Bibliography

Cassava mosaic disease caused by diverse geminiviruses (CMG) and Cassava brown streak disease caused by cassava brown streak virus (CBSV), are key constraints to the cultivation of cassava in Sub-Sahara Africa. We have collected and characterised the diversity of the most prominent geminiviruses from all African cassava cultivation areas and Cassava brown streak viruses from East Africa, to use defined viruses for resistance studies in selected IITA cassava breeding lines and African land races. Virus infections were established in cassava by either graft inoculation with scions of virus infected plants or, by particle bombardment of cloned infectious viruses. Resistance against cassava mosaic geminiviruses was identified in several breeding lines e.g. TME 4, TMS 96/0529 and TMS 96/0160 responding with abortion of virus infections after virus introduction. Other genotypes, e.g. TMS 96/0304, became infected but recovered from symptoms but maintaining the infection status. When cassava clones were analysed for CBSV resistance, it became evident that geminivirus resistance was tightly correlated with susceptibility against CBSV. Geminivirus resistant cassava responded with often severe CBSV infections (e.g. TMS 96/0160) upon virus inoculation. Interestingly, cassava genotypes with an intermediate resistance, TMS 96/0304, initially became mixed infected with CBSV/CMG however subsequently aborting CMG, to establish single CBSV infections. Differential responses were also recorded for strains of CBSV. While CMG resistance in cassava was against all CMG, differential responses were observed in cassava inoculated with CBSV isolates obtained from Kenya and Mozambique. While the CMG susceptible cassava landrace TME 117 was resistant against CBSV from Kenya, it became infected with CBSV from Mozambique. Only the cassava genotype TMS/ 1089A revealed immunity against CMG and also did not establish infections with the CBSV isolates from diverse origins.