1 International Journal of Research in Plant Science 2015; 5(1): 1-9 ISSN 2249-9717 Original Article Spatial and temporal spread of cassava brown streak disease in field grown cassava in coastal Tanzania Gwandu, C. 1&2* , Rwegasira, G. 2 , Ndunguru, J. 1 and Sseruwagi, P. 1 1 Mikocheni Agricultural Research Institute, P.O. Box 6226, Dar es Salaam, Tanzania 2 Department of Crop science and Production, Sokoine University of Agriculture, P.O. Box 3005, Morogoro, Tanzania *Corresponding author: -E-mail: cathygwandu@yahoo.com; Tel: +255 688 26 40 36, Fax: +255 222775549 Received 05 January 2015; accepted 30 January 2015 Abstract A study was conducted to determine the spatial and temporal spread of cassava brown streak disease (CBSD) in field- grown cassava at Chambezi, coastal Tanzania from 2012 to 2013. The experiment was established using CBSD-free tissue culture plants of a susceptible cassava cultivar Albert. Treatments included RT-PCR tested CBSD-affected plants of field- collected cultivar Albert, that were planted in three patterns: inocula centrally placed (IC), inocula diagonally placed (ID) and inocula placed in outer rows (IO) to serve as sources of disease infection. The control plots were planted with CBSD- free plants of Albert. A randomised complete block design (RCBD) with three replications was used. Data were collected monthly on CBSD foliar and stem incidence and symptom severity as well as whitefly (Bemisia tabaci) populations for six months. Representative CBSD-symptomatic leaf samples were collected for laboratory analysis using standard protocols to confirm presence of CBSVs. The results show that there were less effect of treatments on foliar and stem incidence of CBSD. However, the effect of time on foliar and stem incidence was significant (P < 0.001) as expected. In addition, the effect of treatments and time (MAP) were significant (P ≤ 0.05) for CBSD symptom s everity and whitefly populations. Results suggest that external inocula of CBSD-affected cassava plants in the breeding blocks at the station may have served as the main source of new infections into the experimental plots. The high rate of spread of CBSD into the experimental plots was probably also enhanced by the short distances between the experiment and the surrounding environment. © 2015 Universal Research Publications. All rights reserved Key words: Cassava, cassava brown streak disease, epidemiology, incidence and severity. 1. INTRODUCTION Cassava productivity in Tanzania is grossly low estimated at only 5.5 tonnes per ha compared with the world average of 12.8 tonnes per ha (FAOSTAT, 2013). The low yields are attributed to an array of biotic and abiotic stresses whose distribution and impact varies (Eggleston, 1992). Cassava brown streak disease (CBSD), caused by cassava brown streak virus (CBSV) and/or Ugandan cassava brown streak virus (UCBSV) is among the major biotic stresses in Tanzania (Mbanzibwa et al., 2011; Rwegasira et al., 2011). Cassava brown streak disease is transmitted through infected planting materials, grafting of CBSV-free onto CBSV-infected plants and cutting tools (Rwegasira et al., 2012). Whiteflies (Bemisia tabaci) and spiraling whitefly (Aleurodicus dispersus) have also been reported to transmit CBSVs to a very limited extent (Maruthi et al., 2005; Mware et al., 2009). Currently, the management techniques employed by farmers to minimize damage due to CBSD is by planting early-maturing cassava cultivars and use of tolerant varieties. Virus indexing and cleaning through tissue culture techniques have been recommended by researchers to eliminate inocula in the planting materials (IITA, 2000). However, such materials have been observed to get re-infection after a short time in the field. This is particularly common in areas where CBSD inoculum is very high (Hillocks et al., 1999). Natural spread of CBSD in originally uninfected cassava stocks is not uncommon (Kanju et al., 2003). Limited spread of CBSD to initially CBSD-free cassava plots occurs only when the distance is less than 20m away from the source of infection. Thus, in absence of large adult whitefly populations, the likelihood of CBSD spreading to distant cassava fields is lower than in those neighbouring the infected sources (Mware et al., 2009). There is very limited information on the rate of spread Available online at http://www.urpjournals.com International Journal of Research in Plant Science Universal Research Publications. All rights reserved