Association of Rhizoctonia theobromae with cassava witches’ broom outbreak in Brazil and genetic relatedness to Southeast Asian isolates

Background A new cassava disease outbreak was identified in indigenous communities in Oiapoque, Amapá, Brazil, characterized by stunting, proliferation of thin shoots, broom-like leaf formations, and apical dieback. These symptoms are consistent with Cassava Witches’ Broom Disease (CWBD), previously reported in other regions of South America and Asia. Methods Metagenomic profiling, molecular diagnostics, phylogenetic analyses, and multilocus genotyping were used to investigate microbial communities associated with symptomatic cassava plants. Results Rhizoctonia (Ceratobasidium) theobromae was identified as the predominant fungal species associated with symptomatic plants. Genetic analyses indicated a close relationship between Brazilian isolates and Asian reference strains, suggesting a possible transcontinental introduction and supporting an association between R. theobromae and CWBD. This represents the first confirmed report of R. theobromae in Brazil, expanding its known geographic distribution in the Americas. Conclusion The detection of this quarantine pathogen represents a potential threat to cassava production, food security, and preservation of indigenous cassava landraces in Brazil. These findings reinforce the need for surveillance, phytosanitary measures, and further studies on emerging fungal pathogens associated with cassava diseases.