Cassava brown streak disease is endemic to the coastal regions of East Africa and from around 2004 the disease resurged and became epidemic in the Great Lakes Region, where it continues to spread. In both these areas CBSD leaf symptoms occur at high incidences.However, it is the associated symptom of root rot (necrosis) in the starch-bearing tissues that renders the root unfit for human consumption. Because the extent of root necrosis is not known until the crop is harvested and surveys require destructive sampling, root symptoms are much less frequently assessed than are the above-ground symptoms on the leaves and stems. Surveys were undertaken in selected villages in Tanzania, Kenya, Uganda and Malawi to assess the incidence of CBSD leaf symptoms and the incidence and severity of root symptoms, in order to estimate the impact of the disease on household food security and on cassava processing. CBSD leaf symptoms were recorded at high incidences [40 -90% in individual fields] in all fields visited throughout East Africa but root necrosis incidence was lower than would be expected from the high incidence of leaf symptoms. Severe root necrosis at high incidence was found only on a few varieties, usually grown to a limited extent. It appears that varieties that are prone to root necrosis are being abandoned in favour of those with a lower propensity to develop root necrosis after infection by the virus.

Cassava landraces were evaluated for resistance to cassava brown streak disease (CBSD) for two cropping seasons at a disease hotspot area in Naliendele, Tanzania. Based on reactions to CBSD, several landraces including Chimaje, Mfaransa and Supa B were considered to be resistant to the disease while Kikwada, Mbuyu, and Nyoka were tolerant. ANOVA revealed that the largest sum of squares (SS) (41.9 -86.7%) was attributed to the genotype of the cassava landraces, while a smaller proportion of SS (8.1 -38.2%) was due to genotype by environment interactions for all traits tested, which included disease symptoms, root weight, number of roots per plant and dry matter content. Environment accounted for the smallest effect (0.01 -26.3%), however, the mean squares was nonetheless significant for a few genotypes, which indicated that their disease expression was indeed influenced by the environment. Increased CBSD severity was associated with low temperatures and rainfall. Increased rainfall towards harvesting led to higher root weight but lower dry matter content in the first cropping season. Correlation analysis showed that the presence of CBSD symptoms reduces the amount of usable roots, total root weight, and root dry matter content. Many resistant/tolerant landraces also had high root weight and dry matter content, and they can be used by farmers to reduce CBSD losses. The landraces described here form novel sources of CBSD resistance that can be used for breeding disease-resistant cassava varieties with superior agronomic characteristics.

A farm survey was conducted in Kuria East and Suna West sub-counties to determine the incidence, severity and estimated losses of cassava brown streak disease (CBSD) and cassava mosaic disease (CMD) on cassava crops in farmers' fields. The results showed that cassava is the second most important staple crop after maize in Migori County. CMD incidence ranged from 0.0 to 56.7%) in Kuria East and 10.0 to 55.0% in Suna West. CBSD incidences were much higher at 5.0 to 74.0% in Kuria East and 10.0 to 77.5% in Suna West. Both CMD and CBSD had an effect on yield reduction and total root loss ranged from 10.7 to 47.2% in Kuria East and 11.5 to 33.2 in Suna West. The percent mean total root loss in Kuria East was 25.9%; equivalent to 1299.6 US dollars/ha while in Suna East was 24.7%; equivalent to 1259.5 US dollars/ha. The best performing variety with regards to low CBSD and CMD incidence, low root losses and high yield were TMS 30572 and MH95/0183. The findings of this study are expected to provide impetus for the development and promotion of new high yielding, locally adapted and resistant cassava varieties.