[NAFEX] Apple Rootstocks

Ginda Fisher list at ginda.us
Mon Nov 9 23:38:07 EST 2009

Here's something I wrote up for Pomona following the one NAFEX meeting  
I attended.  Professor Cummins spoke about the apple rootstocks Cornel  
has been working on, and it mostly deals with those.  But it has some  
interesting general information.  Oh, and when I was planting some  
stuff, I spoke to Professor Cummins about rootstocks for my backyard,  
in eastern MA, zone 6.  Of the traditional semi-dwarf rootstocks (not  
needing support) he recommended M7.  It's an old reliable workhorse.   
It's not the most fireblight resistant, nor the most hardy, nor the  
most. . . . but it's pretty good in most ways.


Begin forwarded message:
> Jim Cummins on Newer Apple Rootstocks
> Professor Cummins spoke in the first session.  Unfortunately, we  
> started a bit late, and he wasn't able to get to pear rootstocks,  
> but he did talk at length about apple rootstocks - where they were  
> in his childhood, and where they have come today.
> Professor Cummins grew up on an old apple orchard with seedling  
> rootstock.  The trees were spaced on 40'x40' centers, for 27 trees  
> per acre, and the canopy was full, with the trees just meeting  
> overhead.  This was the standard orchard until fairly recently.  In  
> contrast, modern orchards on dwarfing rootstock can have hundreds of  
> trees per acre, up to 1900 (??  this sounds too high, but it's what  
> I wrote down)  with the smallest trellised trees.  Today, most new  
> apples trees are planted on M9, or occasionally M7 for intermediate  
> sized trees.  Very little M26 is planted due to its susceptibility  
> to fireblight.
> These are relatively old rootstocks.  The Malling and MM series of  
> rootstocks became commercially available in the early 1900's, with  
> newer rootstocks such as MM106 coming on line in the late 1960's and  
> dwarfing rootstocks in general becoming widely used by the 1970s.   
> This was the result of a process started back in 1922 when Lord Dowd  
> started a breeding program in England.  Shortly thereafter Dr.  
> Budagovsky started a similar program south of Russia.  Several other  
> programs have since been started (in Canada, the US and Europe) and  
> the program started by Dr. Budagovsky has moved to Poland.
> Most of these programs have looked primarily at horticultural  
> aspects, especially including how easy they are to root.  This has  
> often been the first criteria used to cull seedlings.  In 1968, the  
> Geneva program decided to try something new - they decided to breed  
> first for resistance to the major diseases of rootstocks, and  
> secondarily for horticultural criteria.  One side advantage of this  
> is that their trees, being somewhat harder to root, are far less  
> susceptible to burr knots.  This can be a problem with several of  
> the M and MM series (esp. MM111) and the Russia series.
> The major pests of apple rootstocks are collar rot and fireblight.   
> In order to find resistant plants, the Geneva program would plant  
> out thousands of seedlings from selected crosses.  When the baby  
> trees were 3" tall, they infected them with crown rot.  This would  
> kill between 75% and 100% of the plants.  when the remaining plants  
> reached 1 foot in hight, they injected them with fireblight  
> bacteria.  This would again kill off between 75% and 100% of the  
> plants.  From 100,000 seedlings evaluated, only about 500 survived  
> this treatment.  These ones were reinnoculated and tested for  
> nursery performance, and the most promising of these will be  
> released as the Geneva series of rootstocks.
> It takes a long time to fully evaluate a rootstock, and Dr. Cummins  
> wishes for another 20 years, however, they are beginning to release  
> their roots.  Some of them will eventually come up lacking, but he  
> is certain that some will be winners.
> Three rootstocks have currently been released, G30, G16, and G11.   
> G30 might replace M7 but it has limitations.  It yields early and  
> heavy bearing (20-50% better than M7 in first ten years) and it is  
> very cold-hardy (at least to -30F, probably to -35 or more) but it  
> produces a very brittle graft union, especially with trees that tend  
> towards brittleness, such as Gala and possibly Honeycrisp.  Trees on  
> G30 should have permanent support (an "artificial trunk") to 8'.   
> Another problem with the rootstock is that it is a very poor nursery  
> subject.  In particular, the baby trees have short internodes and  
