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.
Ginda 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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