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Cassava brown streak disease (CBSD) is endemic to the coast of East Africa and the disease continues to spread to new areas in the Great Lakes region. In both these areas CBSD leaf symptoms occur at high incidences. However, it is the associated symptom of necrosis in the starch-bearing tissues that renders the root unfit for human consumption. Where root necrosis occurs, the economic viability of processing into flour is adversely affected. Surveys undertaken in Tanzania, Kenya, Uganda, and Malawi showed that CBSD leaf symptoms were present at high incidences but root necrosis incidence was lower than would be expected. It appears that a form of tolerance to CBSD occurs in which the plants are susceptible to infection by the virus but are less affected by the root symptom. Farmers practise selection for cassava varieties less prone to CBSD root necrosis leading to dependence on a decreasing number of varieties. [ABSTRACT FROM AUTHOR]