Cassava mosaic disease (CMD) causes devastating losses to cassava on the sub-Saharan African continent. CMD is caused by several distinct geminivirus species, including ACMV, EACMV and SACMV, which are transmitted by the whitefly Bemisia tabaci. Cassava brown streak disease (CBSD) has also recently escalated and is now also a major problem in cassava in eastern and southern African countries, but to date has not been reported in SA. CBSD is caused by two distinct ipomoviruses.
Cassava research in our laboratory focuses on five main areas:
1. Development of virus resistance to CMD
This project is the genetic engineering of virus resistance into cassava using RNA silencing as the induced mechanism. Different hairpin constructs have been made and transformed into cassava, and we are testing these for efficacy against SACMV, EACMV and ACMV.
2. Characterization of cassava geminivirus and whitefly diversity
This project involves both discovery and molecular characterization of new and existing geminiviruses in cassava in South Africa and Mozambique.
3. Sub-viral agents associated with geminiviruses
Several sub-viral agents have been associated with geminiviruses, and these are known to modify disease. Amongst these are beta- and alpha-satellites, and defective interfering DNAs. Our laboratory is also looking at discovery, characterization and function of these agents in infected cassava .
4. Functional genomics of geminivirus-cassava host interactions
This project involves next-generation sequencing and functional genomics techniques to examine global interactions between South African cassava mosaic virus (SACMV) and host plants (natural host cassava as well as experimental hosts, Arabidopsis and Nicotiana benthamiana). Transcriptome studies provide information on potential genes involved in susceptibility and resistance in cassava and model hosts, while small RNAs (miRNA and siRNA) data provides clues as to epigenetic mechanisms involved in host responses to SACMV infection. Resuts from this can be used in future to devise strategies for developing natural resistance.
5. Cassava brown streak disease
I am involved with collaborators and PG students outside of SA as CBSD does not occur in SA. Many of these projects are funded by Bill Gates Foundation and I collaborate with MARI in Tanzania, and Danforth Plant Center in the US. I have four PhD students working on aspects of virus diversity; virus evolution and virus resistance (transgenic cassava) in Tanzania, Malawi, Uganda and Mozambique.