Résumé
Phytophthora megakarya is the most virulent Phytophthora species reported on cacao (Theobroma cacao) in Africa. Previous studies have shown that it disperses mainly through rain splash from soil to pod where infection occurs. However, this mechanism takes place in already infected cacao plantations. How P. megakarya arrives in disease free plantations and what determines subsequent successful establishment are largely unknown. Disease monitoring with molecular tools can help to better understand dispersal mechanisms. The objective of this work is to identify the introduction pathways of P. megakarya in young cocoa plantations that could help predict and prevent further spread. This study was carried out in Central-Cameroon on four cacao plantations, located in two distinct agro-ecological regions and established in 2006 on lands free of primary inoculum. These plantations were monitored on a weekly basis, from 2009 to 2016, for the presence of P. megakarya. As soon as first infections occurred, we started to collect P. megakarya in the field and from the surrounding environment. A total of 182 P. megakarya strains were isolated and genotyped using 14 polymorphic SSR markers. Results indicate that disease incidence was relatively low from 2009 to 2016 and restricted to areas most conducive for disease development. The sampled P. megakarya populations showed limited genetic diversity. Thirty Multilocus Genotypes were obtained for all habitats but just one was constant over the years. Based on the spatial disease pattern observed in field and the occurrence of MLGs, it appears that the single constant MLG is the founder genotype which could be the main responsible for disease spread. The number of genotypes shared between the studied plantation and its surrounding environment suggests that inoculum originates primarily from neighboring cocoa plantations. Run-off water seems to be an important dispersal mechanism. The implications of these findings for P. megakarya control are discussed.