Cassava Witches’ Broom Disease in Southeast Asia: A Review of Its Distribution and Associated Symptoms

Abstract

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.

Cut, Root, and Grow: Simplifying Cassava Propagation to Scale

“…The common practice, dissemination of planting materials through an exchange of cuttings between farmers, is associated with the risk of, and, likely, the cause of, cassava brown streak virus disease (CBSD) and cassava mosaic virus disease (CMD) spread in Africa. The introduction and rapid distribution of Sri Lankan cassava mosaic virus (SLCMV) in South-East Asia [ 13 ], with dramatic impact on the national economies of Vietnam, Laos, Cambodia, and Thailand, and the emergence of diseases with unknown aetiologies and cures, cassava witch’s broom disease (CWBD) [ 14 ], cassava frogskin disease (CFSD) [ 15 ], and cassava root necrosis disease (CRND) [ 16 ], provide ample evidence of the constant threat of new diseases that can have severe impact on the production of cassava. Thus, there is an urgent need for a formalized seed system and propagation schemes to furnish farmers with clean planting materials at a low cost.…”

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

“…In these reports, Candidatus phytoplasma asteris and luffae were identified in some of the affected plants. However, the percentage of diseased plants that are positive to phytoplasma (as detected by nested PCR and confirmed by sequencing) is generally not included 4 – 8 and on the contrary high percentages of false positives have been reported 9 . So far, there is no reliable molecular test for CWBD diagnostics.…”

“…The characteristic symptoms of CWBD include dwarfism and proliferation of weak, spindly sprouts on the cassava stems, resulting in the formation of brooms, hence the name. Cassava stems then develop short internodes, and vascular necrosis along the affected parts 3 , 9 . Although such symptoms are identified with phytoplasma infections 10 , 11 , other pathogens can also induce witches’ broom symptoms.…”

“…For example, cacao witches’ broom disease occurring in the Americas, is caused by the basidiomycetous fungus Moniliophthora perniciosa ; it induces a disorganized proliferation of the infected vegetative meristems of axillary shoots, which in time become necrotic, forming a structure named ‘dry broom’ 12 . In fact, an accumulation of dry leaves is commonly observed at the bottom part of the broom in cassava plants with severe symptoms of CWBD 9 . Due to the significant effect that CWBD has on stem development, production of planting material (stem-cuttings) is significantly limited, forcing farmers to acquire stakes from other sources and therefore increasing the risk of introducing and moving around additional pathogens through informal seed exchange networks in the region 13 .…”

“…In addition to a lack of knowledge on the causal agent, management of CWBD in SEA has been hindered by a late development of CWBD symptoms in the field which complicate positive selection 9 , and the presence of co-infecting transboundary diseases such as cassava mosaic disease (CMD) 14 . As with CMD, at low incidence, extension advice has focused on farmers conducting positive and negative selection of harvested stems based on visual symptom, for subsequent replanting within their own fields 15 .…”