Validity of a leaf inoculation test to assess the resistance to Phytophthora megakarya in a cocoa (Theobroma cacao L.) diallel mating design

Cocoa pod rot, caused by different Phytophthora species, is rife in all production zones. Phytophthora spp. control is therefore a major challenge for world cocoa cultivation, and selection of resistant material is a priority research theme for many producing countries. Various genetic trials have been set up over many years in zones affected by the disease. In Cameroon, a 6x6 diallel mating design was set up to study heritability of the resistance trait. Field observations led to the classification of parents according to their ability to transmit resistance to their progenies. These field measurements were long and laborious, so a resistance test based on artificial leaf inoculations was developed. This article studies the correspondence between results obtained with the test and observations of rot rates in the field. There was a significant genetic correlation between pod rot rate in the field and leaf test data, so using this resistance test could shorten cocoa tree selection cycles. This revised version was published online in July 2006 with corrections to the Cover Date.