The M9 rootstock is the main stay of the commerical growers here in the UK. It grows to an approx height of 2 - 2.5m, making it easy to harvest the crop. Its height is mid way between M27 and M26. They can grow on heavy wet soils but can be suseptable to fireblight (which is not much of a problem in the UK). As with all dwarfing rootstocks they will need a permanent stake.
The pedigree is unknown and it was selected in England from a group of French genotypes known collectively as “Juane de Metz” in the late 1800s.