Production of Disease-free Plants
Under normal conditions plants are infected by a wide range of pathogens such as bacteria, fungi, viruses, viroids, and insects like nematodes and insects. Many perennial plants and those propagated by vegetative means are systematically infected with one or more pathogens, which reduce yield, vigour and quality of the plant. If explants for micropropagation are derived from an infected plant, the pathogens can multiply and spread to a large number of plants.
It is, therefore, essential to use disease free stock plants for micropropagation. Eradication of viruses and other pathogens is also desirable from the point of view of international exchange of plant materials. Whereas bacteria and fungi present on the surface of the plant material can be easily eliminated by treatment with surface sterilizing agents, there is no dependable treatment against viruses. Viruses can multiply within the shoots in-vitro without symptoms. Traditionally, thermotherapy has been used for virus elimination but it is not effective against all viruses. Moreover, heat treatment may adversely affect the plant tissues. For some reasons viruses are unable to enter or survive in the apical meristems. Therefore, even in infected plants the apical meristems are generally free of viruses. Taking advantage of this observation, Morel (1950) developed the technique of shoot tip culture to raise virus-free plants from infected individuals. Since then it has become the most effective method of virus elimination. It involves excision of 0.5-1 mm long shoot tips, including apical meristem and one or two leaf primordia and their cultivation on a suitable medium to regenerate whole plants.