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The choice of rootstock can have a major influence on the –production and profitability of a new apple orchard, yet little information is available to help growers choose between the various dwarfing rootstocks, says Tom Auvil, research –horticulturist with the Washington Tree Fruit Research –Commission.

Auvil said some nursery catalogues group all the Malling 9 clones together with Budagovsky 9 in the smallest rootstock category, and many classify M.26 as a larger rootstock.

Replant disease, which can occur when any tree fruits are planted on old orchard ground, reduces tree growth even when the ground is fumigated. Trees on Malling 26 are particularly susceptible to replant disease and can be weaker than trees on M.9 in a replant situation, Auvil said.

"In practice in the Northwest, especially in replant sites, M.26 is an underperforming rootstock." Some M.9 clones, such as EMLA and Nic 29, have proven to be stronger than M.26 in research and grower trials, Auvil reports, and tend to produce better crops of larger fruit.

"I think the biggest challenge for us is to get comfortable with the idea that M.9 rootstocks can pretty consistently grow as well as M.26, but they have better crop density, better fruit size, and tend to have fewer issues with replant disease than M.26."

Dr. Terence Robinson, horticulturist with Cornell University, New York, said M.26 is one of the most susceptible rootstocks to replant disease and is often a disappointment to growers in the western United States. "They expect it to be bigger and plant at a –little wider spacing than M.9, and it’s worse," he said. "In replant sites, M.26 is very poor overall."

Comparison

Auvil has compiled a table comparing the attributes of dwarfing apple rootstocks, based on information from rootstock trials and nursery catalogues. He classes Mark, Bud.9 and M.9-Fleuren 56 as the smallest rootstocks, producing a tree about 25 percent of the size of seedling. Trees on Bud 9 have proven to be –consistently smaller than trees on M.9 at maturity, he reports. They grow nicely as nursery trees, but tend to stop growing after they crop.

M.9-Nic.29 is the most vigorous of the dwarfing rootstocks, at about 45 to 50 percent the size of seedling. In a cropping orchard, it will generate nearly double the amount of one-year wood as Bud.9, yet still provides the benefit of good cropping and large fruit.

M.9-337, the industry standard, is intermediate at about 35 percent of seedling, he noted, and doesn’t always fill the canopy in poor soils.

"So, we have considerably weaker and considerably stronger rootstocks compared to M.9-337, and it would be good to understand when and where to use those," Auvil said. "If you go out and make a replant investment using M.9, there’s probably less risk going with the more vigorous M.9 –selections.

"If you’re not comfortable that M.9 is consistently outperforming M.26 in north central Washington, go ahead and plant the trees on M.9-Nic.29 or M.9-EMLA."

However, Auvil believes that as growers gain confidence in growing young orchards aggressively, they will migrate towards the weaker –rootstocks, such as M.9-Fleuren 56, Mark, and Geneva 41, and they will reserve the more vigorous rootstocks for special problems, such as a low-vigor scion (e.g., Braeburn, Honeycrisp, or Jazz), a low-vigor soil, or a combination of the two. In good soils, the vigorous M.9 clones are –generally too strong for vigorous scions (e.g., Gala or Golden Delicious) and will be difficult and expensive to manage over time, Auvil warned.

True colors

While people commonly believe that the vigor of the rootstock will influence how the trees grow in the first two to three years, Auvil said it’s not until years five through ten that a rootstock’s true colors show. Cultural practices can be a bigger factor than the rootstock in growing the canopy large enough to be efficient and commercially viable.

"There’s not any rootstock that will overcome the need to learn how to grow trees aggressively," he said.

The dwarfing disease-resistant G.41 and G.935 rootstocks from Cornell may be tolerant of replant disease, but Auvil said they need further testing in commercial orchards.

G.41 is immune to fireblight. It offers good hardiness as well as resistance to crown rot and woolly apple aphid, and might be a good option for the fireblight-susceptible variety Cripps Pink. A drawback is that it is difficult for nurseries to propagate.

G.935, a larger rootstock that has resistance to fireblight and crown rot, has been classified as the size of M.26 in the eastern United States, but trials in the Northwest suggest that it settles down after the tree crops and might be smaller than the more vigorous M.9 rootstocks.

Recommendations

Auvil said general recommendations for replant sites in central Washington are to fumigate and consider using M.9 rootstocks. Plant trees no more than 3 feet apart. If the planting is on a V trellis with trees leaned in alternate directions, trees should be spaced 1.5 feet apart, so the trees are 3 feet apart on the trellis wires.

It’s the tree count, rather than the –rootstock, that affects early production, Auvil stressed. "It’s a function of tree number, not genetics of the rootstock, and then as we move into maturity, the rootstock makes management a lot more economical if you have the right rootstock and tree spacing for the soil type and the environment you’re growing in."

He recommends using two or three different rootstocks to compare performance and spread the risk. In many cases, however, growers just accept the rootstocks that are available, rather than pushing for the rootstocks they need, Auvil added.

It’s been suggested that organic growers, who don’t have fumigation as an option, might consider using a stronger rootstock, such as M.106 or B.118, to help overcome replant disease. However, there is no data to support the idea that planting trees on a stronger rootstock will help, Auvil said.

"We’ve had some really significant –disasters with Gala on Bud.118, where the yields are poor, fruit size is poor, and they’re in biennial bearing, so they’re growing a relatively small crop of small fruit every other year. The economics of that are amazingly bleak.

"Perhaps with Honeycrisp or Jonagold, or something that grows tremendously oversized fruit, it’s not such a big deal, but we still have to deal with the crop density and annual bearing on some of these larger rootstocks.

"Yield and fruit quality are such key components of the economics that I don’t see going to fewer trees and not fum–igating being an economically viable –solution."

The dwarfing Geneva rootstocks, which may be tolerant of replant disease, show promise for such situations, he said.