By Tom Auvil, Tree Fruit Research Commission
There is likely not a ‘Best’ apple rootstock. The replant tolerant Geneva rootstocks (G.41, G.214, G.935, G.210, G.969, G.890) are much better than the available standards of Bud 9, Mark, M.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. The precocity of the Geneva rootstocks is better than Mark. 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 and see which scion+rootstock combinations are better in your particular location. The Washington State Tree Fruit Research Commission (WTFRC) is conducting a series of rootstock trials across the state. Here is an update of our preliminary results.
In the spring of 2013 we saw the best return bloom and productivity in blocks with dwarf rootstocks. In contrast bloom and productivity was lowest on seedling and vigorous semidwarf Malling or Budakovski rootstocks.
Malling 9 has some lack of winter hardiness and higher susceptibility to fire blight which has diminished its appeal to some sectors of the Washington apple industry. The M.9 clones will still have a place in the industry in the near future. However, Geneva apple rootstocks have benefits that growers will find advantageous.
|Percent Tree size||Equivalent Malling size||Fire blight Resistant||Replant Tolerance||Woolly Resistant|
|G.11||< 30||M.9 337||Yes||Partial*||NO|
* Partial indicates some tolerance, better than M.9.
** G.16 is not recommended due to its virus sensitivity, lack of replant and woolly resistance.
*** G.222 is a M.9 equivalent in Washington State trials.
**** G.202 is not as productive as other genotypes, and is not shown to be replant tolerant in Washington State.
All the commercially released Geneva rootstocks offer fire blight and phytophthora resistance. The Geneva series replant tolerance has been demonstrated in Washington State trials. An additional benefit is productivity exceeding M.9 and Mark. Several Geneva rootstock genotypes 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’ Geneva products offer fire blight resistance but may have issues to be considered when ordering trees.
Summary from the WTFRC trials to date
G.16 is hyper sensitive to virus infection. Some five and six year old blocks have a steady attrition of 1/2 to 1% of trees that lose vigor and become unthrifty. While resistant to several root pathogens, replant tolerance of G.16 is not the best. G.16 is not a good candidate for use as replacement trees.
G.202 was selected mostly for its ability to root / propagate. It does not offer the crop density nor the replant tolerance. Its fruit size has been noted to be smaller than other rootstocks. It may be suitable for highly precocious scions with a tendency to produce oversize fruit.
G.969 has three evaluation trials underway with the critical first year completed. G.969’s replant tolerance is trending well to date in the more severe conditions of the arid west. For conditions in the arid west, it may only be a bit more vigorous than M.9 337. It is described as a precocious free standing tree suitable for processing varieties. The concept of ‘free standing’ usually indicates horticultural practices that involve heading and delay of cropping. If G.969 is managed in a tall spindle system, it will need a trellis to support the crop.
G.11 produces a smaller canopy volume with lower vegetative. It is usually smaller than M.9-T337. It is very productive and has good fruit size. G.11 is often healthier than M.9 clones in replant conditions. G.11’s productivity reduces its vegetative vigor. G.11 is an excellent rootstock for most planting situations, though use as a replacement tree in non-fumigated orchard sites, especially in sandy conditions, is not recommended.
G.222 is a recent release, G.222 is similar to M.9 in vigor and precocity. It is fire blight, phytophthora and woolly aphid resistant. It is not as replant tolerant as other clones. It is equal to M.9 in productivity and orchard performance. It is rated as a ‘M.26 class’ rootstock in the east, but has not shown that level of vigor in Washington State.
G.41 has become the most widely available of the replant tolerant rootstocks. It has demonstrated winter hardiness and woolly apple aphid resistance. It can grow vigorously as a non-bearing tree and calms quickly with crop. Tree size is about M.9 T337 after 6 years in the field. There has been concern with graft union strength in the nursery. It appears tree caliper is the strongest correlation with likelihood of breaks. The very largest trees have shown the biggest loss. It should be noted that HoneyCrisp on M.26 and Pajam 2 have similar issues of weak unions.
