Predicting Fruit Set

Using the carbon balance model
By Sally Colby

The use of chemical thinners is an accepted method of controlling fruit set, but the method is subject to numerous variables.

"Variation in chemical thinning is due to several factors," said Dr. Jim Schupp of the Penn State Fruit Research and Extension Center in Biglerville, Pa. "Some of those variables the grower has control over, others he doesn't." At a meeting that drew fruit growers from several mid-Atlantic states, Schupp explained that growers can control choice of thinner, rate of application and spray coverage. They can also make decisions about when to spray according to temperature and humidity and can consider cuticle thickness, which can affect spray uptake. What growers can't control is the cropping history of the trees, tree variety and bloom density. Schupp emphasized the fact that continued research supports the finding that one of the most important factors for successful chemical thinning is our ability to determine the initial fruit set - what's actually there.

Schupp said that when blossoms are setting, spur leaves are the only leaves that provide fresh carbohydrate to the tree, so their leaf area and function is critical. Temperature and sunlight also play a major role in fruit set and how fruit trees respond to chemical thinners. Tree vigor is another factor: Is there competition between developing fruits and developing leaves and shoots, and how severe is that competition?

Factors that can reduce chemical thinning response include cool temperatures, sunny weather, light initial crop with resting spurs, and healthy trees with good leaf area. "Those factors all tend to increase fruit set and make it harder to chemical thin," said Schupp. "Conditions that increase the response to chemical thinning include hot temperatures, cloudy weather, heavy initial set on weak spurs and stressed trees."

Although early 20th century growers were aware of factors that affect carbohydrate supply and demand, they didn't have the means to measure such factors. A midcentury change in thinking about crop load management dictated that carbohydrates were considered inert fuel, and that plant hormones were the key.

"Now we're back to a more balanced view," said Schupp as he described the carbon balance model, an experimental method of predicting fruit set. "Dr. Alan Lakso of Cornell University developed the carbon balance model for predicting fruit set, and Dr. Duane Greene of the University of Massachusetts worked on the carbohydrate supply and demand curve with Lakso."

The model estimates carbohydrate supply and demand, and then calculates the carbon balance of the tree. "We know that healthy spur leaves promotes photosynthesis," said Schupp. "If there is a lot of fruit growth and tree vigor, it will affect carbohydrate demand. High light conditions result in [a] surplus of carbohydrates, which makes thinning more difficult. If we shade leaves or have heavy cloud cover, which results in low photosynthesis, potential carbon deficit, and trees [that are] easier to thin."

A tree's ability to maintain photosynthesis is controlled by the external temperatures they are exposed to. Low-light temperatures reduce the biochemical ability for the tree to respirate, so the result is low demand and trees that are harder to thin. "High temperatures mean a very high respiration rate with trees going along at full throttle, and there's a good chance they'll exceed the supply with excess demand. That'll make them easier to thin," Schupp explained.

Schupp said that with chemical thinners, we're upping the ante; imposing transient stress for three or four days, but we need to predict how trees will respond. "All conditions being equal, the ideal window for chemical thinning is fruit at 10 to 12 mm," he said. "This is because it's the point at which demand is reaching crescendo, with active leaves, active shoot growth and root growth. Stems are becoming thicker and fruit is growing."

For this reason, Schupp advises growers to evaluate initial fruit set: Is there a lot of thinning to do, or just a little? "Look for growing fruit," said Schupp. "Growing fruit are setting fruit. Then can we find a good temperature window for thinning applications." Schupp said that daytime highs in the 70s and some sunshine are ideal conditions. But if fruit is 7 mm (smaller than the ideal) and there's an upcoming three-day window of ideal weather, apply thinner.

"Chemical thinning will always carry some risk due to our inability to control, or even predict, the weather," said Schupp. "However, because it is such an important wild card, weather should be the primary focus during the thinning window. The most important days to consider are the two days before making a chemical thinning application and three to four days after application."

Can apple growers more effectively manage the timing of chemical thinner applications? "Here's where the carbon balance model comes in," said Schupp. "The model estimates demand by means of an equation that calculates fruit and vegetative growth based on long-term studies by plant physiologists. It allows us to assign light and temperature-related factors a weighted value and plug them into an equation. We can consider factors that promote photosynthesis and respiration, and look at competition within the tree and use those values to develop a carbohydrate balance model. Is the tree experiencing carbohydrate deficit or a surplus, and what would be the effect of adding an additional layer of stress, such as a chemical thinner, for several days?"

The carbon balance model is based on the physiology of slender spindle trees planted at 800 trees per acre. It's also based on initial fruit set of 300 fruits per tree, which Schupp said is a heavy crop and about twice what most growers would want. Schupp used the model in 2011 to help determine the optimum time for chemical thinning. "It's a relatively easy model to use if you have the necessary software," he said. "Plug in daily maximum and minimum temps and daily solar radiation. The assumption is that the primary source of carbohydrates during this thinning window is the currencies in carbohydrates that the spur leaves are producing."

Although the carbon model has potential, Schupp said that growers should keep in mind that because the model uses forecasted data, the model is only as good as the forecast. "It doesn't cure the situation," said Schupp. "We're getting better at forecasting the daytime high, but light forecasting is another matter. If clouds are forming, where will they go? What about scattered clouds at sundown - are they going to dissipate when the sun goes down, or will clouds persist and keep temperatures up during the night?

The carbon balance model shows promise in eliminating some sources of error in the decision making process and simplifying the variables that will help growers make decisions. "It allows the grower to stay focused on variables that can be controlled, such as chemicals, application rate and initial set," said Schupp, "but many orchards do not fit the assumption of the model, which is slender spindle plantings at a rate of 800 trees per acre. However, it may become more useful as we move toward growing systems that result in more uniform lighting in the canopy."

The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania. Comment or question? Visit and join in the discussions.