Trends in cassava brown streak disease diagnostics for certification of healthy cassava planting materials and future perspectives: A review

Cassava (Manihot esculenta Crantz) is important for food security and livelihoods of more than 500 million people in sub-Saharan Africa particularly for poor subsistence farmers. However, its productivity is hampered by cassava brown streak disease (CBSD), the most devastating viral disease of cassava in East and Central Africa. Cassava brown streak disease (CBSD) often leads to necrosis of storage roots making them unfit for consumption and unsuitable for industrial processing. The disease has been reported to be caused by two species of ipomoviruses; Cassava brown streak virus (CBSV) and Ugandan Cassava brown streak virus (UCBSV) which belong to the Potyviridae family. Cassava brown streak disease is reported to be transmitted by whitefly vectors and disseminated over long distances through farmers’ exchange of infected planting materials and across the borders through trade. This poses great risk to international and domestic exchange of cassava germplasm. Many cassava producing countries in Africa including Kenya have inadequate mechanism for adoption of improved varieties as well as lack cassava seed certification systems. Virus indexing is very key to ensure that only the plantlets that are diseasefree are deployed in the field for further multiplication. The rapidly increasing spread of CBSD to new cassava-growing regions in Africa and the cases of re-emergence of CBSD is a real threat to food security. Recent studies have revealed high sequence diversity of CBSV virus strains. This variability makes diagnostics difficult and there is therefore a need for development of more robust and broad spectrum diagnostic tools for rapid detection of the CBSV virus strains in order to inform implementation of effective control measures. These tools would also be utilized for accurate detection of the viruses to improve certification of cassava planting material in greenhouse and field multiplication. This review provides insights into the trends in cassava brown streak disease diagnostics for certification of healthy cassava planting materials as well as future perspectives in strengthening cassava seed certification. Future research needs and novel genomic approaches will provide basis for improving the existing molecular diagnostic tools or development of new ones suitable for detection of newly discovered virus strains. This will also offer potential to overcome some of the drawback of the current control strategies of the disease.