Understanding how tart cherry trees work is key to quality and cost control

Dr. James Flore

Ask an orchard manager just how good a job do you have to do to keep it producing high quality fruit year after year? We know what perfect is, but what is adequate?

Caught in a squeeze between costs and prices, what are the thresholds? Does it pay to jump high over a hurdle when the bar is actually set lower?

After Dr. James Flore joined the horticulture department faculty at Michigan State University in 1974, he decided to focus on answering those questions for cherry growers, especially for tart cherry growers. Michigan grows two-thirds of the nation’s crop, so it seemed logical that someone at the state’s land grant university should address those questions.

Tart cherries are a difficult crop because of fluctuations in annual yield, Flore said. Prices to the grower can be high in small crop years or low in big crop years, causing the balance between profit or loss to be unpredictable. Tart cherries are grown primarily for processing, in only a few locations, and suffer from erratic annual production because of late spring freezes. Thus, they are difficult to market. Gluts and busts follow each other, so price and supply is erratic.

Moreover, the crop is harvested mechanically, with trunk shakers, which can damage the trees and shorten their life. Many stone fruits respond to injuries with life-shortening canker diseases.

Tart cherries also host cherry leaf spot, a foliar disease so aggressive it can completely defoliate a tree in the short time between bloom in May and harvest in July—reducing tree hardiness and threatening crop yield the next year and the life of the tree itself.

Constrained by price, growers wanted to know exactly how much—and especially how little—they need to do to produce their crop.

“My career has been based on understanding the physiology and using it to help growers make decisions on things that affect yield and quality,” Flore said. “It was a matter of finding thresholds.”

Studying tart cherries

One of the questions tree fruit physiologists began to address in the mid-1970s was, what drives fruit yield? The answer, they found, was light. Light drives photosynthesis, and the highest yields came in orchards that captured the most light. So, for all fruits, that insight led to work with orchard design, tree height and density, row orientation, and pruning for canopy management.

“Does photosynthesis limit yield? Yes. Does increasing photosynthesis increase yield? Not necessarily,” Flore said.

He found that cherries have “feedback inhibitions.” The trees will work hard in the morning, then cut back in the afternoon. “They don’t go full bore all the time unless they are under stress,” Flore said.

This finding had significant implications. He found that photosynthesis in the afternoon increases if the tree “needs” to do the work.

What might cause a cherry or apple tree to need to photosynthesize longer? It might happen if leaves are damaged by mites, tentiform leaf miners, or cherry leaf spot. “As damage increases, trees will photosynthesize more in the afternoon,” he said.

Flore found that leaves could tolerate damage that resulted in loss of up to 30 percent of their surface area and still fix enough carbohydrate each day to support a high quality crop, initiate new flower buds, and store reserves to overwinter and restart the next year. That was true only if the leaf-to-fruit ratio was adequate at the start of the season (greater than four leaves per fruit for tart cherry under Michigan conditions).

The trees have ways to limit photosynthesis. The stomata on leaves through which carbon dioxide is taken up close in the afternoon, or stay open if they need to. They compensate for cloudy days and low-light conditions. Leaves actually cup to reduce their exposure to the sun, he said.

“After trees are shaken and there is no more fruit to support, leaves cup to reduce exposure to the sun,” he said. “Cupping is not a sign of stress. It is a way to reduce heat load and transpiration when there is no fruit to support.”

Over the years, Flore worked with many graduate students to carry out the research. Dr. Desmond Layne, the peach scientist who recently came from Clemson University to Washington State University to head fruit extension work, worked under Flore on the “tree compensation to damage” work.

Damage thresholds

Dr. Paolo Sabbatini, who now works with grapes at Michigan State, worked under Flore to find how leaf stomata work to limit carbon dioxide intake. Other grad students include Dr. Carl Sams, now on the faculty at the University of Tennessee, and Dr. Frank Kappel, who breeds cherries in British Columbia.

Flore and his students were able to determine thresholds. If light is reduced to less than 10 percent of full light, wood on the trees will die.
Light is needed inside canopies for flower bud development.

Tree trunks can tolerate a lot of damage. They found that out by girdling trees, which involves removing portions of the bark. It takes loss of bark three-fourths of the way around the tree before shoot growth is reduced and some trees die, Flore said.

