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Cassava Brown Streak disease (CBSD) has and continues to be a major threat to the cassava industry in Uganda. The most economically damaging symptom of CBSD occurs on the roots as a yellow/brown, corky necrosis. However, the onset and development of this necrosis is not known. Therefore, this study was conducted to understand the progression of CBSD root necrosis.The experiment was conducted at Namulonge (central Uganda), where the CBSD pressure and whitefly population is high. Four CBSD susceptible genotypes (TME204, TMSI92/0067, MH97/2961, and Bamunanika) and five CBSD tolerant genotypes (TME14, NASE 3, NASE 1, MM96/0686 and 28 TME 14) were used. The experiment was laid out in a split-plot factorial experiment with three replicates. CBSD root necrosis was assessed at 4 months after planting (MAP) and, thereafter, at monthly intervals until 12 MAP. Results indicated significant differences (P<0.001)among reaction grades (susceptible and tolerant), genotypes and sampling times. CBSD root necrosis commences as early as 4 MAP in susceptible genotypes with a severity of 2 and incidence of 16.67%. These findings have important implications for CBSD breeding particularly when evaluating seedlings and/or clonal plants that often have different number of roots.