Nov 2, 2015
Timing of mechanical pruning impacts labor, fruit quality

When it comes to mechanical pruning, or hedging, Stefano Musacchi puts an emphasis on improving the size of the fruit, as well as its flavor.

Musacchi, associate professor of horticulture at Washington State University, is working with WSU Extension specialist Karen Lewis to conduct trials in commercial orchards to study the effects of hedging, timing and the impact of sunburn on fruit.

The three-year project, which includes mechanical pruning of cherry trials, is funded by the Washington Tree Fruit Research Commission and has just completed its second year.

“We want to compare mechanical pruning with common hand pruning,” Musacchi recently told members of the International Fruit Tree Association.

Musacchi talked about a mechanical pruning trial on Maslin Cripp Pink apples under cloth. He made a presentation during a stop at McDougall and Sons’ Whispering Rocks Orchard in Mattawa, Washington. It is one of three trial locations, along with a Kanzi apple orchard operated by Columbia Fruit Packers and a trial with Cosmic Crisp (WA 38) at WSU’s research orchard in Prosser. Along with apple, Musacchi is testing mechanical pruning in a McDougall-owned Bartlett pear orchard in Monitor, Washington.

“This is a high-density orchard, planted to be a spindle,” he said. “We want to convert it into a fruit wall with a very thin canopy, and taller than what is a common spindle. It’s a perfectly managed switch from a spindle to a fruit wall.”

He described four different treatments at the beginning of the row, comparing winter pruning with summer pruning with a combination of winter and summer pruning mechanically with the control.

“If you think (mechanical pruning) is just to save labor, that’s not the only reason,” Musacchi said. “When you go inside the canopy, you will see different exposure to the light compared to a common spindle – this makes a big difference in the uniformity of the fruit when you harvest one major. That’s one reason you want to prune mechanically and make (the canopy) really thin.”

When do you prune?

“You can decide to convert an orchard with a winter pruning if you want to be very aggressive. It can have a dramatic decrease in the crop when you’re cutting a lot of bud. Mechanical pruning is really good to minimize or to fix a blind wood issue.

“It’s important in this case because we’re working to find when we prune in the summer, when you have 12 leafs during flower bud time to enhance flower bud induction. It more or less corresponds to 12 leaves, beginning in June in this area.

“We have another trial in another block where we’re trying a timing of 12 to 20 leaves to expand the timing for summer pruning. It’s the reason to prune in the summer – when there’s ethylene production within thin trees. When you’re doing pruning in the summer you’re doing the same thing. You’re usually pushing back vegetation and minimizing blind wood. It allows the buds that are dormant to swell. Every time you prune mechanically you are pushing back close to the stem.”

Musacchi likes to grow the tree a little taller than normal.

“With a thinner canopy you can go three feet higher than a standard orchard due to the volume of the canopy and the light penetration we have,” he said.

When pruning mechanically, “you can see immediately after cutting you have a new bud swelling, producing two more shoots. It’s really important to push back vegetation at the same time you’re producing a new bud for next time.”

Mechanical pruning alone “doesn’t work,” Musacchi said. “You have to make some integration (with hand pruning).”

Shade cloth used in the Whispering Rocks trial helped with canopy development, reducing sunburn on the fruit, Musacchi said.

“The company did a good job in putting in a net. If you summer prune without netting or cooling, you will have sunburn everywhere. Diffused light minimizes the problem of sunburn.”

Bending shoots is an effective technique to help fill gaps in the canopy, Musacchi said.

“You need to grow different branches to fill out the space,” he said.

A certain amount of fruit damage – 6.5 percent in the case of the Washington trials – is to be expected using mechanical pruning methods, Musacchi said.

“With a really good summer pruning, some fruits are wounded,” he said.

“I think we already have a nice level of yield,” he said. “If we expose fruit to the light they are sweeter compared to fruit developed in the shade. I’m thinking we have to look not only to maximize yield but to maximize quality, because quality is the key issue.

“We had a dramatic reduction in yield – more than 10 percent – with winter pruning a lot of buds. Summer pruning was similar or higher compared to the control. The fruit size was a little better. We had a really good result last year on summer pruning. There was some reduction in brix and the acidity of the fruit.

“Maturity was really different,” he said. “Summer-pruned trees show less levels of ripening because they’re exposed to the light until June. They had really good color.”

With more apples being grown on dwarfing rootstock and summer pruning becoming more commonplace, Musacchi expects mechanical pruning approaches to take hold on a wider basis.

He stressed that when shoots and leaves are removed during mechanical pruning, it reduces the total amount of carbohydrate in the tree, but also exposes the fruit to more light. A fruiting wall is typically only 2 feet deep. Narrow canopies can be very efficient, Musacchi said.

Hand pruning is needed immediately after hedging because the vertical blades of the machine can’t cut across vertical branches, or branches oriented parallel with the tree row. Upright or pendant branches have to be removed by hand, but the cost is still less than if the orchard was fully pruned by hand.

Mechanical pruning on angled canopies does not work well, Musacchi said. The hedger can only prune the outside of the trees and not the wood in the middle of the V, so the effect is minimal.

“It doesn’t justify the cost of running the equipment,” he said.

Gary Pullano


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