The Cosmic Gospel: Washington State University continues education on training WA 38 trees.

Horticulturist Stefano Musacchi teaches growers the art of pruning Cosmic Crisp trees at a demonstration in December, 2017, Washington State University Roza trial orchard. Photo by Ross Courtney,  Good Fruit Grower .

Horticulturist Stefano Musacchi teaches growers the art of pruning Cosmic Crisp trees at a demonstration in December, 2017, Washington State University Roza trial orchard. Photo by Ross Courtney, Good Fruit Grower.

Push growth toward the trunk. Tip the end of 1-year-old branches and return the following year to tip again, leaving a few vegetative buds. Girdle or notch blind wood to encourage new shoots.

After several years now of preaching the Cosmic Crisp pruning gospel, the message is largely the same, and Washington State University plans to keep spreading it.

Now that orchardists have planted the first commercial blocks of WSU’s WA 38 apple, researchers and extension specialists are continuing their road show to teach growers how to train and manage the variety, marketed under the trade name Cosmic Crisp.

“We’re learning along with you,” Karen Lewis, WSU’s regional extension specialist, told a group of 20 or so growers huddled in the university’s Roza test blocks outside of Prosser.

The December outreach involved small groups, an hour at a time, watching Stefano Musacchi, horticulturist and endowed chair, explain the pruning, training and canopy management techniques to give Washington growers their best chance at success with the new apple, in which the industry has invested nearly $500 million.

Musacchi advises the click pruning technique, tipping year-old branches to reduce their apical dominance to prompt buds below the cut to swell. That should reduce blind wood and force the fruit closer to the center of the tree, where future mechanization will become easier.

Left on its own, WA 38 has a tendency to tip bear and produce stretches of blind wood, he said. It’s also a vigorous variety and should need little help filling its space in the first few years. He recommends not applying a lot of nitrogen, as long as an orchard has fertile soil.

“This tree will grow,” he told the orchardists. “Believe me. It’s not a Honeycrisp tree.”

The WA 38 also has a habit of “exhausting” branches, so Musacchi suggests cutting older branches back to within a couple inches of the trunk to encourage new growth on a three-year rotation.

Musacchi heads back a tree, reducing it’s height, and using a one-year-old shoot to make a lateral cut.. Photo by Ross Courtney,  Good Fruit Grower .

Musacchi heads back a tree, reducing it’s height, and using a one-year-old shoot to make a lateral cut.. Photo by Ross Courtney, Good Fruit Grower.

Another trick is girdling and notching the trunk in the first or second year. If blind wood starts to develop, use a pair of clippers to gently score the bark and phloem right above a bud at green tip to encourage growth in that bud. Combine that with Promalin (6-benzyladenine and gibberellins) treatments.

However, be careful, he said. Cut too deep and you could snap the trunk. “You have to train your people to not get excited,” he said.

In fact, WA 38 has fragile branches in the winter, as Musacchi accidentally proved during his Prosser demonstration by breaking off a branch as he tried to pull it into place. He suggested waiting until the tree is green to do any tying.

Other tips included pruning more thinly on the inside of bi-axis trees to allow light penetration and setting aside time to prune in the summer to slow vigor.

Musacchi and Lewis plan to continue their demonstrations and field days in commercial WA 38 orchards and are working with WSU’s College of Agricultural, Human, and Natural Resource Sciences to produce instructional videos for each year of tree development up to the first year of cropping, Lewis said.

Article by Ross Courtney, Good Fruit Grower

Cosmic tips to growing WA 38

Stefano Musacchi, Washington State University horticulturist and endowed chair in tree fruit physiology and management, discusses fruit spacing of the Cosmic Crisp in September at the Sunrise research orchard near Wenatchee.. Photo by Ross Courtney,  Good Fruit Grower .

Stefano Musacchi, Washington State University horticulturist and endowed chair in tree fruit physiology and management, discusses fruit spacing of the Cosmic Crisp in September at the Sunrise research orchard near Wenatchee.. Photo by Ross Courtney, Good Fruit Grower.

The first commercial trees were planted this spring of the new apple variety WA 38, following nearly a decade of research into the horticultural traits at four research plantings and the storage behavior of the fruit.

Developed by Washington State University and marketed under the brand name Cosmic Crisp, the WA 38 has unique behavior compared to most scion varieties.

Those of us who’ve worked with the trees in the field have found the variety to be grower friendly, but for those growers who’ve just planted their trees, here are the top five tips for the first year:

Never forget that WA 38 is a Type 4 tree that produces a lot of blind wood, and growers don’t want to sacrifice productive space to blind wood, particularly in a fruit wall. To avoid blind wood, you should…

  • Prune. The variety has the potential to produce feathers, and you should cut them back to four to six buds, cutting the terminal 1 foot from the top of the central leader.
     
  • Never bend the branches below 90 degrees, which both creates blind wood and the potential for the tree to become a biennial producer very quickly. WA 38, in particular, has a huge capacity to produce flowers, and because it’s producing flowers on one-year wood, growers could see an excess of flowers one year, followed by a dramatic reduction the following year.
     
  • Don’t crop in the first year. If you have a wonderful tree, wait until at least the second year to crop.
     
  • And remember, WA 38 usually sets just a single fruit or two fruit per cluster. That means thinning has to be really targeted, on the basis of flower distribution and fruit set.

It’s also important to remember that WA 38 requires a little bit more space, about 10 percent more, compared to other varieties. It’s a vigorous tree and needs adequate light for the bi-color fruit to color, so prune to allow more space to reduce the number of leaves and the amount of shade in the canopy. •

Article by Stefano Musacchi on Good Fruit Grower


Stefano Musacchi, Ph.D., is an associate professor and endowed chair in tree fruit physiology and management at Washington State University in Wenatchee, Washington. He can be reached at stefano.musacchi@wsu.edu