For years, sweet cherry researchers in the U.S. and Canada have been comparing the performance of rootstocks, but through the efforts of a regional research project, they have taken the studies to new levels by factoring in new training systems, cultivars and locations.
The trials, coordinated across North America, have been conceived and carried out under the auspices of the NC140 Regional Rootstock Research Project, which was partially funded by the International Fruit Tree Association.
At least one of the goals: the ability to more precisely recommend to growers what should be planted where and how it should be grown.
Michigan State University horticulturist Greg Lang presented preliminary results from an eight-year trial during the International Fruit Tree Association annual conference in February in Napier, New Zealand.
The study is intended to evaluate the performance of a dwarfing, semi-dwarfing and semi-vigorous rootstocks with three varieties in distinctive canopies and in different locations. All of the varieties are planted on Gisela 3, 5 and 6 rootstocks, in uniform spacings, and in each of the four canopy architectures under review: tall spindle axe, Kym Green Bush or KGB, super slender axe (also called super spindle axe) and upright fruiting offshoots or UFO.
The varieties and locations are:
—Skeena in Summerland, British Columbia, and Kentville, Nova Scotia.
—Benton in Clarksville, Michigan.
—Regina in Hudson Valley and Geneva, New York.
So far, the results of the eight-year trial showed vigor varied by site, as expected, Lang said. One thing became clear though: In high-density systems, root competition controls vigor, too.
“I never like to talk about what is the best system in trees that are two or three or four years old,” Lang said, noting that developing canopies are still dealing with issues of shade and light penetration. “We will take these through several more years.”
For the first two years of the trial, the Michigan and British Columbia trees were performing similarly, but weather issues in Michigan hampered cropping, pushing tree vigor. Generally, though, vigor was very consistent across training systems, he said.
Based on the data from the uniform trial spacings, the research has shown that spacings should be varied depending on the combination of system and cultivar. Lang announced new spacing recommendations by system and rootstock, ranging from 2.5 by 9.1 feet for Gisela 3 rootstocks in a super slender (2,181 trees per acre) to 6.5 by 13 feet for Gisela 6 in a KGB system.
Lang plans to release detailed yield data from trials later this year. However, he said, after eight years, the highest projected cumulative yields, based on actual yields per tree and the suggested tree by row spacings, were as follows:
—For Skeena in British Columbia, UFO on Gisela 3 and Gisela 5.
—For the non-irrigated Skeena plot in Nova Scotia, TSA on Gisela 5.
—For Benton in Michigan, UFO on Gisela 3.
—And for Regina in New York, UFO on Gisela 5 and TSA on Gisela 5.
The trial will continue for two more years to confirm that the current yields on mature trees can be maintained.
Sweet cherry production continues to increase globally, with Washington and Chile leading the way, each with record production last year, and that growth is likely to continue as more growers move to newer rootstocks in high-density systems.
“We have this game-changing plant material that some growers have done well with, some growers have struggled with. We need to figure out how to utilize the genetics of those rootstocks, the traits — the very dwarfing, very precocious or even very vigorous and very precocious, in different kinds of training systems — as we have more and more pressure, labor efficiencies moving toward mechanization and higher and higher bars for fruit quality,” Lang said.
Lang provided a recap of what researchers have learned about the four key cherry systems in previous studies with Gisela rootstocks:
Tall spindle: The challenge with the system that is probably most typical and been around the longest (of the four reviewed) is the complexity of the three-dimensional canopy with different branch lengths, ages, orientations, light exposures and populations of fruit, which means it takes the most time to prune and harvest and has the least uniformity of light distribution and shade, which can affect uniformity of ripening and fruit quality.
The density ranges from 600 to 1,100 trees per acre, lower than other systems because of the vigor of the rootstock. “One of the issues pretty much with all central leader trees is a lot of heavy growth in the top, that we’re always trying to fight and maintain our good fruiting area down below,” Lang said.
Best management practices are to plant a nursery tree, cut off a couple of odd branches and select the wanted buds, removing the lowest, in hopes of stimulating six to 10 branches in the first year of growth.
“Because the more branches we get, the more moderate each of those branches is, the more the tree starts out in balance,” he said. In the second year, prune the last 15 to 20 percent tip of each shoot, to provide more leaf area and even get a little bit of nonspur basal flower bud formation, then make dormant pruning cuts going into year three to maintain good light penetration.
KGB: The architecture of the tree begins to simplify in a KGB system, with vertical leaders that are pruned to maintain some level of light. Canopy is a simplifying mix of fruiting leaders having relatively uniform lengths, orientations and fruit populations (spurs) and somewhat diverse light exposures.
The multiple narrow leaders are an advancement, compared to the TSA system, for simplifying a 3-D canopy, plus are a way to diffuse vigor and avoid the issue of always fighting vigor at the top of a single leader tree, Lang said.
The tradeoff: less precocity because of the wholesale pruning required in the first two years. In addition, the researchers learned they had to stake the leaders — a requirement tied to the growth habit of Gisela rootstocks, which is an insight that was not anticipated at the outset of the trial, Lang said.
“That raises the question: Can we even deal with this kind of training system, where we’re on weak rootstocks, soils or climates?” he asked. “We simply need to balance tree spacing and the number of leaders with the vigor of the rootstock or training system.” Density is about 500 to 825 trees per acre.
SSA: The system features a central leader with short, lateral branches, formed new every year. “Basically, you renew the entire tree very year,” Lang said.
The canopy is a simplified mix of fruiting branches having uniform lengths, ages, orientations and light exposures, and because they’re all about the same size, fruit populations.
“You’re fruiting mostly on these basal flower buds, which gives you the best fruit population. But it’s also a limited population. So to get yields up, you have to plant these trees at a very high density.” Densities range from 1,145 to 1,930 trees per acre.
UFO: The extremely narrow fruiting wall not only allows the crop to be picked from one side, but has the best light uniformity and most adaptable canopy for mechanization of tasks such as hedging and flower thinning, and since (like KGB) it distributes vigor with multiple leaders, it is adaptable to rootstocks that range from dwarfing to vigorous by simply proportionally altering the tree spacing and leader number.
By contrast, the other two-dimensional fruit wall systems, SSA, is absolutely unsuitable for vigorous or semi-vigorous rootstocks since the single leader has no way to diffuse vigor. Densities range from 850 to 1,620 trees per acre.
Because the real challenge in utilizing the Gisela rootstocks early on was their precocity and high productivity, crop loads got out of balance for many growers Lang said. Many complained that dwarfing cherry rootstocks led to dwarf fruit.
“As physiologists, we had proven that was not the case, but we had not shown how to utilize traits effectively to maintain good crop loads and maintain good fruit quality,” he said.
With the next round of research, there’s still more to learn, but Lang noted that growers who want a first-hand look at one of the coordinated trials will have an opportunity during the IFTA summer tour in British Columbia in late July. •
—by Shannon Dininny