Tuesday, September 1, 2015

Apple rootstock, Part II

I received a comment on the last post regarding apple seeds.  It came via email and it was from somebody who has practical orchard experience, nursery/propagating experience and has done much heavy lifting in the breeding for rootstocks.  In other words, he knows five hundred times more than I do about apple trees and rootstocks.

In his email he asked, "Why plant seeds and get a big unmanageable tree?  Why??"  That is a very good question.

The next generation

It is a little known fact that a tidal wave of new apple rootstocks are being released.  The previous generation was based on  apple rootstock that were "collected" and categorized in Britain in the 1920s.  Those rootstock were given names like M-9, M-7 and such.  Some of those rootstock, clones actually, were several hundred years old.  Only a few of them were suited for the rigors of North America.

Around 1970, it was realized that improvement required more than the haphazard application of the scientific method.  A team developed a list of 12 essential priorities (primarily resistance to diseases and early/heavy/quality fruit production), another 8 that were important and 4 that would be helpful.  They established benchmarks for each priority and started identifying genetic material that might contribute toward meeting those goals.

It was a very long slog.  Priorities changed as new problems became economic concerns.

Are seedlings tougher?

At one time a fruit grower could confidently say that seedlings were tougher than the commonly available size controlling rootstock that had been semi-randomly selected under western European climatic and pest pressure conditions.

That is not a good call when looking at these new rootstock coming onto the scene.

In a typical year, the "Geneva" program produces more than 20,000 seeds.  They are from controlled crosses where elite size controlling rootstock were crossed with dream-dates that have the potential to compliment the elite rootstocks weaknesses and buttress their strengths.  An example would be Ottawa 3 (a dwarfing rootstock) crossed with Robusta 5 (an incredibly tough crab apple).

Collar rot killed ornamental plants
The seeds are planted in trays and even before their first leaves emerge they are water boarded.  That is, they are immersed in water that is contaminated with multiple strains of Phytophthora cactorum (collar rot) zoospores.  Early crosses resulted in 95% mortality.  That attrition rate has been dropping as the breeders became more familiar with sources of resistance and as they started "stacking" traits.  By 1983 the attrition rate had dropped to 25%.  So 8000 seedlings a year survived the water boarding in the early 1980s.
Fire blight can kill the roots, which then results in tree death.
Then the seedlings were subjected to multiple, direct injections of Erwinia amylovora (fire blight) spores.  Like the P. cactorum, a cocktail of multiple, virulent strains was used to assess survivability.

That knocked the number of survivors down to 2000.
Woolly apple aphids.  They also go below ground and attack the roots.
Then the tiny seedlings were swarmed with assorted insect pests.  One rarely considered the ramification of growing a plant that is inherently attractive to bugs is that every bug bite is a potential conduit for disease or a potential source of viral infection.  There is even a name for this resistance, i.e. unattractiveness to biting insects, "Klenducity".  I think we have all been outside in a group of people and noticed that some people are more bedeviled by mosquitoes and biting flies....they lacked klenducity.  That same variation exists in individual plants.

That knocked the number of seedlings from 20,000 to less about 500.  From there they are culled for leaf diseases, smooth wood, winter kill and thorns.  By the end of the first year the breeders have culled 99% of the seedlings while a ruthless producer of seedling rootstocks might have rouged out a whopping 80%.

That is, the team at the "Geneva" project exert 20 times more selection pressure than even a ruthless producer of seedling rootstocks. 

"Why plant seeds and get a big unmanageable tree?  Why??"

So the question remains.  I am not growing seedlings because I think they are tougher.
This is what 850 "Ben Trio" seeds looks like.

I am growing them because I can.  I have the seeds.  I have the garden space.  I don't have oodles of money to buy size controlling rootstock.  And frankly, I am in a holding pattern until these new "Geneva" rootstocks become more available.
A typical set of yards in Fabulous Acres.
A final reason for growing big trees is because one of my "customers" is a guy with community gardens in Lansing.  He is based in an older neighborhood named "Fabulous Acres".  The houses are tall and close together.  The yards are small.  There are many mature shade trees.  Sunlight is at a premium.

I am not growing seedling rootstocks because they produce more/sooner.  Almost any size controlling rootstock is capable of producing 30 pounds of fruit per tree in its third growing season on a good agricultural site under best management practices.  A seedling rootstock is doing well to produce its first fruit by the fifth growing season.

I am growing those big, unmanageable apple trees because Fabulous Acres is not a good agricultural site and its residents are not professional farmers.  Those big, unmanageable trees might be able to survive and and grow tall enough to find some sunlight.  They might actually produce some fruit in that environment.

I intend to share some of these seedling rootstock with Marcus, if he wants them.


  1. Just goes to show that everyone looks at things with a different perspective. What is perfect for one may be not acceptable for another.