This apparatus, located at Utah State University, measures the force required to break each graft. The trees were tested after a year of growth and treatment with a plant growth regulator as part of an experiment to assess ways to improve graft strength. <b>(Courtesy Stuart Adams, Willow Drive Nursery)</b>

This apparatus, located at Utah State University, measures the force required to break each graft. The trees were tested after a year of growth and treatment with a plant growth regulator as part of an experiment to assess ways to improve graft strength. (Courtesy Stuart Adams, Willow Drive Nursery)

Keeping up with continued demand for new, disease-resistant, dwarfing rootstocks has been an ongoing challenge for nurseries in recent years, but one of the most-sought cultivars, Geneva 41, creates additional challenges for nurseries because it’s prone to graft union breaks.

“We’ve just had a tough time. If we get a big wind event or any time there is a lot of handling of the trees, they just break easily at the graft union,” said Stuart Adams, who runs research and development at Willow Drive Nursery in Ephrata, Washington. “So, we wanted to do some research to see what was going on.”

Adams, who was pursuing his master’s degree at Utah State University, collaborated with scientists from Cornell University and the U.S. Department of Agriculture program in Geneva, New York, that developed the rootstock, to test a variety of rootstock and scion combinations at Willow Drive Nursery, which largely funded the research, to assess how graft strength varies.

Some of the standard rootstocks, such as Budagovsky 9 or Malling 9, don’t really have breakage issues unless there is an incompatibility with the scion, Adams said, but G.41 can experience breakage with many varieties.

“When we break MM.106 (Malling-Merton 106), you almost can’t tell where the union is because it’s so fibrous, but these dwarfing rootstocks always break at the union,” he said.

Problems with weak graft unions are not new to the industry, but Gennaro Fazio, the apple breeder who heads the rootstock program at the USDA in Geneva wanted to find out the best way to address the graft weakness many nurseries were reporting on G.41. It’s just a particularly brittle rootstock, he said, and prone to breakage when paired with a brittle scion.

The problem isn’t relegated to just traditional nurseries. As more growers begin operating their own nurseries to speed the process of planting new trees, the problem is likely to gain more attention.

The hidden truth

To understand the underlying cause of the weakness in certain grafts, the researchers began sending graft unions in for CT scans. The results show the fibers that grow to form the transition from rootstock to scion, in some weak cases, are not as well organized as seen in stronger grafts, Fazio said.

“What we’re learning is that there are a few molecular reasons for that, one being hormone confusion,” Fazio said.

Hormones are the tree’s main means of communication, but if there is a mismatch between how the rootstock and scion interpret or produce certain hormones, the message — in this case to build a strong union — can get lost in translation.

So, could a hormone application help the graft union grow stronger?

That’s what Adams set out to test, using a variety of commercially available plant growth regulators, PGRs, on G.41 grafted with Gala and Scilate, known under the brand name Envy. Scilate tends to be more of a problem, because it gets big and branchy quickly and is more likely to catch the wind than spindlier Honeycrisp, Adams said.

The experiment cast a wide net in terms of chemicals and techniques because it was the first attempt to look at this issue. “This research was very early, and we just took a stab at it,” Adams said. “The trick of it all is getting the PGRs to the part of the plant you want; it turns out to be really difficult.”

Trees grafted in the fall were treated with PGRs the following summer and harvested for testing that fall.

The chemicals that showed promise included benzyl adenine and abscisic acid, 1-napthaleneacetic acid and prohexadione-calcium. Adams also tested a variety of application methods, including graft paint and leaf spray, on the trees grafted and grown at Willow Drive Nursery in 2014 and 2015.

After the growing season, all the trees were then cut and taken to Utah State University for strength measurements.

Each tree was broken in a three-point bend apparatus, which recorded the force required to finally snap the stem.

The hormones chosen aimed at different means of strengthening the graft union, including encouraging growth directly at the graft and slowing down scion growth since a smaller caliper scion would be more flexible and put less stress on the graft.

Although that could increase the graft’s strength, it’s a trade-off nurseries may not want to make, Adams said. Moreover, the gains in strength he found from different treatments in the experiment weren’t large enough to make a meaningful difference in the nursery.

“Maybe there is an easier way to get the PGR to the graft union or a different application rate that would work. You could spend years and years exploring this. … In the end, what we concluded is that while the PGRs are promising, in my opinion, it’s trying to mess with how G.41 grows,” Adams said. “It’s just a naturally brittle rootstock, and if you can baby it and get it into an orchard, it’s going to be fine.

Handle with care

So, workers at Willow Drive have adopted a more obvious solution: putting the baby trees on a temporary trellis and ensuring careful handling to keep them from flopping too much and stressing the new union. That’s the best strategy for now until more research is done to optimize the use of hormones to increase graft strength.

“Sometimes, you do all this research and you find the old-school solution,” Fazio said. That’s good enough for Willow Drive at this point, but Fazio is planning a new round of experiments to further understand the genetic basis for graft union strength, which could potentially lead to new strategies, starting this summer, he said.

And Fazio says that this weakness in G.41 rootstocks is actually a result of one of its strengths: supporting vigorous scion growth.

The rootstock is designed for trellised trees that have lots of support, and the young trees are no exception.

A recent trial of Geneva rootstocks in Maryland lost 28 trees to graft breaks because a wind storm came through before the new trees had been tied to the trellis, Fazio said.

“The trees of the same scion were growing so much better on the G.41 rootstock, they were creating bigger branches that become like sails and catch the wind,” Fazio said. “Something that’s a good thing in the orchard; it’s not a good thing in this weak graft union situation in the nursery.”

Adams agreed. He said that orchardists want G.41 because of the high yield and disease resistance it offers and the graft weakness is really just a concern for the nursery, where the young trees have to be handled.

“G.41 puts out a tree pretty quickly and puts its energy into growth before it develops a fibrous graft union,” Adams said. “If you can get it into an orchard and get it stabilized on a good trellis, it’s going to be a good tree, it’s just a headache for the nursery.”

And the grafts do grow stronger over time as well. Fazio said that they haven’t seen many problems after two years and none after four years. •

– by Kate Prengaman