Novel sources of witches’ broom resistance (causal agent Moniliophthora perniciosa) from natural populations of Theobroma cacao from the Brazilian Amazon

Witches’ broom is a severe disease of Theobroma cacao L. (cacao), caused by the basidiomycete Moniliophthora perniciosa . The use of resistant cultivars is the ultimate method of control, but there are limited sources of resistance. Further, resistance from the most widely used source (‘Scavina 6’) has been overcome after a few years of deployment. New sources of resistance have been intensively searched for in the Amazon basin. Here, we evaluated for witches’ broom resistance, cacao accessions from various natural cacao populations originally collected in the Brazilian Amazon. Resistance of 43 families was evaluated under nursery and/or field conditions by artificial or natural infection, respectively, based on disease incidence. Screening for resistance by artificial inoculation under nursery conditions appeared to be efficient in identifying these novel resistance sources, confirmed by natural field evaluation over a nine-year period. The increase in natural field infection of ‘Scavina 6’ was clearly demonstrated. Among the evaluated families with the least witches’ broom incidence, there were accessions originally collected from distinct river basins, including the Jamari river (‘CAB 0371’; ‘CAB 0388’; ‘CAB 0392’; and ‘CAB 0410’); Acre (‘CAB 0169’); Javari (‘CAB 0352’); Solimões (‘CAB 0270’); and from the Purus river basin, the two most outstanding resistant accessions, ‘CAB 0208’ and ‘CAB 0214’. The large genetic diversity found in cacao populations occurring at river basins from Acre and Amazonas states, Brazil, increased the chance that the selected resistant accessions would be genetically more dissimilar, and represent distinct sources of resistance to M. perniciosa from ‘Scavina 6’.