Detection of cassava brown streak viruses in coastal Tanzania

Abstract: Cassava brown streak disease (CBSD) is a factor which can decrease cassava production up to 70%. Cassava is an important crop, especially in sub-Saharan Africa for small-scale farms, as it is a staple food with tolerance to local conditions such as draught. The disease is caused by two viruses: Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). Symptoms of the disease can be seen as chlorosis, especially along secondary veins in leaves, and as brown streaks along the stem. A severe symptom is root necrosis, which makes the starchy tissue unsuitable for human consumption. The CBSD is not always visible as symptoms on leaves or stems and can be very hard to detect and it is difficult to know the extent of losses before the plants are harvested. In this study two field trips for collecting samples were made, one along the coast towards Tanga and the other one to Kibaha, also located in Tanzania’s coastal region. Leaf samples were collected in a plant press as dry samples. Cassava was sampled to compare virus composition between the two areas and also other species of plants were sampled to search possibilities of them as alternative hosts for the viruses. An alternative hosts can function as a reservoir for the viruses. Knowing them would limit spreading of CBSD. RT-PCR was used to detect the viruses in non-cassava plants, and seven of the tested plants gave positive results. The RT-PCR products of the seven samples were sent for sequencing, but the sequencing results were of poor quality with high background. The determined sequences were compared to the sequences in GenBank through BLAST and no CBSV was found. Thirty extracted cassava samples were tested with both RT-PCR and Real-Time RT-PCR and some differences in virus composition were found in the two different areas. In this study two main findings were made: i) more mixed infections of CBSV and UCBSV in Kibaha than in Tanga ii) UCBSV was found in Mwamkongo, Muheza close to the border of Tanga.