ABSTRACT
In the context of this chapter, the term ‘virus’ will include virus-like agents that share certain characteristics such as persistence in the tree and graft transmissibility. Therefore, the comments to follow apply to viruses, viroids and phytoplasma, as well as agents of graft-transmissible diseases that have yet to be identified. Many viruses and virus-like agents that infect apple do not elicit acutely obvious symptoms. Consequently, they are frequently and inadvertently propagated and dispersed, creating a negative impact on production and the economic sustainability of the industry from the nursery to the consumer. Unlike many insect or microbial infections, viruses persist within infected tissues, and thus, exert their effects throughout the life of an orchard tree, which may span 20 years or more. Most viruses encountered in apple production do not have insect vectors known to transmit the pathogen – perhaps the long history of vegetative propagation has allowed the viruses to evolve without the need to associate with insect or other vectors other than humankind. For this reason, many diseases caused by virus-like agents can be managed most effectively by planting trees that are tested and found free of pathogens. The effectiveness of this strategy was demonstrated in the 1950s with fruit tree and grape programmes and is currently reinforced by a system of clean plant programmes operating throughout the world (Gergerich et al., 2015). Planting stock that is tested and found free of virus-like agents based on prescribed protocols is referred to by one of several terms, including foundation, nuclear or G1 stock. The strategy of building the industry on nuclear stock is particularly critical in the face of rapid globalization of fruit tree production, increased global competitiveness and the exchange of large volumes of planting and breeding material in the search for new and desirable product characteristics.
