(Some Inconvenient Observations about Apple Rootstocks)

By Mark Weaver — December 2012

In the last 20 years, productive apple trees and productive home orchards have proliferated in Southcentral Alaska to an extent once thought impossible. In large measure, this has happened because of the willingness of a few Alaskan and Canadian growers—real pioneers in the pomological wilderness — to share their hard-earned apple experiences, and often as not, their scion wood. Perhaps the most significant pearl of wisdom passed on by these pioneer orchardists has been the importance of using “ranetka” rootstock — the ranetka having proven itself in the far north as vastly superior to antonovka, and to many other supposedly hardy rootstock apples, most of which fail to survive our cold summer soils and sub-zero winter frosts.

The beneficial impact of ranetka rootstock on Southcentral apple growing can hardly be overstated. It has acted as a catalyst, almost single-handedly enabling the production of hundreds of viable bench grafts each year at the APFGA spring grafting workshop, and a remarkable increase in the trees and cultivars represented at our autumn apple-tasting event.

At the same time, because of the relative success of northern growers using ranetka, there is a corresponding risk that we will become complacent in our thinking about rootstocks. I, at least, have been guilty of subscribing to a convenient myth — a myth so easily conceived and so compelling that it may have attracted the allegiance of others also. By way of warning (the faint of heart should stop reading here!), I choose to call this “The Ranetka Myth.”

The Ranetka Myth goes like this: Malus ranetka is a species of crabapple having M. baccata as one parent and M. prunifolia, or possibly Dolgo, as the other. It is hardy, it is vigorous, it is almost perfectly reliable as the rootstock of choice for Alaskan conditions, and it is conveniently available in seedling form, at low prices, from various nurseries in the lower 48 and from APFGA at the April grafting workshop. Use ranetka, and your orchard will prosper!”

I will move on to a key point of this article, which I call:

Heresy #1: Believing in the Ranetka Myth is a bit like believing in the Tooth Fairy. Or, to be more botanical, like believing that all brown mushrooms are edible. Why? Because the Ranetka Myth is dangerously overstated. It defies common sense and several well-established genetic, taxonomic, and economic principles. It ignores, or misinterprets, a considerable body of local experience. And, most significantly, it is potentially lethal — at least to apple trees.

Let me add some related points, each of them troublesome:

Heresy #2: As far as I can tell, none of the rootstock imported by APFGA and others in recent years is M. ranetka, or even M. x ranetka. Careful scrutiny of Westwood’s Temperate Zone Pomology (3^rd ed., Timber Press, Portland, 1993) suggests there is no such species as M. ranetka. But if, by ranetka, we simply mean crosses of M. baccata x M. prunifolia, or possibly M. baccata x Dolgo, the rootstocks that we have been calling “ranetka” in recent years are not those either! Most likely, they are seedlings of open-pollinated crabs that may themselves be seedlings of open-pollinated M. baccata. Or they are seedlings of open-pollinated seedlings of open-pollinated seedlings of open-pollinated M. baccata … etc., etc.

Heresy #3: It is a mistake to give any domestic, seedling-propagated population of apple rootstocks a formal name, other than seedling #1, seedling #2, seedling #3, and so on. A formal name implies uniformity of characteristics like hardiness and onset of dormancy, and uniformity in apple seedlings (except in some wild populations) is a perilous illusion. Apple seedlings are inherently diverse.

As apple-growing readers doubtless realize, apples which are full siblings can be quite different in key characteristics. Norland and Parkland, for example, are full siblings–they are both Rescue x Melba crosses. Yet they differ in tree structure, taste, appearance, and other traits, including hardiness. Seedlings derived from trees that are open-pollinated, and that flowered in places where many cultivars — and species! — of apple are potential pollen donors, are likely to be even more diverse.

Heresy #4: The imported “ranetka” seedlings many of us have been using for rootstock are, not just theoretically but in fact, genetically and phenotypically diverse. They are variable in a number of observable traits, including leaf color, vigor, tree form, onset of dormancy, and most significantly, winter-hardiness. Moreover, a significant percentage of them are not hardy enough to withstand a Southcentral Alaska test winter — a winter with precipitous drops in temperature and little or no snow cover.

For emphasis, I will present the last statement in the form of a prediction:

Heresy #5: If we get a Southcentral Alaska test winter in the foreseeable future, there will be, come spring, an unfortunately large number of dead apple trees. Many will fail because of “ranetka” rootstock. Trees grafted on to the APFGA “ranetka” class of 2010 and 2011 will be particularly vulnerable, but some from earlier years will also die — if they are not already dead.

One more point, before I offer support for these pomological heresies (or am besieged by emails from angry apple-grafters): I sincerely hope I am wrong. Or, if not wrong, I hope that global warming will mean no test winters and thus few rootstock failures — in which case, my predictions can be laid to rest as the debunked rantings of an amateur. And I will not be offended; I will be tending my apple trees.

