Elimination of Cassava Brown Streak Virus from Infected Cassava

Abstract

Cassava brown streak disease (CBSD) is an economically important disease of cassava (Manihot esculenta crantz) caused by Cassava brown streak virus (CBSV). Use of clean planting material is one of the strategies for disease management. However, obtaining clean planting material for some farmer-preferred varieties is often difficult. This research was aimed at evaluating the effect of meristem tip sizes, effects of varying concentration levels of ribavirin and salicylic acid and determining the efficacy of thermotherapy in combination with either meristem tip culture or chemotherapy in the elimination of CBSV from infected cassava. CBSV infected cuttings of Guzo variety collected from Coast province of Kenya were established and maintained in a greenhouse at the Plant Quarantine Station in Kenya Plant Health Inspectorate Service in Muguga were used as test plants. Cassava leaves were sampled from eighteen cassava plants of Guzo variety and virus indexing was done using Reverse Transcriptase-Polymerase Chain Reaction with virus specific primers and those that tested positive for CBSV were used as initiation materials. From the in vitro plantlets established, the second sub-cultures were subjected to the virus elimination procedures. In vitro meristems (0.5mm, 1mm, 2mm and 10mm) were obtained and cultured in modified Murashige and Skoog media. For chemotherapy, nodes were cultured in MS media supplemented with antivirals at 0mg/l, 10mg/l, 20mg/l, 30mg/l. In the combination treatments single nodal plantlets were subjected to thermotherapy at 38ºC for twenty one days then excised meristem tips (1.0mm) with some plants being subjected to ribavirn treatments at (10mg/l, 20mg/l and 30mg/l). Data was analysed using Genstat 13^th edition (2013). The regeneration of plants established from 0.5mm was 63% while 2mm was 88%. In chemotherapy survival of shoots was observed to decrease with increase in the antiviral concentrations. Ribavirin at 10mg/l recorded the highest rate of survival compared to the other treatments. On the other hand salicylic acid exhibited the least survival rate compared to ribavirin. The number of plants testing negative was observed to increase with increase in concentration for both chemicals. At 30 mg/l of ribavirin and salicylic; 88.8% and 100% of virus free plantlets were produced respectively. Thermotherapy (38°C) combined with meristem tips (1mm) resulted in 68% of regenerated plants with 84% being virus free. Invitro plants that had been thermo treated and then subjected to chemotherapy did not give the expected results since all plants died. Thermotherapy at (38°C) for a period of twenty one days combined with meristem tip culture can be used for production of virus free cassava.

Keywords: Cassava brown streak virus, thermotherapy, chemotherapy, meristem tip culture, Virus elimination.