Identification of problems and prioritising breeding objectives based on those problems are essential first steps in a rootstock improvement program. For all tree fruits, incorporating resistances to critical diseases and pests will facilitate fruit production in a social environment demanding reduction in pesticide usage. Diseases caused by variousPhytophthoraspecies are important and can be catastrophic for all major tree fruit crops; breeding for resistance toPhytophthorahas generally been successful. Very large initial seedling populations are required to permit suitably rigorous early screening; the breeding team should anticipate odds of l:104to 1:106that any given seedling will be commercially successful. In preliminary orchard tests, positive selection for dwarfing and precocity induction can be made by the fifth year. More rigorous second tests should be made with a number of commercial varieties in several locations. Although almost all rootstock improvement programs now rely on conventional breeding methods, through application of genetic engineering, the Mailing 26 apple(Malus domestica)rootstock has been successfully transformed from being highly susceptible to fire blight to being moderately resistant.