From coconut to cassava: the coconut lethal yellowing phytoplasma is worsening the threat to food security in Côte d’Ivoire

Background. Cassava (Manihot esculenta Crantz) is a major staple food in the developing countries whose total world production reached approximately 278 million metric tonnes in 2017. Côte d'Ivoire produces around three million tonnes of cassava every year and reached 4.54 metric tonnes in 2017. It is typically consumed as 'attieké', which is currently exported on regional and international markets. Cassava crop is now threaten by the Côte d'Ivoire lethal yellowing disease (CILY) first reported and associated with a phytoplasma in Grand-Lahou in 2013. CILY destroyed over 400 ha of coconut groves in smallholder coconut farms where women farmers started planting cassava as an alternative food and cash crop in coconut lands devastated by the disease. Methods. Symptoms of leaf mosaic, curling and yellowing were observed in cassava orchards intwo coconut-growing villages located in the south coastal littoral of Grand-Lahou. Leaf samples were collected from symptom-bearing and symptomless cassava plants and subjected to total DNA extraction. PCR with phytoplasma universal 16S rRNA primers, and group-specific primers for subgroup 16SrXXII-B, ‘Candidatus Phytoplasma palmicola’-related strains; and with specific primers for African/Eastern cassava mosaic viruses (ACMV, EACMV). Amplicons were purified, cloned and sequenced. Sequences were compared to those of reference in NCBI (https://www.ncbi.nlm.nih.gov) and used for phylogeny analysis of phytoplasma and virus strains, respectively. Results. Phytoplasma DNA was amplified from six out of 12 symptom-bearing samples, five of which co-amplified virus DNA. Phytoplasma sequences showed 99% identity to those of 16SrXXII-B phytoplasmas as confirmed through phylogeny analysis. One cassava plant was co-infected with ACMV, closely related to the Angola strain, while the other four showed co-infection with both the ACMV (Angola) and an EACMV strain from Madagascar. All cassava varieties were phytoplasma-begomovirus co-infected, except the Yacé variety. Conclusions. Cassava plants in Grand-Lahou orchards were found infected by CILY phytoplasma (group 16SrXXII-B) and ACMV/EACMV virus strains. Results indicate that cassava may be an alternative host for the CILY phytoplasma, which may play a role spreading and worsening CILY epidemic. Prompt actions are required while waiting for a suitable resistant coconut cultivar. Short-term solutions may include replanting cassava yards with newly developed cassava varieties that enhance plant resilience against the coconut phytoplasma and ACMV/EACMV viruses to help supporting food production and improve livelihoods of smallholder coconut farmers in Grand-Lahou.