Phil Schwallier

Phil Schwallier

Thinning rules
In an article in Good Fruit Grower in 2009, Cornell University’s Dr. Terence Robinson laid out some general rules for fruit thinning:
•    Dark, cloudy weather of more than one day reduces the carbohydrate supply, resulting in greater natural drop and greater response to chemical thinning.
•    Nighttime temperatures above 65°F increase carbohydrate demand, thereby increasing natural drop and chemical thinning response.
•    Daytime temperatures above 85°F increase ­carbohydrate demand and result in excessive thinning.
•    Cool daytime temperatures, below 65°F, reduce fruit ­carbohydrate demand, resulting in poor ­thinning response.

In the last ten years, Michigan apple growers have experienced big crops following small crops, almost every year. No other state has had this problem this dramatically, and the problem will get worse unless growers take strong action this spring. Phil Schwallier, Michigan State University district horticulture agent and director of the Clarksville Horticulture Experiment Station, has been taking this ­message, and his advice for ­solutions, to grower meetings across the state this winter.

In a nutshell, his message is: Prune hard to reduce capacity, and then thin, thin, thin.

Growers in New York State and Ontario, where crops were greatly reduced by freezes last spring, face a similar situation.

“Last year’s events have set us up for more years of inconsistent annual cropping,” he told growers at Southwest Michigan Horticulture Days in Benton Harbor in February. “Michigan’s climate may be the most adverse in North America. Our production history says that, and we have to address it. This is the year you’re going to have to start, or we will set 30 million bushels this year, and that’s too many apples.”

Throwing 30 million bushels of small apples into the market this year will not be cure for the problems of last year, he said.

The freezes last year and the absence of crop have set the stage for prolific flowering this spring and the setting of an excessive crop. That will mean a small crop again in 2014 and an ongoing ­biennial bearing cycle.

Some varieties, such as Golden Delicious, Fuji, Paulared, and Honeycrisp, have a strong tendency to  bear biennially. Special attention must be paid to them, Schwallier said. Rome, Jonagold, Red Delicious, Empire, and McIntosh are mildly biennial, and Gala, Granny Smith, Ginger Gold, and Idared have low tendency to bear biennially.

Growers have tools to address the problem. The first is pruning intensity, and he advised them to prune hard this winter to adjust crop potential. Then follow with chemical thinning during the best thinning windows, and follow that with rescue thinning when fruits reach 20 millimeters, if needed.

If all fails, hand thinning may be required. If the set is still too heavy, return bloom sprays can help adjust crop potential for 2014.

Variety differences

Apples vary in their sensitivity to thinners. Schwallier rates Ginger Gold, Empire, Cortland, Idared, Mutsu, Jonagold, nonspur Rome, and Jonathan as easy to thin.

He rates McIntosh, Paulared, Honeycrisp, Spartan, and Stayman as moderately sensitive to thinners. Spur Delicious, Gala, Fuji, Rome, Golden Delicious, and Cameo are hard to thin, he said.

For the easy-to-thin varieties, he recommends lower rates of thinners and a single application when fruitlets are 10 to 12 millimeters in diameter.

For moderate varieties, he recommends two applications of medium rates of thinners in a tank mix with carbaryl, starting at petal fall if bloom is heavy.

For hard-to-thin varieties, he recommends multiple applications and aggressive thinning, starting with blossom or petal fall thinners, using the highest label rates and adding oil and carbaryl at the 10-12 mm size, and following with a third spray of ethephon or carbaryl if needed.

The basic theory of fruitset says that it’s all about stress and the supply of and demand for carbohydrates.

“Carbohydrates come from plant reserves and from this year’s photosynthesis,” Schwallier said. “About the time fruitlets reach ten millimeters in size, a crisis occurs with carbohydrate levels. Reserves are used up, and photosynthesis may not provide adequate levels of carbohydrates. In this competition, leaves and shoots win, and fruit loses. Fruit fails to grow and falls off.”

Temperature and light conditions add to or reduce stress, making thinning easier or harder.

On bright days when light conditions are good, there is an increased supply of carbohydrates from photosynthesis, and fruit is hard to thin. Under low light ­conditions, fruit is easier to thin.

Low temperatures create low demand for carbohydrates, so fruit is hard to thin when it’s colder. High temperatures create high demand for carbohydrates, and fruit is easier to thin.

The best conditions for thinning are when light is low and temperature is warm (low supply and high demand for carbohydrates), and the worst conditions are when there is lots of light, but it’s cool (high supply, low demand).

As leaves and fruits get larger, leaves produce more energy and fruit becomes harder to thin.

The best time to thin is when a warming trend is ­forecast and temperature is rising.

Help with thinning

Over the last few years, researchers have put thinning theories into advice that can help growers as they go through the season. Researchers at Cornell University, especially Dr. Alan Lakso and Dr. Terence Robinson, have led the effort to create a carbohydrate model, called MaluSim, that has been integrated into grower advisory services. In Michigan, where Schwallier cooperated in validating the model, that information is part of the ­EnviroWeather network.

Similar networks are in New York and in Pennsylvania, where work on the model was also done.

Schwallier advised that growers apply their thinners, wait four or five days to see results, then thin again if necessary.

Another tool they can use to see how thinning is progressing was developed by Dr. Duane Greene at the University of Massachusetts. Realizing that growers are reluctant to make repeated thinner applications without knowing how effective their treatments have been, he developed his “fruitlet model.”

This involves marking and measuring apples before thinning and again seven to ten days after applying the thinner. Those apples growing at less than half the rate of the fastest growing fruitlets will fall off at some point, probably during June drop.

The method was described in the April 15, 2010, issue of Good Fruit Grower magazine and can be accessed in the archives at goodfruit.com.

“Thinning predictions from 13 experiments over four years confirmed that a prediction can be made within seven to ten days with near 95 percent accuracy if 130 to 140 heating degree units (using a base of 50°F) have been accumulated after the thinner application,” he said then.

“We followed the growth rate of thousands of fruit, and slow fruit growth is highly correlated with fruit abscission. Rapid fruit growth is highly correlated with fruit set.”

When thinning, growers need a target. Schwallier suggests that target should be eight apples per square centimeter of trunk cross-sectional area for most varieties; five apples for Honeycrisp. That can be calculated on a limb-by-limb basis for mature trees, but on younger trees, it can be done from the trunk, especially if the trees are being pruned on a renewal pruning system.

Alternatively, growers can calculate backward by dividing expected yield in bushels per acre by number of trees per acre. Figure 100 apples per bushel, multiply by bushels, and divide by trees per acre to get apples per tree.