But now, the observations that I believe support the previous points:

Observation #1: “Malus ranetka” appears to be a misnomer, unless somewhere there are wild native populations of ranetka apples which have been taxonomically catalogued and named. Moreover, any seedling resulting from a cross between Malus baccata (which is a species, having a wild population with remarkably uniform characteristics and botanical descriptors that have been catalogued) and any other apple, should not be considered a species. It should be considered, like many apples, a seedling hybrid, and unless clonally propagated, it can be expected to vary substantially in both genotype and phenotype from other siblings resulting from the same cross.

Observation #2: Decades ago, Robert Garner, dean of grafting and longtime chief propagator at the East Malling Research Station in England, noted the disadvantages of seedling rootstock: “The chief disadvantages of seedlings are their propensity for variation and the impossibility of predicting the limits of this variability” (emphasis added). Garner went on to suggest that “the unpredictable variability of seedling rootstocks will probably not much longer be tolerated” (The Grafter’s Handbook , 5^th ed., Cassell, London, 1988, 70, 76).

Observation #3: Many seedlings labeled “Malus ranetka” and/or “Malus x ranetka” (suggesting they were open-pollinated seedlings of a species named “Malus ranetka”) have been imported and distributed by APFGA and others from Lawyers Nursery in Montana. Two years ago, in December 2010, I telephoned Lawyers and inquired about the source and the parent material of the rootstock they had been selling as “ranetka.” I received a written reply, the complete text of which is (exact quote): “Thank you for your call. The Malus ranetka source is the Kiev region in the Ukraine. No additional information is available.” (Again, note the use of “Malus ranetka” as an apparent genus/species indicator, although as far as I can discover, no such species exists.)

Observation #4: Bernie Nicolai, an Edmonton apple grower, has on more than one occasion pointed out that the term “ranetka” is a source of semantic confusion. He wrote the following in 2004 (emphasis his, quote taken from NAFEX online correspondence, Feb. 24): “[A]ccording to a friend in Siberia, ranetka is the generic term for ANY seedling that has Siberian Crab as one parent. They usually take the extreme hardiness from the Siberian Crab, and have small inedible fruit, and make great rootstocks in cold climates. The ranetka seeds Lawyers Nursery in Montana sells come from the Ukraine, and are simply Siberian x Domestic apples.”

I believe that Nikolai is correct in his assertion about what “ranetka” means, and what Lawyers sells. But I also believe he misses a key point about hardiness, which is:

Observation #5: Pomologists who study winter hardiness in apples have found that cold tolerance is genetically determined and additive. That is, in any population of apple seedlings, a majority will inherit cold hardiness traits that range between those of the two parents in a distribution curve centered on the mean (Janick, Cummins, Brown, and Hemmut, Apples; from Janick & Moore, Fruit Breeding; 1996, http://www.hort.purdue.edu/newcrop/pri/chapter.pdf, 40). Thus, a few seedlings will inherit traits that are approximately the same as one or the other parent. A very few may be hardier than the hardiest parent or less hardy than the least hardy parent. But almost all of the apple seedlings originating in Kiev — or in any other place — will be less hardy than their hardiest parent, and about half of the seedlings will be less hardy than the average of their parents.

Hardiness is obviously a complex cluster of traits, more complicated than simple cold tolerance, and more difficult to measure. Nonetheless, the implication of the above genetic principle as applied to imported seedling rootstocks is staggering. Consider the following:

Observation #6: Recent plant hardiness maps indicate that the Kiev region of Ukraine — the apparent source of many of our “ranetka” — is generally comparable to U.S. zone 5. If this is accurate, winters in Kiev are much like those in Iowa and southern Wisconsin. They are less severe than those in most of up-state New York and in most of the apple-growing areas of Minnesota. Obviously, many cultivars of apple (and hence, many pollen donors and many mother trees) that survive in Kiev that would not survive in Southcentral Alaska. Moreover, part of the Kiev region, less than a hundred miles south of the city of Kiev, is mapped as zone 6!

Observation #7: In the mid-1980s, Catherine Wright, an experienced plant propagator at the State Plant Materials Center in Palmer, Alaska, performed apple rootstock trials. Her results are published and available on the Internet. Notable findings include these: Survival rate of “M. ranetka”: 48% (more than 40 specimens tested). Survival rate of M. baccata: 61% (more than 70 specimens tested). Most fatalities were on three-year-old plants after the winter of 1985-86, which Wright considered a test winter. As far as I can tell, the rootstocks tested were seedlings obtained either from Lawyers Nursery or from government plant facilities in British Columbia and Washington state, but I have been unable to locate Wright to confirm this.

