Cassava brown streak virus disease: Past, present and future.

Cassava has been an important success story in Africa's developing agriculture over the past decade. The crop's inherei'lt capacity to cope with marginal growing conditions and unpredictable rainfall, coupled with determined efforts of a number of countries to move away from over-reliance on maize, have helped establish it as the continent's number one crop in terms of total fresh production. The hope provided by these gains is now threatened, however, by the devastating impact of the virus diseases, cassava mosaic and cassava brown streak. Both have been known for many years, but seem to be becoming increasingly damaging, and pose an ever greater threat to the livelihoods of the millions of Africans who depend on cassava as a food staple. Substantial effort has been directed towards understanding and managing cassava mosaic. By contrast, cassava brown streak disease (CBSD), which has a more limited distribution, and causes most severe damage in the coastal lowlands of Eastern and Southern Africa, has received little attention. In order to address this deficiency, an international workshop was convened at Mombasa, Kenya. This meeting brought together a diverse range of cassava stakeholders with the twin aims of reviewing current research into cassava brown streak and developing a plan to guide future research for development initiatives. The IS papers presented in these proceedings provide a useful and informative summary of the history and current status of CBSD, recent research initiatives and management options for the worst affected countries - Malawi, Tanzania, Kenya and Mozambique. Outputs of the research and on-farm working group sessions are also included. lt is anticipated that this information will serve both as a useful technical resource as well as an essential planning tool for scientists, development workers and others with an interest in the management of CBSD and the development of cassava in Africa

Impact of brown streak necrosis on cassava value addition

The main aim of this study was to investigate the quality and quantities of cassava flour produced and value chain development as a result of cassava brown streak disease (CBSD). Analysis of flour prepared from roots affected to various degrees of CBSD were assessed for various parameters such as starch and carbohydrate content, viscosity, pasting temperature, colour and pH. Values showed significant differences between the grades. There was a strong correlation (R2=0.904) between flour viscosity breakdown and pH value, and with peak viscosity and pH value (R2=0.737). A 25% level of CBSD root necrosis not only increased the conversion ratio of fresh roots to final product, but also presented an additional $41-47/t for the cost of fresh cassava roots if necrotic tissue were removed, representing a significant economic loss that makes cassava processing unprofitable

Transcriptional response of virus-infected cassava and identification of putative sources of resistance for cassava brown streak disease

Cassava (Manihot esculenta) is a major food staple in sub-Saharan Africa, which is severely affected by cassava brown streak disease (CBSD). The aim of this study was to identify resistance for CBSD as well as to understand the mechanism of putative resistance for providing effective control for the disease. Three cassava varieties; Kaleso, Kiroba and Albert were inoculated with cassava brown streak viruses by grafting and also using the natural insect vector the whitefly, Bemisia tabaci. Kaleso expressed mild or no disease symptoms and supported low concentrations of viruses, which is a characteristic of resistant plants. In comparison, Kiroba expressed severe leaf but milder root symptoms, while Albert was susceptible with severe symptoms both on leaves and roots. Real-time PCR was used to estimate virus concentrations in cassava varieties. Virus quantities were higher in Kiroba and Albert compared to Kaleso. The Illumina RNA-sequencing was used to further understand the genetic basis of resistance. More than 700 genes were uniquely overexpressed in Kaleso in response to virus infection compared to Albert. Surprisingly, none of them were similar to known resistant gene orthologs. Some of the overexpressed genes, however, belonged to the hormone signalling pathways and secondary metabolites, both of which are linked to plant resistance. These genes should be further characterised before confirming their role in resistance to CBSD

Cotton diseases and their control in the 21st century

Disease is a significant cause of crop loss in all the world’s cotton growing areas although the components of the disease complex vary according to prevailing climate, soil type and varieties grown. Disease resistance forms the mainstay of integrated control strategies, combined with chemical control for nematodes and seed dressing for seedling disease control. Marker-assisted selection and other tools in biotechnology are beginning to impact on disease management and new varieties engineered for disease and nematode resistance will begin to be commercially available in the next 3 – 5 years. This Chapter presents a summary of changes in disease incidence and management since the early 1990s and, reviews current trends in cotton disease management and research

Bionomics, morphometrics and molecular characterization of a cassava Bemisia afer (Priesner & Hosny) population

Bionomics, morphometric measurements and partial mitochondrial cytochrome oxidase I gene (mtCOI) sequences were examined for a population of Bemisia afer (Priesner & Hosny) (Hemiptera: Aleyrodidae) collected from cassava in Uganda. The development of the eggs, first to fourth instar nymphs and adults required respectively, 12, 31 and 10 days, with a total life duration of approximately 53 days on the cassava cultivar Ebwanateraka. Each female laid one egg per day for 13 days and about 40% of the eggs failed to develop into adults. The male: female sex ratio was 1:4. The length and width of the four nymphal instars were positively correlated (correlation coefficient=0.97). Females were bigger than males and the sexes differed significantly based on body length (P<0.001) and width (P<0.001). Overlaps in their body sizes, however, makes sex assignment based on size alone unreliable. The B. afer population shared 68% mtCOI sequence (817 nucleotides) identity with a cassava Bemisia tabaci population (Namulonge) from Uganda. This is the first study on B. afer fecundity and morphometrics, and the variability that might exist in this species is discussed

Number of reads generated from the RNA-Seq analysis and the corresponding gene expression range obtained for resistant Kaleso and susceptible Albert cassava varieties.

<p>Number of reads generated from the RNA-Seq analysis and the corresponding gene expression range obtained for resistant Kaleso and susceptible Albert cassava varieties.</p

Number of plants infected with CBSV or UCBSV for the three cassava varieties during the time course of virus transmission by grafting.

<p>- grafting was not done since all plants expressed symptoms from the previous grafting at this time point.</p

Relative changes of virus titre in cassava plants for a) CBSV and b) UCBSV.

<p>Virus quantities were normalised to the concentration of virus detected in Albert at the first week time point.</p

Fold enrichment of NAC-protein genes in infected cassavas as measured by RNA-Seq and RT-qPCR.

<p>Fold enrichment of NAC-protein genes in infected cassavas as measured by RNA-Seq and RT-qPCR.</p

Number of cassava plants infected with CBSV or UCBSV in roots and leaves over a period of 12 weeks after virus inoculation by grafting as confirmed by RT-PCR.

<p>Number of cassava plants infected with CBSV or UCBSV in roots and leaves over a period of 12 weeks after virus inoculation by grafting as confirmed by RT-PCR.</p