Results and Discussion Conclusion and recommendations EFFECT OF INITIAL FIELD SOURCES OF INFECTION ON THE SPREAD OF CASSAVA BROWN STREAK DISEASE Katono1, K., Alicai2 , T., Baguma2, Y., Edema1, R., Bua2, A., and Omongo2, C.A 1Department of crop science, Makerere University, P. O. Box 7062, Kampala, Uganda 2National Crops Resources Research Institute (NaCRRI), Namulonge, P.O.Box 7084, Kampala, Uganda Abstract Cassava brown streak disease (CBSD) is a major threat to cassava production in Uganda. Since the outbreak of the CBSD epidemic in Uganda in 2004, knowledge on its spread in the field is still limited. In this study, three cassava genotypes with varying levels of tolerance to CBSD were used to evaluate the effect of CBSD inoculum, disease pressure and whitefly vector populations on disease spread in different disease pressure zones in Uganda. Disease incidences (%), apparent infection rate (r), area under disease progress curves were determined and populations of the whitefly vector, Bemisia tabaci monitored on a monthly basis. Starting inoculum level significantly affected CBSD incidence (P = 0.001), AUDPC and r (P = 0.05). At Namulonge and Kamuli, where disease pressure is high and moderate respectively, disease incidence by seven months was high (>75%) in all the treatments. Even in Lira where disease pressure and vector population are low, there was considerable spread with higher initial-field inoculum levels resulting to higher incidence levels which signify the importance of inoculum source in disease spread. The high rate of disease spread in Lira despite low vector numbers shows the importance of the vector in dissemination of the disease and further validates the significance of using infected cutting in the spread of CBSD. Introduction Cassava brown streak disease (CBSD), is a major threat to cassava production in Uganda. It is caused by two viruses; CBSV and UCBSV (Mbazibwa et al., 2009b) that are transmitted by the whitefly vector, Bemisia tabaci (Maruthi et al., 2005). The disease is also spread by propagation of infected cuttings (Legg et al., 2007). However, knowledge on epidemiology of CBSD is limited. This study was designed to understand the relative influence of CBSD inoculum, disease pressure and whitefly vector populations on the spread of disease in Uganda. Materials and methods Cassava genotypes TME 204, MH 97/2961 and I92/0067 were planted at the three sites of varying disease pressure. Infected cuttings of these genotypes were planted randomly among virus free plants to provide four levels of initial inoculum (0, 5, 15, and 30 %). A RCBD with split plot arrangement and three replicates was used. Data on CBSD foliar severity (Figure 1) and incidence was collected on a monthly basis for 12 months while whitefly numbers were counted monthly on the top five fully expanded leaves of a representative shoot on 15 randomly selected plants per plot for 8 months. Figure 2: Disease progress curves of CBSD incidence (%) with varying inoculum levels on the cassava genotype MH97/2961 in three locations of Uganda. Table 1: Apparent infection rates ( r) for CBSD on three cassava genotypes with varying levels of inoculums sources at three locations of Uganda Genotype Inoculum level (%) Location Lira Kamuli Namulonge I92/0067 0% 0.270 0.774 0.700 5% 0.260 0.813 0.795 15% 0.263 0.786 0.801 30% 0.165 0.760 MH97/2961 0.394 0.658 0.336 0.370 0.783 0.512 0.288 0.757 0.713 0.416 0.752 TME 204 0.382 0.779 0.728 0.499 0.196 0.788 0.753 0.12 0.756 0.745 Mean 0.30 0.764 Lsd0.05 0.030 0.010 0.040 Figure 1: CBSD leaf, stem and root symptoms Results and Discussion Starting inoculum level had significant effect on cumulative CBSD foliar incidence (P=0.001), AUDPC and r (P = 0.05) (Figure 2 and Table 1). Disease pressure and genotype also significantly affected AUDPC (P = 0.001) and r (P = 0.001 and P = 0.05 respectively). Disease spread faster in Kamuli compared to Namulonge and Lira (incidence in the 0% plots at 7 months was 93.5, 78.8 and 6.4% respectively; Figure 2). The differences in infection rate is as a result of variation in whitefly vector populations with the highest numbers recorded in Kamuli (140.4) and the least numbers in Lira (9.7). This is also as a result of differences in the prevailing disease pressure. However, the high incidence levels in Lira shows the importance of using infected cuttings in enhancing CBSD spread. and the different infection rates in the different locations further demonstrates the importance of prevailing disease pressure in CBSD spread. Conclusion and recommendations In conclusion, results from the study show that presence of starting inoculum promotes spread of CBSD. High disease pressure, using susceptible genotypes and high whitefly vector populations increase the rate of spread. The difference in disease pressure calls for different control measures i.e. in low pressure areas, sanitation alone is adequate while in high/moderate pressure zone the use of tolerant varieties is a pre-requisite. Acknowledgements MSI- Uganda, under grant No. MSI / WAI / 02/ 08. National Crops Resources Research Institute ( NaCRRI) Makerere university References Legg, J.P and Pheneas, N, 2007. New spread of Cassava Brown Streak Virus Disease and its implications for the movement of Germplasm in the East and Central African Region. Crop Crisis Control Project, May 2007 Maruthi, M.N, Hillocks R.J, Mtunda K, Raya M.D, Muhanna M, Kiozia K, Rekha A.R, Colvin J, Thresh J .M. 2005. Transmission of cassava brown streak virus by Bemisia tabaci (Gennnadius). Journal of Pytopatology 153:307-312. Mbazibwa, D.R; Tian, Y.P; Tugume, A.K; Mukasa, S.B; Tairo, F; Kyamanywa, S; Kullaya, A; Valkonen, J.P.T. 2009b. 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 154: 353 – 359.