There is likely not a “best” apple rootstock, though the replant-tolerant Geneva rootstocks (G.41, G.214, G.935, G.210, G.30, G.890) are much better than the available standards of Budagovksy 9, Mark, Malling 9 clones, M.26 and the semidwarf rootstocks.
In replant situations, it is common to have lack of vigor contributing to an undersized, low-yielding orchard. The most impressive trait of the replant-tolerant Geneva rootstocks is their combination of productivity and vigor.
When the precocity of the replant-tolerant Geneva rootstocks is combined with the ability to grow a canopy, exceptional yields have been generated.
Experience will show which scions will do better with which rootstock in a given site. It is likely that particular scion-rootstock combinations will demonstrate superior results. The best practice is to plant three or four of the replant-tolerant, especially the woolly apple aphid resistant Geneva rootstocks with each scion in each site to gain experience to show which combinations are better.
As the spring of 2013 demonstrated, return bloom and productivity in general are better in blocks with dwarf rootstocks, and worst in blocks with seedling and vigorous semidwarf Malling or Budagovksy rootstocks.
Lack of winter hardiness and susceptibility to fire blight has diminished the appeal of Malling 9 in some sectors of the Washington apple industry. The M.9 clones have a place in the industry for the near future, but Geneva apple rootstocks have benefits that growers will find advantageous.
All the commercially released Geneva rootstocks offer fire blight and phytophthora resistance. Replant tolerance has been demonstrated in trials in Washington State with an additional benefit of greater productivity than M.9 and Mark. Several Geneva rootstocks have demonstrated winter hardiness and woolly apple aphid resistance.
In addition to the replant-tolerant Geneva rootstocks, there are a few commercially released Geneva rootstocks that offer advantages over other commercially available rootstocks. These other rootstocks offer fire blight resistance but may have issues to be considered when ordering trees.
• G.16 is hypersensitive to virus infection. Some blocks five or more years old have a steady attrition of ½ to 1 percent of trees that lose vigor and become unthrifty. While resistant to several root pathogens, G.16 does not have the best replant tolerance and is not a good candidate for replacement trees.
• G.202 was selected mostly for its ability to root, which makes it easier to propagate. It does not offer the crop density or the replant tolerance that other Geneva rootstocks provide, and its fruit size has been noted to be smaller than with other rootstocks. It may be suitable for highly precocious scions that tend to produce oversized fruit.
• G.969 has not been trialed in the western United States. Its replant tolerance is unknown in the more severe conditions of the arid West. It is described as a precocious freestanding tree suitable for processing varieties. It could also be suitable for weak soils and/or weak-growing scions. The concept of “freestanding” usually indicates horticultural practices that involve heading and delay fruiting. If G.969 is managed in a tall spindle system, it will need a trellis to support the crop.
• G.11 produces trees with a smaller canopy volume and lower vegetative vigor, usually smaller than M.9-T337. It is a very productive rootstock and produces good fruit size. G.11 is often more healthy than M.9 clones in replant conditions. The productivity of G.11 reduces its vegetative vigor. It is an excellent rootstock for most planting situations, though not for replacement trees in unfumigated orchard sites.
• G.222 is a recent release that is similar to M.9 in vigor and precocity. It is resistant to fire blight, phytophthora, and woolly apple aphids. It is not as replantolerant as other clones. It is equal to M.9 in productivity and orchard performance. G.222 is rated as an M.26-class rootstock in the East, but has not shown that level of vigor in Washington.
One challenge in evaluating the replant-tolerant rootstocks is judging their relative vigor and canopy volume. In difficult replant sites, the Geneva rootstocks with replant tolerance will out-grow more vigorous Malling or Budagovksy rootstocks.
Tree size can be evaluated only by comparing relative vigor in loam or clay loam soils that have been fumigated and reconditioned for new trees.
In most cases, the appearance of the trees on replant-tolerant rootstocks is surprisingly similar in fumigated and unfumigated plots. In all replant trials, all the fumigated plots initially out-yield the unfumigated plots, regardless of rootstock genotype. Over time, the replant-tolerant Geneva rootstocks in unfumigated plots out-yield M.9, Bud 9, or M.26 in fumigated plots.
The next larger group of tree vigor/canopy volume than M.9 is termed “M.26 class.” In Washington, M.26 has typically not been larger than the vigorous M.9 clones (M.9 EMLA, M.9 Nic 29, or Pajam 2).
In replant sites, M.26 often fails to fill its allotted space unless aggressive preplant practices such as fumigation and deep ripping of the soil are employed. Malling 26 does not fill canopy volume as consistently as M.9, nor does it yield as well as M.9 or crop as regularly as M.9. The M.26-class Geneva rootstocks have crop density similar to or better than M.9 or Mark.
Reports from eastern U.S. trials tend to overestimate Geneva rootstock vigor/canopy volume potential for the arid conditions of Washington. Several of the Geneva rootstocks grow very vigorously as nonbearing trees, but lose vegetative vigor as cropping intensifies.
