Cassava Brown Streak Virus Disease

Taxonomy and distribution
Cassava brown streak virus disease (CBSD) is caused by Cassava brown streak virus (CBSV) from the family Potyviridae and genus Ipomovirus. CBSV particles are sub-microscopic flexuous rods, approximately 750nm in length, which can only be viewed with an electron microscope. The genome comprises a single-stranded particle of RNA, enclosed within a protein coat.

CBSD occurs most commonly along the coastal strip of East Africa, from Kenya in the north to Mozambique to the south (Figure 1). Incidence is greatest at low altitude, and the disease is rarely observed above 800m.a.s.l. CBSD also occurs along the shores of Lake Malawi in both Malawi and Tanzania, and has been reported from some locations in Uganda and Zambia.

 
 
Figure 1. Distribution of cassava mosaic geminiviruses in Africa.

 

Symptoms and economic importance
CBSD symptoms may be expressed in leaves, stems, fruits and tuberous roots. The combination of symptoms expressed varies between varieties although there is an association between the range and severity of aboveground symptoms and the severity of symptoms in tuberous roots. Leaf symptoms are most pronounced in mature leaves and comprise a blotchy yellow chlorosis, in some varieties clearly associated with minor veins (Figure 2).


Figure 2.
Characteristic
chlorotic mosaic
symptoms caused
by CMD.		 There is no distortion of the lamina. Symptoms on stems are most apparent on upper green portions, and consist of dark brown ‘streaks’ with necrotic lesions on leaf scars, in severe cases leading to shoot dieback. Fruits of infected plants may be covered by black blotches unevenly distributed over the pericarp, and tuberous roots may be misshapen, externally fissured and have sepia brown to black dry necrotic lesions in starch storage tissues. Symptoms are most pronounced during the dry season and aboveground symptoms may not be apparent during periods of wet weather. Economic losses result both from damage to the aboveground plant parts associated with dieback and from the spoilage of roots resulting from dry necrotic rot.
 


Total crop loss may occur where susceptible varieties are grown in areas where CBSD is prevalent. The disease is the most economically important constraint to cassava production in southern Tanzania and northern Mozambique although no quantitative assessments have been made of these losses.

Control
The prinicipal approaches to CBSD management include the use of disease-free planting material and resistant varieties. Varieties have been identified in southern Tanzania and Malawi which are either resistant to infection by CBSV or express very mild symptoms which do not have an effect on yield. No information is currently available on mechanisms of resistance, and there has as yet been no major effort to use conventional breeding approaches to develop CBSV-resistant germplasm.

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