G.214 is one of the best replant performing rootstocks. It is very productive. Cold hardiness has not been proven though its siblings have been quite hardy. G.214 does not have commercial rootstock liner production yet. It is coming. Tree size is about large M.9.
G.814 was released in 2014. It has a more upright growth habit and better fruit size than G.214. G.814 is virus sensitive and will require certified budwood for budding or grafting.
G.935 is a very reliable replant tolerant rootstock. It does not have woolly aphid resistance but does have winter hardiness. G.935 grows very vigorously as a non-bearing tree and will crop very heavily. Fruit size may be impacted due to over-cropping with the first crop. Tree size is about large M.9.
G.210 has shown excellent replant tolerance so far (one Washington planting in Wapato). Growers should consider this rootstock in trial quantities (+/- 100 trees) to further evaluate its potential. Eastern data show G.210 as a M.7 size canopy. The Wapato data, with many trees in the plots, strongly show it is not as big as G.30 which is ‘M.26 class’ in Washington State. Tree size is equivalent to large M.9 in Wapato with Gala as scion.
G.30 is an ‘M.26 class’ rootstock with excellent replant tolerance. G.30 is not woolly aphid resistant. G.30 has limited liner production and some nurseries are discontinuing propagation.
G.890 has limited trials in Washington State, but did well in one tough replant site demonstration with sandy soil. G.890 is ‘M26 class’ in vigor. It has moderate to high resistance to woolly aphid.
One challenge in evaluating the replant tolerant rootstocks is judging the relative vigor / canopy volume of each genotype. In difficult replant sites, the Geneva rootstocks with replant tolerance will out grow more vigorous Malling or Budakovski rootstocks. Only by comparing relative vigor in loam / clay loam soils that have been fumigated and reconditioned for new trees, can tree size be evaluated. In most cases, the appearance of the trees on replant tolerant rootstocks is surprisingly similar in fumigated compared to 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.
“M.26’ class is the next largest group (tree vigor/canopy volume) to M.9. In Washington State, 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 canopy volume unless aggressive pre-plant practices such as fumigation and deep ripping of the soil are employed. M.26 does not fill canopy volume as consistently as M.9. M.26 does not yield as well as M.9 nor have annual cropping as regular as M.9. The ‘M.26 class’ Geneva rootstocks have crop density similar to or better than M.9 or Mark.
Reports from Eastern US trials over-estimate rootstock vigor / canopy volume potential for the arid conditions of Washington State. Several of the genotypes grow very vigorously as non-bearing trees, but lose vegetative vigor as cropping intensifies. G.214, G. 935 and G.210 are reported by Cornell University website as approaching, equal or larger than ‘M.26 class vigor. In Washington State 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 WTFRC trials are ‘M.26 class’ rootstocks in canopy volume with much better precocity.
Another issue in eastern rootstock trials are the planting density of the trials. Most of the trials have been in densities less than 500 trees per acre. Washington trials have been 1300 to 2500 trees per acre. Horticultural practices have been observed to have a significant impact on productivity and overall canopy volume. The more vigorous Geneva rootstocks have high (and more annual) precocity / crop density not observed in any other rootstock families.
Trellis engineering for trees on newer rootstocks
The high yields from these rootstocks can cause the scion to slide off the rootstock, especially when the trellis fails. Trellis engineering should assume 100 bins per acre as the typical load for vertical trellis. Limbs should be attached to wire with five wraps of electrical tape to prevent trunks from sliding down the trellis or twisting in high winds. Post intervals down the row should be less than 40 feet. Wire tension is critical to trellis strength. A properly tensioned wire should not deflect more than 3/4” with substantial force applied. It is too common to encounter trellises where the top of the tree can be pushed a foot or more from center. Collapse is likely with lose wires when the crop load reaches 70 or more bins per acre. Another major flaw of many trellises has been anchor placement too close to the end post. The anchor must be at least as far away from the end the post as the height of the top wire. For example, if the top wire is at 10 feet, the anchor must be 10 feet from the end post.