Observation #8: (complicated but critical) Several years ago, I began to apply principles of “forward funding” to my apple rootstock supply. Instead of immediately cutting and bench-grafting onto the bare root seedling “ranetka” I obtained from APFGA each spring, I simply planted the bareroot whips in pots, let them grow outside for a summer, discarded any weak or obviously problematic plants, and wintered-over the rest by tipping the pots before freeze-up to prevent ice damage. I planned to use them for rootstock the following spring. My thinking at the time was that I would get more reliable and vigorous graft growth on ranetka that had developed well-branched root systems and proven their vigor.

What I got—quite unexpectedly—was a high rate of winter mortality. This was not due to icing, or voles, or moose. It was due to simple winter-kill, resulting from a lack of hardiness. In the spring of 2011 (the wintered-over seedling class of 2010), the survival rate was only 50% on a group of about 20 whips. (Is an exposed-pot a tough winter test? Of course. But so is a winter in which the temperature drops below zero in late October with no snow cover.)

Observation No. 9: In 2011-12 (last winter), I again wintered over a “ranetka” seedling class from the previous spring (2011). In addition, I deliberately wintered-over more than 20 clonally propagated rootstock whips which I obtained by layering two 3-year-old “ranetka” (APFGA class of 2008) that had survived several winters in untended pots in a neglected part of the garden (think “well winter-tested” here). This gave me two additional groups of whips that were physiologically the same age as the new class of “ranetka” seedlings, but were genetic copies of winter-tested survivor “A” and survivor “B” of the class of 2008.

I treated the three groups identically throughout the growing season. As late fall 2011 approached, the pots of the clonally propagated whips were placed next to the pots of the seedling whips, so that winter conditions, especially amount of snow cover, would be identical. Visually, the three groups of plants—the two groups cloned from winter-tested plants and the new seedlings—were quite similar in size and apparent vigor.

In late March of 2012—last spring—I dug the three groups out of the snow and put them into the garage to warm up. The survival rate of the 2011 “ranetka” seedlings? Exactly 15 percent. Only 3 of 20 seedlings had survived. (The group originally numbered 25. Five were discarded for poor vigor before winter even began.) The survival rate of the “ranetka” clones, from my two winter-tested plants? 100 percent. Every single clone survived.

The conclusion is obvious: Not all “ranetka” are created equal. Some are significantly hardier than others, and those hardiness traits can be selected for and replicated through cloning.

Not All Ranetka Are Created Equal

 

Observation #10: One added point of interest with respect to the two clone groups: the members within each group appear, in branching habit and vigor, remarkably similar to each other — that is, all the “A”s look alike, and all the “B”s look alike, as one would expect because of their identical genotype. However, as can be seen in one of the accompanying photographs, when Group A ranetka are compared to Group B ranetka, differences in habit and vigor become obvious. There are also differences in the timing of seasonal adaptation. Last autumn, all the plants in Group A turned color and lost their leaves at exactly the same time. Ditto in Group B. However, leaf fall in Group B was two weeks earlier than in Group A. This suggests that even in the winter-tested plants, there is a significant genetic difference that affects the rate at which the plants transition into dormancy. Obviously, this could have important implications for survival in an early winter.

Since this article is already long, I will not review other details nor belabor the fine points of logic that led to my opening heresies. Astute readers can analyze the observations and compare them to their own experiences and research. If observations or logic are found wanting, readers are welcome — encouraged! — to point out errors.

Meanwhile, I will state as succinctly as possible what I (tentatively) think this all means:

Many of the seedlings we have been calling “ranetka” are not particularly hardy — and are a poor rootstock choice for Southcentral Alaska.

Many of the seedlings we have been calling “ranetka” are hardy — and are a good rootstock choice for Southcentral Alaska.

Some of the seedlings we have been calling “ranetka,” especially from older importations which have been winnowed in the orchards of various apple-growing pioneers, are very, very hardy, have been tested repeatedly by harsh winters, and have for years supported strong grafts and produced good apples. We should identify these, clone them, propagate the clones, call them something like Alaska Pioneer No.1, Alaska Pioneer No.2, etc., make them available for rootstock, and begin to minimize the use of seedling ranetka in Alaska except for experimental purposes or landscaping situations where significant winter mortality is acceptable.

I am not suggesting that the value of “ranetka” to the Alaska orchardist is in any way diminished. Far from it! But it is time to refine our thinking — and our propagation techniques — and to begin the next step in our selection of Alaskan rootstock materials. Cloning individual rootstocks that are productive and have been winter-tested is a means to accomplish this. As Robert Garner pointed out years ago, seedlings are variable; it is impossible to predict the limits of this variability; and seedling rootstocks should not much longer be tolerated!