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Durability of Field Resistance to Black Pod Disease of Cacao in Papua New Guinea

Black pod disease of cacao, caused by various species of Phytophthora, is one of the most serious causes of cacao yield losses worldwide. In Papua New Guinea, such losses on local smallholdings are estimated at more than 40% of annual production. Genetic improvement of cacao resistance is the most promising way of reducing the impact of these pathogens. Field infection of 25 selected Trinitario × Upper Amazonian hybrid progenies in a factorial design (5 by 5) was monitored for five consecutive years at the Cocoa and Coconut Research Institute in Papua New Guinea. The resistance durability of the various hybrids was analyzed using a factorial split-plot model with years as the main factor. The reaction of the hybrid progenies remained stable and similar throughout the 5 years. Similarly, the general combining ability ranking of the parents was also the same for each year. Moreover, data accumulated over several years provided a clearer distinction between the parents. The analyses clearly showed female inheritance under field conditions, and the progenies from what are believed to be resistant females showed good resistance.