International Case Study
Within the context of an existing disease (Cassava Mosaic Disease, CMD) and an emergent disease (Cassava Brown Streak Disease, CBSD), The Great Lakes Cassava Initiative (GLCI) had an overall goal of distributing clean planting material of disease tolerant or resistant varieties to 1.15 million farmers to six countries—Burundi, Democratic Republic of Congo, Kenya, Rwanda, Tanzania and Uganda—during the four years of the project period.
The target farmers were poor and vulnerable, though not exclusively, and the project policy was zero tolerance to contributing to the spread of CMD and CBSD by distributing diseased material.
With this goal in mind, GLCI was set up with five components—partnership, disease, training, seed and farmer groups.
Cassava is a major subsistence crop in many parts of the world, providing more than half of the dietary calories for half of the population of sub-Saharan Africa. In recent years, cassava crops in the east African countries of Kenya, Tanzania and Uganda have seen an emergence of cassava brown streak disease (CBSD), with reported yield losses up to 70%.
Identification of CBSVs based on symptoms is reported as unreliable because the symptoms are inconsistently expressed in leaf, stem and root and are difficult to distinguish from mite damage and nutrient disorders. Also, there is no evidence of any symptom differences between CBSV and UCBSV, making differentiation of CBSVs by visual symptoms impossible.
For these reasons, a diagnostic test is required to accurately identify the presence of viruses for research, policy (e.g. quarantine) and planting material multiplication purposes.
The first RT-PCR assay for the detection of CBSV was developed using only the small number of sequences available at the time, and recently, gel-based conventional RT-PCR assays for the detection of both viruses and a real-time RT-PCR assay for CBSV have been developed. Real-time PCR is widely recognized as providing greater sensitivity than both ELISA and conventional PCR.
However, the most significant advantages of the technique are realized in routine testing of samples: the absence of a gel electrophoresis step enables larger sample numbers to be processed at a reduced cost and the closed tube system effectively eliminates post-PCR contamination and resultant false positive results.
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