Genetic mapping of resistance factors to Phytophthora palmivora in cocoa

Phytophthora palmivora causes pod rot, a serious disease on cocoa widesprea d throughout the producing regions. In order to ascertain the genetic deter mination of cocoa resistance to P. palmivora, a study was carried out on tw o progenies derived from crosses between a heterozygous, moderately resista nt Forastero clone, T60/887, and two closely related and highly susceptible Forastero clones, one completely homozygous, IFC2, and one partially heter ozygous, IFC5. The cumulative size of both progenies was 112 individuals. P lants were subjected to natural and artificial inoculation of P. palmivora in Cote d'Ivoire. The genetic maps of T60/887 and of IFC5 were constructed using amplified fragment length polymorphism (AFLP) markers and microsatell ites. The map of T60/887 comprised 198 markers assembled in 11 linkage grou ps and representing a total length of 793 cM. The map of IFC5 comprised 55 AFLP markers that were assembled into six linkage groups for a total length of 244 cM. Ratio of rotten over total number of fruit under natural infect ion was measured for each tree over two harvests. Artificial inoculations w ere performed on leaves and pods. These tests were weakly correlated with t he pod rot rate in the field. Five quantitative trait loci (QTLs) of resist ance were detected for T60/887 but none were common between the three trait s measured. Stability and reliability of the experimental procedures are di scussed and revealed the difficult use of these artificial tests on adult t rees for a good prediction of field resistance.