G.214, G.935, and G.210 are reported by Cornell University website (http://bit.ly/1kyosm6) as approaching, equal, or larger than M.26 in vigor. In Washington, these Geneva rootstocks are comparable to large M.9 in fumigated or new-to-perennial-crop sites.
G.890 and G.30 are rated by Cornell as M.7 or larger, but in trials conducted by the Washington Tree Fruit Research Commission, they are M.26-class rootstocks in canopy volume with much better precocity. •
Geneva apple rootstocks are demonstrating the ability to increase reliability of growth in new orchards. This trait can change the perspective of performance and give growers the opportunity to adopt different production systems and use different nursery products.
The current general practice for orchard establishment is to plant a feathered tree with at least five small-caliper branches well distributed around the trunk. Often the best performing of these trees are 1/2-inch caliper due to reduced transplant shock and fewer broken graft unions.
The replant-tolerant Geneva rootstocks (G.41, G.214, G.935, G.210, G.30, and G.890) have the ability to grow vigorously and establish a very productive canopy from a whip or unbranched tree when planted. A trial with Fuji at a Vantage, Washington, orchard demonstrated the growth and productivity of these rootstocks when bench-grafted and planted in place in the orchard.
The increasing reliability of high-performing rootstocks reduces the risks of trying smaller and more delicate plant materials when establishing new orchards, especially in replant sites.
In California, several nurseries are growing containerized plants for grapes, citrus, stone fruits, and almonds. The plants are initiated in a tissue culture lab and budded in the greenhouse. One of the big advantages to growers is not losing a significant portion of the roots when they are dug up for transplanting in the orchard.
One of the logistical or production issues is learning how to handle plants that are leafed and growing when planting the orchard, compared to the traditional dormant tree. Some Hood River, Oregon, growers are using containerized nursery stock to establish new pear blocks with dramatic improvement of consistency and quantity of first-year growth.
When contemplating two-leader trees, the idea of using a one-year nursery-grown bench graft on a replant-disease-resistant Geneva rootstock that is a 7/16- to 1/2-inch whip is very interesting. The timeline of deciding which scion to place on the rootstock becomes much shorter.
The ability to better manage the canopy into the form needed for the production system is easier. The tree price in addition to the shipping, handling, and planting logistics are much more favorable than with 3/4-inch trees.
Availability of Geneva rootstocks
All dwarf rootstocks are in short supply. Geneva rootstocks in particular have a long waiting list at some nurseries.
• G.11 is the most widely available Geneva rootstock. It is not suitable for replacement trees, nor should it be considered replant tolerant in Washington State.
• G.41 is the most widely available replant-tolerant Geneva apple rootstock. There is some concern about weak bud unions at the nursery and during orchard planting, but it has not been a problem after establishment, other than the broken union from trellis collapse. Risk of union breakage appears to increase as the size of the nursery stock increases.
• G.935 production is increasing but not likely to increase as rapidly as G.41. G.935 has also had some union breakage but, as with G.41, cultural practices in the nursery may be the leading issue. Its only flaw is lack of woolly apple aphid resistance.
• G.214 is being planted spring of 2014, with some liner availability in 2015. This genotype has exceptional replant tolerance and productivity. It tends to grow a more flat and spreading tree form.
• G.210 and G.890 are new releases that do not appear to have as many nurseries licensed to produce them as G.41. Both of these rootstocks may propagate more easily than G.41. G.210 may sucker more than some growers want to manage, but the replant and propagation traits are good enough to encourage trial planting and consideration for wider use.
G.210 and G.890 may allow the establishment of two or three leaders per tree stump at 20 to 30 inches between leaders down the row, needing fewer stumps per row. There is some risk of needing to manage excess vigor with these more vigorous, replant-tolerant rootstocks.
Historically in replant-challenged sites, this is not a risk but an opportunity. These rootstocks do not have the number of trials supporting the data as G.41, G.935 or G.214, but the performance in replant sites make them very worthy candidates for grower trial, especially for growers interested in multileader trees at sites that can not be fumigated.
• G.30 is a very high-performing rootstock in the orchard but somewhat miserable to manage in the nursery. Most of the high-volume liner nurseries do not produce G.30, and several finished-tree nurseries avoid it.
• G.969 will be trialed for the first time in Washington State in 2015. Its vigor class has been moved smaller over the last three years by New York researchers. Currently it resides in the M.26 class, but may slide towards the large M.9 category in Washington State. G.969 is in commercial production, and roots are planted to be budded in 2014.
• G.202 and G.16 should be avoided. G.16 has virus sensitivity issues, and G.202 is available only because it propagates easily. It does not have productivity nor replant reliability in Washington State. They may be available, but should be used only if desperate for fire blight resistance.
Production and thus the availability of all the Geneva rootstocks is rapidly changing. Checking with the various licensees regularly will keep interested growers in the loop on availability.
The website http://www.cctec.cornell.edu/plants/index-test.php#apple-rootstocks has a list of Geneva rootstocks in commercial production. When a rootstock is selected from the website list, it will show the licensed rootstock liner nurseries for the rootstock. These lists were not complete as the magazine went to press, but were expected to be updated soon.