> are very thorny.  These thorns need to be removed if the plant is to  
> be grafted.  They can be removed early on by hand ("thumbing them  
> off") somewhat later with sheers, or after the plants have been  
> dug.  The third is easiest, but it results in lots of open wounds on  
> the nursery plants, and they tend to dry out and die.
> G16 might compete with M26.  It has no thorns or burrknots, and  
> doesn't sucker at all - it yields a perfectly smooth trunk.  It is  
> more productive than M9, winter hardy, with heat and drought  
> tolerance similar to M7.  Curiously, it gets this tolerance through  
> a totally different mechanism than M7.  M7 has a vertical root  
> system, and the roots often find water by heading down.  G16 has  
> very heavy, aggressive roots that go everywhere.  It appears to be  
> adequately winter hardy but has not yet been subjected to a test  
> winter.  The problem with this otherwise very exciting rootstock is  
> that it is very susceptible to common latent viruses, including  
> applestem grooving virus.  It is critical that it only be grafted to  
> clean scion wood, or the tree will die.  Because apple viruses are  
> not transmitted sexually (e.g. by pollinators) or by leaf-eating  
> insects, the tree is unlikely to be naturally infected in the  
> field.  (Nematodes do transmit some apple viruses, such as tomato  
> ringspot, but this tree is not susceptible to those.)  On the other  
> hand, it could be infected by pruning equipment - this is a tree to  
> treat carefully.
> There was an interesting side discussion about why G16 is dwarfing  
> given the high vigour of its roots.  Dwarfing is not well  
> understood, and the "weaker roots" theory is not backed by any solid  
> evidence.  Dr. Cummins and others suggested two possible mechanisms  
> for why this tree might be dwarfing.  The first is that "wider  
> pipes" in the tree's plumbing might limit the hight to which  
> nutrients flow.  Research done in Israel has shown that M9 has very  
> large pipes and they appear to reduce the capillary action that  
> helps bring sap up the tree.  The second theory is that precocity is  
> itself dwarfing - once a tree has started to fruit (even if you  
> remove the young fruits) it simply puts less energy into growing  
> trunk and branches.
> G11 has just been released this year.  It is in the same size class  
> as G16 and M26.  It is not as fireblight resistant as the other two,  
> but it is probably good enough for most uses (comparable to M7) and  
> has adequate resistance to phytopthera.  There are (so far) no known  
> virus problems with this rootstock.  It is also an early and heavy  
> bearing variety, and is also a good nursery subject.
> At this point Dr. Cummins ran out of time, and quickly ran through a  
> couple of "things to come" from the Cornell-Geneva program, and  
> answered some questions about the other new rootstock series.
> CG3041 is more dwarfing than M9, and much less brittle.  CG5935 is a  
> bit more vigorous than M26.  Both are still undergoing testing  
> (along with G5202, described in his handout.)  According to the  
> handout, CG5202 is also highly resistant to wooly apple aphid, and  
> has been introduced in New Zealand, where WAA is a major pest.
> Bud9 (or red leafed paradise) is similar to M9 but more cold hardy,  
> with a smooth shank.  It has better fireblight tolerance than M9.   
> It was introduced in Russia in 1948 and is now widely available in  
> the US.  Bud118 produces a tree about 90% as large as standard.   
> Like M111, it seems to delay the onset of fruiting, but once it  
> starts it is very efficient and very well rooted.  He believes it  
> would be good under Honeycrisp.
> He has had trouble propagating the Polish series (coincidentally the  
> "P" rootstocks, although the "P" stands for the Polish word for  
> rootstock, not for Poland.) and had little to say about it.  He is  
> skeptical of the German supporter series, which he doesn't think has  
> been adequately tested outside of Germany.  He feels that program is  
> under pressure to release stuff.  He has found this series to be  
> susceptible to fireblight.  The least susceptible to fireblight of  
> the lot (Supporter 4) is still somewhat susceptible, and is also  
> susceptible to collar rot like M9 and seemed to yield poor cropping  
> in his tests.  Some trees died from apple aphid.
> At this point we really ran out of time for the first session.

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