Effect of cultivar and thermotherapy combined with meristem-tip culture on eliminating prevalent viruses infecting potato (Solanum tuberosum L.) in Uganda
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Virus infections in potato (Solanum tuberosum L.) have negative effect on yield through accelerating seed tuber degeneration. Several methods have been proposed for virus elimination, however in-vitro thermotherapy combined with meristem-tip culture has proved effective for the elimination of potato viruses. This technique had not been used for the elimination of potato viruses in Uganda. Therefore, the aim of the study was to evaluate and determine appropriate procedures for eliminating selected viruses from infected potato cultivars in Uganda. Specifically, the study was designed to; (i) establish the incidence, distribution and farmer perception of major potato viruses in key potato producing districts of Uganda and then (ii) establish the optimum temperature exposure period for maximum virus elimination from selected viral-infected potato cultivars. Farmer perception and survey for potato viruses was conducted in farmer’s fields in south-western Uganda (Kabale, Kisoro and Kanungu districts) and Eastern Uganda (Mbale and Kapchorwa districts). From each district, 40 farmers’ potato fields were selected and 20 plants in each field sampled. Leaf samples were analyzed using double antibody sandwich-enzyme linked immunosorbent assay (DAS-ELISA) aided by a plate reader at 405 nm wavelength. Tubers of farmer-preferred cultivars; Kinigi, Rwangume and Victoria were collected from farmers and planted in plastic pots. Two weeks after emergence, they were indexed for the presence of major potato viruses; Potato virus X, Potato virus S, Potato leafroll virus, Potato virus M, Potato virus A and Potato virus Y. Two weeks after planting, auxiliary and terminal buds were excised from plants only positive for PVX or PVS and initiated into tissue culture to obtain the required number of plantlets for virus elimination. In vitro plantlets were subjected to four heat treatment exposure periods of 0, 2, 3 or 4 weeks at 37-400 C; 16 hours of light and 30-340 C; 8 hours of darkness. Control plantlets of the same cultivars and virus infection were not exposed to heat treatment. Meristem tips (0.2-0.5 mm) were excised from both heat-treated and control plantlets and cultured on basal media containing 0.4 mg/l 6-benzyl amino purine (BAP) and 1.0 mg/l Gibberellic acid (GA3). Two months after culture, fully grown meri-clones were indexed for virus infection using DAS-ELISA. Field survey results indicated that Potato virus X (PVX), Potato virus S (PVS), Potato virus M (PVM) and Potato leafroll virus (PLRV) were incident at 64.9, 78.9, 3.1 and 21.2 % respectively across districts. Potato virus A and PVY were not detected in any of the samples tested. Incidence of all potato viruses except PVM varied significantly (P0.05) between cultivars and districts. Incidence of all potato viruses except PVX varied significantly (P0.05) between the two agro-ecological zones. Significant (P0.05) interactions were observant between season and district on incidence of PVX, PVS, and PVM. In terms of farmer perception of viruses, more farmers in Kisoro recognized symptoms of leaf roll while in Kanungu more farmers identified mottling. Farmers rated poor seed quality as the major cause of symptoms observed as well as yield loss and hence regarded changing seed as a measure to prevent more loss in yield. In vitro thermotherapy combined with meristem-tip culture resulted into the recovery of virus free potato plantlets. Virus elimination efficiency significantly (P≤0.05) varied between the four virus elimination treatments, cultivars and viruses. Significant (P≤0.05) interactions existed between thermotherapy exposure duration and virus type on virus elimination efficiency. The highest virus elimination efficiency for PVS was attained at four-week thermotherapy exposure duration (72.1 %) while for PVX, the highest elimination efficiency was attained at three-week (49.4 %) than four-week thermotherapy duration (11.1 %). These results confirm the occurrence of potato viruses on many farmers’ preferred potato cultivars hence, the risk of increased spread. Results also indicated that farmers linked virus infection to poor seed quality and hence the need to apply in vitro techniques for virus elimination. Thermotherapy combined with meristem-tip culture enhanced elimination of PVX and PVS thus providing an option for management of potato viruses. This technique can be applied to clean popular but virus-infected potato cultivars.