Cassava Witches’ Broom Disease in Southeast Asia: A review of its distribution and associated symptoms

Cassava witches' broom disease (CWBD) is one of the main diseases of cassava in Southeast Asia (SEA). Affected cassava plants show reduced internodal length and proliferation of leaves (phyllody) in the middle and top part of the plant, which results in reduced root yields of 50% or more. It is thought to be caused by phytoplasma; however, despite its widespread distribution in SEA still little is known about CWBD pathology. The overarching goal of this study was to review and corroborate published information on CWBD biology and epidemiology considering recent field observations. We report the following: (1) CWBD symptoms are conserved and persistent in SEA and are distinct from what has been reported as witches' broom in Argentina and Brazil. (2) In comparison with cassava mosaic disease, another major disease of cassava in SEA, symptoms of CWBD develop later. (3) Phytoplasma detected in CWBD-affected plants belong to different ribosomal groups and there is no association study available indicating phytoplasma as the causing agent of CWBD. These findings are essential clues for designing surveillance and management strategies and for future studies to better understand the biology, tissue localization and spatial spread of CWBD in SEA and other potential risk areas

Occurrence of CWBD in 3 countries in Southeast Asia

The monitoring was conducted during 2020 in the main cassava producing areas in the countries of Laos, Cambodia, and Vietnam. The monitoring protocol was defined by CIAT's Virology and Crop Protection group and is part of the project "Sustainable cassava disease solution in Southeast Asia". A sample size of 60 photographs per hectare was defined following an X or Z pattern across the field, a photograph was taken of each plant and 500-1000 milligrams of young leaves were collected on paper towels, the tissue was stored in hermetically sealed plastic bags with silica gel. Data was uploaded to the PestDisPlace platform where visual recognition of CWBD symptoms was identified for each photograph

SLCMV and CMD-field survey dataset using a standard BSP for Cambodia, Laos and Vietnam

Standard surveillance and diagnostic protocols need to be evaluated and implemented at regional level, for early warning and coordination of rapid responses among neighbor countries. This dataset provides a description of 2020 surveillance data for cassava mosaic disease (CMD) in Laos, East Cambodia and Northern Vietnam, indicating location of the surveyed fields, and presence/absence of visual symptoms. The image data was accessed and validated through the PestDisPlace platform by experts working with CMD. All data was collected using standard protocols (e.g. 60 observations per Hectare). Detailed information is available upon request

Ceratobasidium sp. is associated with cassava witches’ broom disease, a re-emerging threat to cassava cultivation in Southeast Asia

Abstract Cassava witches' broom disease (CWBD) is a devastating disease of cassava in Southeast Asia (SEA), of unknown etiology. Affected plants show reduced internodal length, proliferation of leaves and weakening of stems. This results in poor germination of infected stem cuttings (i.e., planting material) and significant reductions in fresh root yields and starch content, causing economic losses for farmers and processors. Using a metagenomic approach, we identified a fungus belonging to the Ceratobasidium genus, sharing more than 98.3–99.7% nucleotide identity at the Internal Transcribed Spacer (ITS), with Ceratobasidium theobromae a pathogen causing similar symptoms in cacao. Microscopy analysis confirmed the identity of the fungus and specific designed PCR tests readily showed (1) Ceratobasidium sp. of cassava is strongly associated with CWBD symptoms, (2) the fungus is present in diseased samples collected since the first recorded CWBD outbreaks in SEA and (3) the fungus is transmissible by grafting. No phytoplasma sequences were detected in diseased plants. Current disease management efforts include adjustment of quarantine protocols and guarantee the production and distribution of Ceratobasidium-free planting material. Implications of related Ceratobasidium fungi, infecting cassava, and cacao in SEA and in other potential risk areas are discussed