—story by Kate Prengaman
—photos and video by TJ Mulllinax

Washington State University tree fruit physiologist Stefano Musacchi stub-cuts newly planted WA 38 trees in May 2022 at the Sunrise research orchard in Rock Island. He left the feathered nursery branches untouched on other trees, to compare how the different pruning and training approaches impacted the trees’ productivity and propensity for blind wood development. (TJ Mullinax/Good Fruit Grower)
Washington State University tree fruit physiologist Stefano Musacchi stub-cuts newly planted WA 38 trees in May 2022 at the Sunrise research orchard in Rock Island. He left the feathered nursery branches untouched on other trees, to compare how the different pruning and training approaches impacted the trees’ productivity and propensity for blind wood development. (TJ Mullinax/Good Fruit Grower)

While WA 38 trees require no thinning and produce high-quality fruit that stores extremely well, the cultivar is also prone to blind wood.

Many growers clamored to crop the new variety in its inaugural seasons, chasing sky-high prices, but that early crop load came at a cost: development of blind wood. 

In the few years since then, the industry has learned a lesson: “It’s a tree that you can’t crop until the third leaf,” said Stefano Musacchi, endowed chair for tree fruit physiology at Washington State University. “You need one more year to build your tree.”

To show growers how their pruning approaches matter for WA 38, the Washington Tree Fruit Research Commission funded Musacchi and WSU collaborators Sara Serra, Karen Lewis and Bernardita Sallato, to set up demonstration plantings in which WA 38 trees, paired with different rootstocks, are pruned in different styles from planting to production. 

The conclusions from the project, along with other best practices for WA 38 production, will be published by WSU Extension this year, including a video of the different pruning approaches and resulting tree performance over the three-year study (developed in partnership with Good Fruit Grower). 

To develop this “textbook for WA 38,” the researchers needed years of data from trials and commercial production, said Ines Hanrahan, executive director of the research commission. 

The takeaways from Musacchi’s trial offer several strategies for WA 38 growers to implement as they establish new blocks: Start with stub cuts after planting, rather than retaining the original feathers; score leaders to encourage more branching; and use click pruning to push spur development and keep the fruiting wood close to the trunk.

“This is the natural habit for the variety,” Musacchi said, referring to the tendency for WA 38 to push its crop away from the trunk, clearly visible on this second-leaf tree on M.9 rootstock. That pairing appears to exacerbate blind wood development in this WSU trial planting. (Kate Prengaman/Good Fruit Grower)
“This is the natural habit for the variety,” Musacchi said, referring to the tendency for WA 38 to push its crop away from the trunk, clearly visible on this second-leaf tree on M.9 rootstock. That pairing appears to exacerbate blind wood development in this WSU trial planting. (Kate Prengaman/Good Fruit Grower)

Start with stubs

For this project, in 2022, WSU planted WA 38 on B.9, B.10, G.11, G.213, G.41, G.935, G.969, G.890 and M.9-T337 rootstocks in a single-leader style. Musacchi pruned the trees with either his recommended approach — stub cuts at planting, followed by click pruning in the second year — or a “control” approach that left the initial feathers to grow on the nursery trees, only pruning out large branches and simplifying the tree structure in later seasons. 

The control approach highlights WA 38’s natural habit to push the crop out to the branch tips. It also shows that leaving the larger branches on the tree allows them to compete with the leader, reducing its caliper compared to the trees that were stub cut after planting. 

“What we learned from this is that if you leave the scaffolds from the nursery, you can have a good early crop, but you’ll start to build up the blind wood and you’ll never fill up completely the space at the top,” Musacchi said.

When researchers returned in 2023, they compared the two approaches. Looking at four random branches, they found 10 to 14 centimeters of blind wood in the unpruned branches, depending on rootstock. M.9 showed the most blindness. 

In 2023, he added a third treatment designed to recover the blind wood developing in the control trees, using stub cuts — albeit stub cuts three times longer than those used on the nursery trees in 2022 (to account for the trees’ growth in that time). 

Some rootstocks, such as G.213, showed more response to this “recovery” approach than others, but even in those cases it’s a struggle because some of the branches need to start over. 

“We are losing one year compared to the original click pruning,” Musacchi said. “If you don’t start correctly, to retrofit after one year is already a challenge.” 

WA 38 pruning comparisons. (Source: Stefano Musacchi/Washington State University; Good Fruit Grower illustration by TJ Mullinax and Kate Prengaman)
WA 38 pruning comparisons. (Source: Stefano Musacchi/Washington State University; Good Fruit Grower illustration by TJ Mullinax and Kate Prengaman)

Setting spurs

WA 38 wants to be a tip-bearing apple, but growers need to fight that tendency to keep high-density plantings productive.

Click pruning “postpones the yield by one year, because we are taking out the buds with the click, but we are building the tree,” Musacchi said.

He cuts back all 1-year-old shoots, taking off the flower buds at the tips. Yes, that’s sacrificing crop for this growing season, but when those buds are left to set, the weight of the apples bends the branch down, causing the buds behind to go blind. Cutting them back forces the tree to carry fruit on spurs instead. 

Growers want uniform fruit with consistent maturity, but if a WA 38 tree sets fruit on its shoot tips (or, in Musacchi’s native Italian, brindilla) and along the length of new shoots (what Musacchi calls ramo misto), as well as on spurs, the result will be mixed size and maturity. The spurs bloom a week before those young laterals, and they result in larger apples. Surveys of mature production orchards show that the click-pruning approach pushes most of the fruit production to spurs, which gives the benefit of consistency, while other approaches can yield lots of variability. 

So, how does click pruning keep WA 38 in balance and bearing uniform fruit?

It’s best to think of click pruning as a cycle. The first year’s stub cuts usually elicit a response of two vegetative shoots, giving the pruner options for which positions to keep. 

“It’s better to have too many than too few,” Musacchi said. He selects one to cut short again and another to leave longer — but still cut back — to crop. 

The process also includes common pruning practices, including simplifying branches, though a flat cut is only used to remove large, competitive branches. Working up the young trees, he stubs the 1-year-old shoots and scores the trunk just above buds he wants to see break. He holds the pruners at an angle, only rotating around three-quarters of the trunk, which maintains stability. 

As the trees enter their third leaf this season, it’s the first year that both the click-pruned and control trees will carry a crop. WSU plans to hold field days at the trial block, so growers can see for themselves how the different rootstocks and pruning treatments perform. •

Click pruning WA 38 (Source: Stefano Musacchi/Washington State University; Good Fruit Grower illustration by TJ Mullinax and Kate Prengaman)
WA 38 pruning comparisons. (Source: Stefano Musacchi/Washington State University; Good Fruit Grower illustration by TJ Mullinax and Kate Prengaman)
Washington State University tree fruit physiologist Stefano Musacchi stub-cuts young WA 38 trees in the spring of 2022, 2023 and 2024 at the Sunrise research orchard in Rock Island. Watch his pruning method on one of the trees over three years to improve productivity and reduce blind wood. (TJ Mullinax/Good Fruit Grower)