Elucidating identity of cassava witches’ broom vectors in Vietnam: A step-wise approach

Cassava witches’ broom (CWB) is a disease caused by 16SrI phytoplasma that is affecting cassava crops throughout Southeast Asia. In early 2014, CWB attained outbreak levels in cassava-growing areas in western Thailand, central Vietnam and throughout Cambodia. The disease brings about sharp reductions in cassava yield and root starch content. With cassava being regionally cultivated on >4 million ha by approx. 8 million small-scale farmers, CWB constitutes a substantial threat to farmers’ livelihoods, food security and rural economies. CWB-causing phytoplasma is thought to be vectored by insects, and more specifically by one or more groups of Hemiptera. At present, a step-wise approach is being adopted in southern Vietnam to identify CWB vectors. Research is carried out to delineate temporal population fluctuations and species composition of Ciccadellidae, Delphacidae, Membracidae, Psyllidae and Cixidae in CWB-infested cassava fields. Species presence and abundance data are then linked to CWB incidence levels on a field basis. Next, potential vectors are identified through nested and Real Time PCR approaches. In a final phase, plant-based and membrane transmission trials are used to reveal which of the different insect species actually transmits CWB phytoplasma from infested to healthy plants. Clarity on the identity of CWB vectors and an in-depth understanding of their population dynamics in local cassava fields constitute most valuable components for a future integrated disease management strategy.