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Malling 9 rootstocks, shown in layer beds at Willow Drive Nursery, Washington, have proven to be the most efficient rootstocks in most regions. The various clones differ considerably in size, with 337 among the smallest and Nic.29 the largest.

Malling 9 rootstocks, shown in layer beds at Willow Drive Nursery, Washington, have proven to be the most efficient rootstocks in most regions. The various clones differ considerably in size, with 337 among the smallest and Nic.29 the largest.

This is the second of a series of articles covering all aspects of planning and establishing a competitive orchard.

by Tom Auvil, Washington Tree Fruit Research Commission

Size-controlling rootstocks that offer resistance to pests and diseases are here now, with some commercially available for planting or soon to be commercialized.

Rootstock trials conducted by the Washington Tree Fruit Research Commission in collaboration with Dr. Gennaro Fazio, apple rootstock breeder with the U.S. Department of Agriculture in Geneva, New York, indicate that big gains in orchard efficiency, together with a reduction in risk, are possible with the Geneva rootstocks. Overall, the elite Geneva rootstocks have higher yields, can mitigate pests and diseases, and can tolerate winter freezes. Some genotypes are tolerant of replant disorders.

What follows is an overview of dwarfing apple rootstocks best suited to Washington State conditions, from smallest tree size to largest.

Budagovsky 9 is resistant to fireblight, but in medium- to light-textured soils it may stunt. In head-to-head comparisons, Bud.9 is at least one-third smaller than Malling 9 NAKB-T337 (337 clone). Bud.9 is a very precocious rootstock, equal to Mark in Washington conditions, and should be planted with 2 to 3 feet between trees for vertical or upright trellis systems or 1 to 1.5 feet between trees for V or angled-canopy systems in good soils. Tree spacing should be reduced in weak soils, and this rootstock should not be used at all in light sandy soils (beach sand). A drawback to Bud.9 is the root-suckering tendency that is worse than with nearly any other rootstock.

Mark has found a niche in plant-in-place orchards where rootstocks are planted in the orchard and scion budwood grafted on top. Mark is a dwarfing rootstock with a vigor range between M.9 and M.26. Mark transplants well, and has consistently high take of buds or bench grafts. It heals the graft or bud uniformly, and starts growth very uniformly. Mark grows vigorously as a nonbearing tree and nearly stops vegetative growth when cropping. Scion varieties on Mark have high crop density. Very little blind wood is observed. Repeated yields (consecutive annual yield) approaching 150 bins per acre have been reported. The knock against Mark has been drought sensitivity and a root tissue proliferation that was a concern in the 1980s. At tree spacings of 3 feet in vertical systems or 1.5 feet in V or angled-canopy systems, Mark has done very well, producing annual high yields well above the Washington industry average.

Geneva

Performance of four Geneva rootstocks (G.11, G.41, CG.4214, G.935) in a plant-in-place trial with uncertified (untested for virus) scion wood was competitive with Mark.

G.11 is a bit less vigorous than M.9 337, but has better yield and better fruit size than other M.9 clones. Availability of G.11 is increasing, and the rootstock should be readily available in a year or two. Orders for G.11 should be made today. G.11 is a high-performance dwarfing rootstock, but it does not have woolly apple aphid resistance. In replant sites, G.11 should be planted at 3 feet or less between trees in vertical systems or 1.5 feet in angled-canopy systems.

G.41, also smaller than M.9 337, is resistant to woolly apple aphid and fireblight, and is tolerant of replant problems and cold temperatures. It has done well in most unfumigated trials. G.41 was released for commercial propagation in 2004, and commercial volumes should be available in 2011. G.41 survived winter cold better than M.9 or M.26 in New York trials. G.41 is a high-performance dwarf rootstock with cold hardiness, disease (fireblight, phytophthora, and replant) tolerance, and insect resistance. It will be most successful in higher density plantings with 3 feet or less in-row spacing in vertical systems, or 1.5 foot in-row-spacing in angled canopies.

CG.4214 is slightly larger than M.9 337 and very productive. The “C” indicates a Geneva rootstock that has not been released for commercial propagation. It is hoped that CG.4214 will be released in the very near future. CG.4214 is the best-performing rootstock in unfumigated trials in Washington State, and it has also done very well in fumigated trials. CG.4214 has been evaluated with Gala, Fuji, and Honeycrisp as scions. Resistance to woolly apple aphid needs confirming. Propagation of CG.4214 is easier than G.41. CG.4214 is the best of the best in Washington trials.

G.935, in Washington State trials, is comparable to the large M.9 class (M.9 EMLA, M.9 Nic 29, Pajam 2) for rootstock vigor. G.935 responds to cropping, slowing its vegetative growth to a near standstill. It is vigorous as a nonbearing tree. G.935 is very productive and resistant to fireblight. G.935 is suitable for use in replacement trees to fill those holes in the orchard created by fireblight loss. G.935 is not resistant to woolly apple aphid. There has been some concern with fruit size from G. 935 trees. It is apparent that with G.935, fruit size is a function of fruit numbers, but when the appropriate fruit-to-leaf relationship is established, fruit size on G.935 is similar to reasonably cropped M.9 clones.

M.9 family of rootstocks has been proven to be the most efficient in most growing regions in the world. The combination of crop load, fruit size and annual bearing has increased the average yield in commercial apple orchards by 40 to 60 percent in every region where M.9 is now the dominant rootstock. The biggest drawback to M.9 is its susceptibility to fireblight. Also, there is a large range in vigor, varying from 25 to 45 percent of the size of seedling. M.9 337 is about 30 percent of seedling. M.9 Nic.29, Pajam 2, and M.9 EMLA are on the larger end of the spectrum, with M.9 Nic.29 the largest. M.9 Nic.29 will develop a larger tree than M.26. M.9 Nic.29 is the second-worst root-sucker producer after Bud.9. However, M.9 Nic.29 tends to remain vigorous after the canopy is grown and full crops are being produced. Vegetative growth management is likely to be a challenge with M.9 Nic.29, even with weaker scions and very sandy soils.

M.26 has several traits that reduce its value as a rootstock under Washington conditions. All of the weaker scions, like Honeycrisp or Braeburn, do better on vigorous M.9 rootstocks than on M.26. The rootstock has inconsistent vigor and less annual cropping than the more dwarfing rootstocks. M.26 is susceptible to a range of root rots that M.9 is not. M.26 and M.9 are both highly susceptible to fireblight. In most comparisons, M.9, and especially the larger-size clones, tend to be significantly larger trees than M.26. When adding all of the M.9 benefits togethermore reliable growth, phytophthora resistance, better annual cropping, and more consistent (larger) fruit sizeM.9 is clearly a better choice and a more reliable investment than M.26.

Many of the new scion varieties are susceptible to fireblight. Tree loss due to fireblight is a plague in some locations. Fireblight-resistant rootstocks will reduce tree loss dramatically, and will allow a more merciful management strategy.

Rootstocks larger in size than M.9 are less annual in production. Biennial scions, such as Cameo, Fuji, and Honeycrisp, will be even more biennial on semidwarf rootstocks.

Higher-vigor rootstocks such as Bud.118, M.7, or M.106 will not mitigate replant disorders. All semidwarf rootstocks have yield and/or crop efficiency problems that make them slower to come into production and yield less.