Cassava production in eastern and Central Africa is severely threatened by the current epidemic of cassava brown streak disease (CBSD). The disease is caused by cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) of the genus Ipomovirus, family Potyviridae. Using virus-free planting material is one effective way to reduce CBSD yield losses in farmer fields. The effects of chemical and heat treatment on tissue cultured plants was investigated to eliminate CBSV and UCBSV infections in three cassava varieties with differing levels of resistance to CBSD. Cassava node buds taken from young shoots of the varieties Kaleso (CBSD resistant), Kiroba (tolerant) and Albert (susceptible) infected with CBSV and UCBSV were grown in vitro in standard Murashige and Skoog tissue culture media, which alone eliminated viruses from up to 30% of the plants. To increase the number of virus-free plants, two-week old virus-infected in vitro plants were exposed to constant temperature regimes of 30, 35, 40 or 45°C for three weeks. Heat treatment at 40°C had the optimum effect as up to 50% virus-free plants were obtained for Kiroba; however, incubation at 45°C was lethal to all three varieties while the lower temperatures produced fewer virus-free plants. Chemical treatment of in vitro plants was performed using the antiviral chemical ribavirin at various concentrations. Ribavirin at 0.10 mM generated up to 40% virus-free plants in all three varieties but concentrations of 0.21 mM were lethal. These findings indicate important methods to produce virus-free planting material; their use by breeders and in the field, will help develop effective control strategies for CBSD management.
Key words: Africa, cassava, chemical and heat treatment, viruses, tissue culture, disease resistance.
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