests, preference is given to genetic control using resistant varieties, and sources of resistance are actively being sought, particularly in wild cacao trees. Surveys were undertaken in the natural cacao tree populations of south-eastern French Guiana between 1985 and 1995 and an abundant amount of plant material belonging to a particular genetic group, the “Guiana” group, was collected. A great deal of work has shown the merits of this genetic group as a source of resistance to Phytophthora palmivora and megakarya . We describe here the results of a global study to assess the resistance of the 186 clones in the “Guiana” group “core collection” to a Guianese strain of Phytophthora capsici (strain Reg 2-6). This study, which used an efficient methodology (fifteen series of tests on leaf discs and a statistical test adapted to the ordinal nature of the basic data), showed that the “Guiana” genetic group is a major source of resistance to P. capsici. Strain Reg 2-6 proves to be particularly virulent, as the Scavina 6 control, an international reference for resistance to Phytophthora , is not resistant to it. However, 24 clones of the “Guiana” group are, and 92 have proved to be more resistant than Scavina 6, thereby showing the interest of the group in genetically controlling P. capsici . Thus, of the clones in the Guiana group that are more resistant to P. capsici than Scavina 6, some, which are also resistant to P. palmivora and/or Phytophthora megakarya , and also displaying some other notable qualities, could be incorporated into cocoa genetic improvement programmes in countries where P. capsici is rife on cacao trees.