Incidence and molecular variability of cassava brown streak disease in Rwanda
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In Rwanda, despite the importance of cassava as staple food, its production is greatly limited by biotic and abiotic constrains leading to poor yields. One of the most important constraints to cassava production is the cassava brown streak disease (CBSD). Cassava Brown Streak virus Ugandan valiant (UCBSV) was the first reported to occur in Rwanda in 2009. However, the Rwandan isolates of CBSVs have not been fully characterised which is a big gap in knowledge on epidemiology and management of CBSD in Rwanda. In addition no studies have been conducted in the country to understand the potential role of non-cassava plant species acting as virus reservoirs or alternative hosts for the viruses causing CBSD. This study therefore aimed at determining the incidence and severity of CBSD in the ten major cassava producing districts located in different agriculture zones of Rwanda. It also aimed at determining the diversity of cassava brown streak viruses; and to identify the possible alternative hosts for CBSD causal agents in Rwanda. In a survey conducted in 2015, 279 cassava leaf samples were collected from 93 cassava young field (3 to 6 MAP) and 101 leaf samples from shrubs and herbs with virus-like symptoms growing in or around cassava fields, as well as from areas with no nearby cassava fields. Sampling was undertaken in the ten districts of Rwanda: Bugesera, Nyagatare, Kayonza, and Kirehe (Eastern agriculture zone), Rusizi, Nyamasheke, (Western agriculture zone), Gisagara, Nyanza, Ruhango and Kamonyi (Southern agriculture zone) that grow large acreages of cassava. The CBSD incidence was calculated as a percentage number of plants diseased relative to the total number of plants assessed, while a scale of 1 to 5 was used to determine the disease severity where 1 = no visible CBSD symptoms and 5 = severe foliar symptoms and/or defoliation and plant die. Total RNA was extracted using CTAB (cetyltrimethyl ammonium bromide) method and amplified using reverse transcriptase polymerase chain reaction (RT-PCR). Positively amplified samples were partially sequenced using the Sanger method. Findings from this study showed that there were a significant differences (P<0.05) in the incidence of CBSD between surveyed districts. The disease was widely distributed in the surveyed areas with an average incidence of 27.39%, which is a marked increase from the 18.8% in 2012. There were no significant differences (p>0.05) in CBSD severity between the surveyed districts mean severity score was 2.4 indicating that all the districts had moderate symptoms. Based on the RT-PCR and sequencing results, this study revealed further spread of UCBSV, reports for the first time a new appearance CBSV and UCBSV- CBSV co-infections. For alternative host, no CBSV was detected in the study materials. However, five species: Asystacia gangetica (Acanthaceae), Physalis peruviana L. (Solanaceae), Carica papaya L. (Caricaceae), Sida cordifolia L. (Malvaceae) and Senna spectabilis (DC.) (Caesalpinioideae) tested positive for UCBSV. This is the first report of non-cassava plant species as hosts for UCBSV in Rwanda. The findings suggest that providing clean (virus-free) planting material alone might not offer an effective solution to management of CBSD in areas where such alternative host plants are rampant. It is therefore also advisable to create awareness on the importance of alternative host plants in the management of CBSD, with emphasis to reduce or remove the plants in cassava fields and the surrounding environments.