enabled rootstock breeding at the tetraploid level via sexual
hybridization, which can yield maximum genetic diversity
in zygotic progeny upon which to impose selection for the
many traits required in improved rootstock cultivars,
including disease and insect resistance, broad adaptation,
tree size control, and the ability to consistently produce
high yields of quality fruit. Recent progress and successful
examples of these applications are discussed. Finally, a
discussion of the genetic potential of somatic hybrids as
breeding parents, including meiotic behavior and inheritance
is provided.