The following observations regarding replant-tolerant Geneva rootstocks have been made as a result of trials in Washington:
• The high yields from these rootstocks can cause the scion to slide off the rootstock, especially when the trellis fails. Trellis engineering should consider 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 three-fourths of an inch 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, allowing a swing of two feet.
Collapse is likely with loose wires and a crop load of 70 or more bins per acre. Another major flaw of many trellises is that the anchor is placed too close to the end post.
The anchor must be at least as far away from the end post as the height of the top wire. For example, if the top wire is ten feet from the ground, the anchor must be ten feet from the end post.
• When replacing trees in established orchards, the replant-tolerant Geneva rootstocks have consistently demonstrated their superior characteristics.
• The M.26-class Geneva rootstocks are far more productive than M.26, M.7, or M.106, especially as the trees become older with larger-caliper wood. “M.26 class” is the term applied to the rootstocks that will grow significantly more than the large M.9 group.
Unfortunately, M.26 seldom grows larger than M.9 in replant sites and can fail to develop a commercially viable canopy. The large M.9 rootstocks (Nic .29, EMLA, and Pajam 2) have been more reliable in growing an M.26- class canopy than M.26 in fumigated replant sites.
• 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 nonbearing tree and calms quickly with cropping. Tree size is about the same as M.9 T337 after six years in the field.
There has been concern with graft union strength in the nursery. It appears tree caliper is the strongest correlation at this point with the very largest trees showing the very 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-performing replant-tolerant rootstocks. It is very productive. Cold hardiness has not been proven though its siblings have been quite hardy. G.214 is not in commercial rootstock liner production—yet. It is coming. Tree size is comparable to the large M.9 clones.
• G.935 is a very reliable replant-tolerant rootstock. It does not have woolly aphid resistance but does have winter hardiness. G.935 grows vigorously as a nonbearing tree and will crop very heavily. Fruit size may be impacted due to overcropping with the first crop. Tree size is about the same as with large M.9 clones.
• G.210 has shown excellent replant tolerance in its one planting with only one scion variety in one trial site. Data from a Gala rootstock trial at Wapato, Washington, are very encouraging, and growers should consider this rootstock in trial quantities (around 100 trees) to further evaluate its potential. Eastern data show G.210 as an 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. It is difficult to propagate, but some nurseries are expanding production. G.30 is not woolly aphid resistant.
• G.890 has been in limited trials in Washington State, but had an excellent demonstration on a tough replant site with sandy soil. G.890 is M26 class in vigor as indicated in a sandy soil replant trial and a heavy loam soil, fumigated, in Wapato. It has moderate to high resistance to woolly aphid. •