Hot water therapy and meristem shoottip culture techniques for elimination of cassava viral pathogens
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Cassava Mosaic Disease (CMD) and Cassava Brown Streak Disease (CBSD) are two main viruses affecting Cassava productivity in many agro ecologies of Uganda. Phytosanitary procedures to eliminate these viruses from infected cassava materials have been employed. However, none of the reports in Uganda has recorded 100% virus elimination. In this study, leaves and stem sections of varieties TME 204, NASE 3, Alado Aldo, Bao and Magana showing symptoms of CBSD and CMD were collected from the experimental field. Sampled leaves were tested for the presence of CMV and CBSV using conventional and RT- PCR respectively. Stems of positively confirmed leaf samples were subjected to Hot water therapy at varying temperature conditions (0oC - 60oC) and time intervals (10 - 30 minutes) followed by two rounds of meristem tip culture (MTC) on modified Murashige and Skoog medium. Data was analyzed using ANOVA (SPSS v23). Results indicated that plantlet survival and viral load decreased with increase in temperature treatments. Significant interactions (P=0.00) also existed between temperature treatment and number of regenerating shoot (P=0.00), shoot regeneration and cultivar (P=0.00), temperature treatment and cultivar (P=0.00), disease severity and temperature (P=0.00) and disease severity and cultivar (P=0.00). However, no CMD or CBSD free plants were obtained after hot water treatment at any temperature. Hot water therapy and first round of MTC produced CMD free plantlets only at 55oC. Alado emerged as the best responding variety in-vitro followed by Magana, Bao, TME 204 and then NASE 3 as the least responding variety. A combination of hot water therapy, first and second round of MTC produced 100% CMV free plants and a greatly reduced viral load of CBSV invisible on treated weaned plants at 55oC and 60oC as confirmed by PCR and qRT-PCR respectively. This demonstrated that Hot water therapy followed by subsequent rounds of meristem shoot tip culture can produce cassava plantlets free of CMV and CBSV.