The genetic and symptom diversity of six virus isolates causing cassava brown streak disease (CBSD) in the endemic (Kenya, Mozambique, and Tanzania) and the recently affected epidemic areas (Uganda) of eastern Africa was studied. Five cassava varieties; Albert, Colombian, Ebwanateraka, TMS60444 (all susceptible) and Kiroba (tolerant) were graft inoculated with each isolate. Based on a number of parameters including the severity of leaf and root symptoms, and the extent of virus transmission by grafting, the viruses were classified as either severe or relatively mild. These results were further confirmed by the mechanical inoculation of 13 herbaceous hosts in which the virulent isolates caused plant death in Nicotiana clevelandii and N. benthamiana whereas the milder isolates did not. Phylogenetic analysis of complete coat protein gene sequences of these isolates together with sequences obtained from 14 other field-collected samples from Kenya and Zanzibar, and reference sequences grouped them into two distinct clusters, representing the two species of cassava brown streak viruses. Put together, these results did not suggest the association of a hypervirulent form of the virus with the current CBSD epidemic in Uganda. Identification of the severe and milder isolates, however, has further implications for disease management and quarantine requirements. 1. Introduction Cassava brown streak disease (CBSD) is endemic in areas along the Indian Ocean coast of eastern Africa, from the northeastern border of Kenya across the Tanzanian border down as far as the Zambezi River in Mozambique, and it was widespread around the shore of Lake Malawi. In the endemic areas, CBSD was confined to altitudes below 1,000 metres above sea level [1–3]. More recently, CBSD has been reported at midaltitude levels (1200–1500 meters above sea levels) in Democratic Republic Congo , Uganda , and the Lake zone areas of Tanzania [6, 7], which were not considered to be at risk by the disease previously. This is a serious concern because the disease incidences of up to 100% were recorded , and in sensitive varieties the disease causes rotting of tubers, reducing both the quality and quantity of tubers available for consumption [1, 2, 9]. A moderate infection by CBSD (10–30% damage to root surface area) decreases the market value of cassava tubers drastically by 90%, fetching under US $5 per tonne, as opposed to $55 for fresh healthy cassava root . Severely diseased roots are completely destroyed and unfit for market or family use. Recent estimates indicate that CBSD causes
R. J. Hillocks, M. D. Raya, and J. M. Thresh, “Distribution and symptom expression of Cassava brown streak virus disease in southern Tanzania,” African Journal of Root and Tuber Crops, vol. 3, pp. 57–62, 1999.
S. C. Jeremiah and J. P. Legg, “Cassava brown streak virus disease: farmers’ perspectives on a new outbreak of this disease from the Lake zone of Tanzania,” 2008, http://www.youtube.com/watch?v=nCJdws9CnUw.
J. P. Legg, S. C. Jeremiah, H. M. Obiero et al., “Comparing the regional epidemiology of the cassava mosaic and Cassava brown streak virus pandemics in Africa,” Virus Research, vol. 159, no. 2, pp. 161–170, 2011.
R. J. Hillocks, M. D. Raya, K. Mtunda, and H. Kiozia, “Effects of brown streak virus disease on yield and quality of cassava in Tanzania,” Journal of Phytopathology, vol. 149, no. 7-8, pp. 389–394, 2001.
R. J. Hillocks, M. Raya, and J. M. Thresh, “The association between root necrosis and above-ground symptoms of brown streak virus infection of cassava in southern Tanzania,” International Journal of Pest Management, vol. 42, no. 4, pp. 285–289, 1996.
S. McSween, T. Walker, V. Salegua, and R. Pitoro, “Economic impact on food security of varietal tolerance to cassava brown streak disease in costal Mozambique,” Research Report 1E, Mozambican Institute of Agricultural Research, Mozambique, Research Paper Series, 2006.
W. A. Monger, T. Alicai, J. Ndunguru et al., “The complete genome sequence of the Tanzanian strain of Cassava brown streak virus and comparison with the Ugandan strain sequence,” Archives of Virology, vol. 155, no. 3, pp. 429–433, 2010.
D. R. Mbanzibwa, Y. Tian, S. B. Mukasa, and J. P. T. Valkonen, “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, vol. 83, no. 13, pp. 6934–6940, 2009.
D. R. Mbanzibwa, Y. P. Tian, A. K. Tugume et al., “Genetically distinct strains of Cassava brown streak virus in the Lake Victoria basin and the Indian Ocean coastal area of East Africa,” Archives of Virology, vol. 154, no. 2, pp. 353–359, 2009.
S. Winter, M. Koerbler, B. Stein, A. Pietruszka, M. Paape, and A. Butgereitt, “Analysis of Cassava brown streak viruses reveals the presence of distinct virus species causing cassava brown streak disease in East Africa,” Journal of General Virology, vol. 91, no. 5, pp. 365–376, 2010.
B. Mware, R. Narla, R. Amata, et al., “Efficiency of Cassava brown streak virus transmission by two whitefly species in coastal Kenya,” Journal of General and Molecular Virology, vol. 1, pp. 40–45, 2009.
D. Deng, P. F. Mcgrath, D. J. Robinson, and B. D. Harrison, “Detection and differentiation of whitefly-transmitted geminiviruses in plants and vector insects by the polymerase chain reaction with degenerate primers,” Annals of Applied Biology, vol. 125, no. 2, pp. 327–336, 1994.
B. L. Patil, E. Ogwok, H. Wagaba et al., “RNAi-mediated resistance to diverse isolates belonging to two virus species involved in Cassava brown streak disease,” Molecular Plant Pathology, vol. 12, no. 1, pp. 31–41, 2010.
S. K. Hahn, J. C. G. Isoba, and T. Ikotun, “Resistance breeding in root and tuber crops at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria,” Crop Protection, vol. 8, no. 3, pp. 147–168, 1989.
M. A. Lodhi, G. N. Ye, N. F. Weeden, and B. I. Reisch, “A simple and efficient method for DNA extraction from grapevine cultivars and Vitis species,” Plant Molecular Biology Reporter, vol. 12, no. 1, pp. 6–13, 1994.
M. M. Abarshi, I. U. Mohammed, P. Wasswa et al., “Optimization of diagnostic RT-PCR protocols and sampling procedures for the reliable and cost-effective detection of Cassava brown streak virus,” Journal of Virological Methods, vol. 163, no. 2, pp. 353–359, 2010.