Trees need to retain about 70 percent of their foliage as they enter the fall season, or winter hardiness can be reduced. By punching holes into leaves and studying their photosynthetic ability, they found no effect at all at 20 percent area removal if tissue was lost between the veins, as with disease or leaf miner damage. However, photosynthesis could be affected with less area removed if veins were damaged, as by chewing insects. Growers are now advised to keep their fungicide program going even after the fruit has been harvested to keep foliage healthy, at least until September.

They found the threshold for mite damage and stated it in mite-days. In the shorter season around Traverse City, trees can tolerate about 1,000 mite-days per leaf. Father south in southwest Michigan, they can tolerate 1,500.

Trees will reduce photosynthesis if the crop is light. They reduce photosynthesis when “the sinks are full.”

They found that Montmorency cherry trees need two to four leaves per fruit to produce a quality crop. Effects of not enough leaves, or foliage damage, are small fruit size, less red color, lower sugar and soluble solids, and greater fruit removal force.

Healthy foliage affects winter hardiness. Trees in high vigor, as rated by shoot growth longer than eight inches, were hardy to -23˚F, but only to -15°F if they were in low vigor, with less than four inches of growth.

Over the years, Flore and his students used whole-tree balloon chambers, herbicides, and hole punches to simulate foliage damage, and isotopes of carbon to study the sinks for carbon dioxide.

All of these studies could result in savings to growers if they knew when they needed to spray for insects, mites, and diseases and when enough was enough. “It doesn’t have to be pristine,” he said. •

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Recent research

Much of Dr. Jim Flore’s basic tree fruit physiology work was done before the year 2000. In recent years, Flore has focused attention on what makes sweet cherries crack, how mist cooling can be used to delay onset of bloom to avoid spring freezes, and how Michigan sweet cherry growers can mechanically harvest more cherries with stems on—since stem-on Maraschino cherries are four times more valuable.

Flore is convinced that sweet cherries crack simply because of a change in osmotic potential. When two solutions come into contact, the concentration of solids in them will equalize. Water moves through the skin of cherries to dilute the sugars inside. “The solids can’t go out, so the water must go in,” he said, the fruit skin cannot expand any further, and therefore it cracks.

There is some debate about whether water in the soil can be taken up and blow up cherries from the inside, or whether rain water outside can be modified by addition of salt to stop its uptake through the skin. Flore believes that under Michigan conditions, it’s rain—even heavy fog or dew—that causes the problems and that adding salt—
calcium chloride is the cheapest—will reduce cracking. “We have been able to make it work,” he said.

He’s developed a technique in which a salt spray is applied automatically during rain events. It would use the same technology that is now being developed, in a
project involving Michigan State, Washington State, and Cornell University, called the solid-set canopy spray delivery system. Fixed-in-place plastic irrigation lines can be used to spray pesticides, growth regulators, foliar fertilizers, plain water as mist, or salty water.

Most of Flore’s mist cooling work has been done on apples.  This is an idea that was tried in the mid 1970s, but was dropped because sprinklers applied excessive water, which resulted in poor fruit set, more disease, and soggy orchard soil.

“However, it proved that cooling delays bud break,” he said. “It’s based on evaporation, and by misting and applying only enough to keep the surface moist, we can cool buds and extend dormancy and perhaps prevent frost damage by delaying bloom.”

The new twist to this idea came with the application of mist using the solid set delivery system in the orchard, and the use of modern control systems that deliver the mist based on temperature and humidity. This has resulted in delayed bloom and reduced water application and its negative side effects.  More work is still needed.

He also worked to help sweet cherry growers mechanically harvest sweet cherries with more stems on—since the payout is greater for them.

“We found that when fruit removal force was greater than 300 grams, we could make the fruit abscise near the leaf rather than at the fruit,” he said. “There is a three- to five-day window where 80 percent come off with stems. It’s based on having a large enough fruit and enough velocity generated by the shaker to apply enough force to the upper abscission zone so that the stem rips from the upper spur.”

Some growers using Flore’s technique now get the better price.

“I’ve had a really great career at Michigan State,” Flore said. “They’ve allowed me to pursue my interests and be the best that I can be. There were rarely any
roadblocks.”

Read more:

Flore’s tart cherry physiology studies meshed with work in other fruits. Read the exclusive online story “An era of physiological discovery.”