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Introduction

Cassava brown streak virus disease (CBSD) is a damaging disease of cassava plants, and is especially troublesome in Eastern Africa. It was first identified in 1936 in Tanzania, and has spread to other coastal areas of East Africa, from Kenya to Mozambique. Recently, it was found that two distinct viruses are responsible for the disease: cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). Both belong to the genus Ipomovirus in the family Potyviridae ((+)ss RNA), and produce generally similar symptoms in infected plants (Vanderschuren et al, 2012 AND Legg, 2011, and Winter 2010). Root rot renders the cassava tuber inedible, resulting in severe loss of economic value; therefore, current research focuses on achieving cultivars that do not develop the necrotic rot (Legg et al, 2011).

Symptoms

CBSD is characterized by severe chlorosis and necrosis on infected leaves, giving them a yellowish, mottled appearance (Winter et al, 2010). Chlorosis may be associated with the veins, spanning from the mid vein, secondary and tertiary veins, or rather in blotches unconnected to veins. Leaf symptoms vary greatly depending on a variety of factors. The growing conditions (i.e. altitude, rainfall quantity) plant age, and the virus species account for these differences (Mohammad et al, 2011). Brown streaks may appear on the stems of the cassava plant and along the seed clusters. Finally, a dry brown-black necrotic rot of the cassava tuber exists, which may progress from a small lesion to the whole root. Typically the affected plants do not possess all of these characteristics, but those that are severely affected may (Legg et al).

Vector and Disease Cycle

After a period of ambiguity among researchers, the consensus is that the most likely candidate of CBSD vector is B. tabaci , (specifically the B biotype) the silverleaf whitefly (Alicai et al, 2006, Mware et al 2009, Legg, 2010). It is also sometimes referred to as Bemisia argentifolii (Jiu et al, AC belotti). This whitefly species is also considered to be the vector of the cassava mosaic virus (CMV). It is suggested, however, that B. tabaci whiteflies transmit CBSVs less effectively than CMVs (Legg et al, 2010). Also, the CBSD retention period in B. tabaci may not exceed more than 24 h, but more research is needed to confirm this (Legg, 2011). The adult B. tabaci lives an average of sixteen days, and the maturation process from egg to adult is thirty days (hoddle, mark). Both immature and adult whiteflies feed on the phloem of the leaves by inserting a sucker mouthpart into the leaf, thereby transmitting the virus to the plant (AC Belotti, Hoddle, Mark (bio control site). Eggs may be deposited haphazardly or in a spiral fashion on the leaf undersides.

Resistance

Treatment/Control Strategies

Economic Importance

References

1. Pheneas Ntawuruhunga and James Legg (May 2007). "New Spread of Cassava Brown Streak Virus Disease and its Implications for the Movement of Cassava Germplasm in the East and Central African Region" (PDF).
2. Mohammed, I. U. (NaN undefined NaN). "The Symptom and Genetic Diversity of Cassava Brown Streak Viruses Infecting Cassava in East Africa". Advances in Virology. 2012: 1–10. doi:10.1155/2012/795697. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: unflagged free DOI (link)
3. Mbanzibwa, D. R. (NaN undefined NaN). "Cassava Brown Streak Virus (Potyviridae) Encodes a Putative Maf/HAM1 Pyrophosphatase Implicated in Reduction of Mutations and a P1 Proteinase That Suppresses RNA Silencing but Contains No HC-Pro". Journal of Virology. 83 (13): 6934–6940. doi:10.1128/JVI.00537-09. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
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5. Wydra, Kerstin. "Occurrence of cassava diseases in relation to environmental, agronomic and plant characteristics". Agriculture, Ecosystems & Environment. 93 (1–3): 211–226. doi:10.1016/S0167-8809(01)00349-8. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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• Alicai, T. "Re-emergence of Cassava Brown Streak Disease in Uganda". Plant Disease. 91 (1): 24–29. doi:10.1094/PD-91-0024. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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