Effect of thermotherapy duration, virus type and cultivar interactions on elimination of potato viruses X and S in infected seed stocks

Virus infection in potato reduces yield through seed degeneration. This can be reduced by use of virus-free seed tubers. However, novel approaches are required to ensure availability of virus-free stocks, especially in developing countries where the seed potato system are evolving. Consequently, a laboratory experiment was conducted at Kachwekano ZARDI in Uganda to evaluate and determine appropriate procedures for cleaning valuable potato cultivars infected with the most prevalent potato viruses; potato virus X (PVX) and potato virus S (PVS) which are frequent in the potato farming system. Thus, 20 in-vitro potato plantlets from cultivars Victoria, Kinigi and Rwangume each batch infected with either PVX or PVS in three replicates were grown in a thermotherapy chamber for two, three and four weeks at 37 to 40°C in 16 h of light and 30 to 34°C for 8 h of darkness per day. An equal number plantlets and replicates of the same cultivars and virus infection combinations were grown in a standard tissue culture (TC) growth room at 16 to 18°C with 16 h of light without thermotherapy as controls. Results indicated that plantlet survival after in-vitro thermotherapy decreased with increasing duration of heat treatment. Virus elimination efficiency significantly (P ≤ 0.05) differed between heat-treated plantlets and the controls but not between the duration of thermotherapy treatment. However, the highest proportion of virus-free plantlets was obtained after three weeks of thermotherapy. Significant (P ≤ 0.05) interactions were observed between thermotherapy duration and virus type on virus elimination efficiency where significantly (P ≤ 0.05) more PVS-free than PVX-free plantlets were obtained at the same thermotherapy duration. Three weeks of thermotherapy of virus-infected in-vitro plantlets, particularly for PVS, offered an equilibrium duration for adequate plantlet survival and maximum meri-clone regeneration to obtain the highest proportion of virus-free plantlets. The use of thermotherapy to obtain a high proportion of PVX-free plantlets from infected potato seed stock showed no clear trend and needs further investigation.   Key words: Latent virus infection, meristem tip culture, plant virus heat therapy, recalcitrant viruses.