Transformation of fruit tree species can be accomplished and resistance demonstrated in a relatively short period; the agronomic evaluation of transformed plants for fruit production however requires many years. Therefore experiments need to be carefully planned. The genetic variability of individual viruses is an important consideration when choosing sources of genes, and when making evaluations of resistance, once transformation is achieved.
Although several efforts are underway to produce virus resistant fruit trees, papaya is the only one successful example to date that made its way into commercialisation. This is in part due to technical factors; however, there are also many legal and IPR issues behind this situation (Laimer 2005).
It should also be remembered that prevention and avoidance are basic components for the management of fruit tree viruses. Many of the current virus disease problems in fruit trees could have been prevented by eliminating the distribution and propagation of infected propagating materials, and by wiser use and movement of germplasm.
In the case of perennial trees challenging questions remain to be solved in future:
What confers more sustainable protection: recovery, resistance or immunity?
Should we go for prevention of local virus infection or systemic disease resistance?
How can we achieve pyramiding of resistance genes in fruit trees?
And obviously the question on how to gain public acceptance for these products.
Acknowledgements. Special thanks to E. Maiss (University of Hamover) for valuable discussions. Financial support by the BMBWK and BMLFUW project "characterisation of transgenic fruit trees and analyses of direct and indirect biological interactions" and the EU project "RESISTVIR" is kindly acknowledged.
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