Identifying the insect culprit behind a regional cassava disease pandemic

In Southeast (SE) Asia, cassava (Manihot esculenta Crantz) is a prominent staple, bio-energy and industrial crop grown on approx. 4 million ha. While local cassava crops have historically been free of limiting biotic constraints, their production is now threatened by a number of invasive pests and diseases. Cassava witches’ broom (CWB), a systemic and debilitating disease, has spread fast throughout the region and is currently reported from approx. 60% local cassava fields.  Capable of causing yield reductions of 30-35%, CWB is thought to be caused by phytoplasma and vectored by one or more species of phloem-feeding leafhoppers, planthoppers or psyllids. In this study, we document the community of sap-feeding insects that inhabits SE Asian cassava fields, and shed light upon the identity of insect vectors of CWB. More specifically, we conducted season-long nightly light trapping in 10 fields and late-season trapping in 10 heavily-infested fields in southern Vietnam. Furthermore, we collected live individuals from CWB-affected fields and conducted laboratory-based membrane-feeding assays, to assess their phytoplasma vectoring ability. A speciose community of sap-feeding insects was documented, composed of 73 morpho-species and primarily consisting of Cicadellidae and Delphacidae. PCR assays showed that nine morpho-species retained phytoplasma in whole insect bodies. Next, transmission trials showed that  four morpho-species of Cicadellidae and two delphacids readily transmit CWB phytoplasma to 17 Tris-EDTA (TE) buffer solutions. These candidate vector species were observed at differing abundance in local fields, with two of the ciccadellids reaching significantly higher abundance in CWB-infected fields than phytoplasma-free plots. While in planta transmission work is still required to further confirm CWB vector identity, our findings are an important contribution to ongoing multi-country efforts to develop sustainable, environmentally-sound management tactics for this fast-spreading plant disease.