plantcollector rootstocks are used for helping with diseases but only Soilborne diseases. This is the part of the tree in the soil so thats all it helps with, as far as diseases. plants are bred for resistance to foliar problems like powdery mildew ect. rootstocks also provide growers the option to grow plants that have picky soil types without having to change their soil to accommodate the plant. for example allowing growers in heavy clay to grow cherries. Rootstocks also control tree height reducing pruning. Cuts and wounds on trees are open invitations to pest and diseases so some rootstock will dwarf a tree minimizing the pruning cuts.
if you airlayer a plant you will loose your rootstock unless the plant is self rooted to begin with. over the past thousand or so years most trees that can be air layered can and have been rooted via cuttings and often don't have a rootstock. At the same time most of the plants on rootstock don't root easily from cuttings or air layers so they are grafted. This however will be changing in the coming years as more and more plants grown for their fruits or flowers more than soil borne diseases resistance are being grafted on superior rootstocks. Even vegetables are being grafted nowadays.
when we came to america we brought grapes over with us to grow, true vitis vinifera grapes and they all failed year after year they tried and failed. It wasn't until years later we figured out that we had phylloxera in our soil and it would kill all the table and wine grapes we were use to. finally someone grafted a vitis vinifera on a native grape and we can now grow vitis vinifera everywhere with it grafted. Thanks to breeders crossing native grapes with the old world grapes we can now grow hybrids without the need of a rootstock and these can now be rooted or air layered. However still to this day if you airlayer or root a ture vitis vinifera once it gets planted out phylloxera will kill it.
I hopes this helps.
