Plot based heritability estimates and categorization of cassava genotype response to cassava brown streak disease.

Cassava brown streak disease (CBSD) caused by Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) is a threat to food security in sub-Saharan Africa, where the disease persistently reduces overall root quality and quantity resulting in up to 100% yield losses. Complexities in CBSD symptom expression and the damage caused on leaves, stems and roots throughout the 12 months of cassava growth require that appropriate ways of categorizing genotype response and optimal stages of evaluation be identified. This study aimed at: 1) determining plot based heritability of CBSD based on symptom expression and 2) categorizing genotype resistance to CBSD based on symptom expression. Herein, 41 genotypes were evaluated for two years at Namulonge with an additional evaluation conducted across three locations. Evaluations were done at three, six, nine and twelve months after planting. Genotype responses to CBSD varied significantly. High broad sense heritability estimates of up to 0.81 (incidence) and 0.71 (severity) were obtained. Average disease severity scores had higher broad sense heritability estimates (0.53 and 0.65) than maximum disease severity scores (0.33 and 0.61) for root and foliar severities respectively. These findings are important in choosing an appropriate evaluation method for CBSD. Genotypes displayed differing CBSD responses in type, locality and severity of symptoms. This suggested that genotypes had differences in mechanisms of resistance that can be exploited in CBSD resistance breeding.

Re-emergence of Cassava Brown Streak Disease in Uganda.

During November 2004, veinal chlorosis on mature cassava leaves, typical of cassava brown streak disease (CBSD), was observed at Mukono in central Uganda. Five out of 11 cultivars at the site showed CBSD symptoms (incidence range 4 to 64%). In a survey of farmers' fields, CBSD was observed in Wakiso and Mukono districts. Incidence of cassava mosaic disease was also recorded and averaged 60% for landraces (range 16.7 to 100%) and 20% for resistant varieties (range 0 to 65%). Leaf samples of plants with CBSD symptoms produced an amplicon of 222 bp using reverse transcription-polymerase chain reaction with primers that amplify a fragment of the coat protein (CP) gene of Cassava brown streak virus. Sequence comparisons based on the amplified CP gene fragment indicated that the isolates have 77 to 82.9% nucleotide and 43.9 to 56.8% amino acid identity with those from Mozambique and Tanzania. There was 95.9 to 99.5% nucleotide and 85.1 to 90.5% amino acid identity among the Ugandan isolates. These results confirm the re-emergence of CBSD in Uganda after it was first observed in the 1930s in cassava introduced from Tanzania and controlled by eradication. Prior to this report, CBSD was known to be restricted to the coastal lowlands of East Africa.