A Strategy For Precision Crop Load Management In Apples

Editor’s Note: The following is a condensed version of an article that ran in the Summer 2013 issue of the New York Fruit Quarterly newsletter. The article was written by Cornell University horticulturist Terence Robinson, with input from Steve Hoying and Alan Lakso at Cornell, and Duane Greene at the University of Massachusetts.


Crop load management is the single most important yet difficult management strategy that determines the annual profitability of apple orchards. The number of fruit that remain on a tree directly affects yield, fruit size, and the quality of fruit that are harvested, which largely determine crop value. If thinning is inadequate and too many fruits remain on the tree, fruit size will be small, fruit quality will be poor, and flower bud initiation for the following year’s crop may be either reduced or eliminated. Consequently, poor or inadequate thinning will reduce profitability in the current year and result in inadequate return bloom in the following year.

Management of crop load is a balancing act between reducing crop load (yield) sufficiently to achieve optimum fruit size and adequate return bloom without reducing yield excessively. Identifying and then achieving this optimum crop value is often very difficult for apple growers. It is hard to know the economic impact of not achieving the optimum crop load without having various levels of thinning each year. The difference between the optimum crop load and under thinning or over thinning can sometimes be a difference of thousands of dollars per acre. Thus growers often fail to capture the full crop value possible without knowing how much “money they left on the table.” More precisely managing crop load will help growers achieve the optimum crop load and maximize crop value.

Crop Load Management Approaches

There are three management practices that have a large effect on crop load: 1) pruning, 2) chemical thinning, and 3) hand thinning. In recent years growers have relied primarily on chemical thinning to adjust crop load with a lesser reliance on hand thinning to reduce labor requirements. In other countries hand thinning is still the primary means of adjusting crop load. A few progressive growers have also begun to view pruning as a means to adjust crop load.

Precision crop load management utilizes all three management approaches to adjust crop load. It begins with precision pruning to leave on the tree a preset bud load, followed by precision chemical thinning to reduce initial flower number per tree to as close as possible to a preset fruit number per tree, and ends with precision hand thinning to leave a precise number of fruits per tree.

Chemical thinning remains an unpredictable part of apple production, with large variations from year to year and within years due to weather. The interactions of the environment with thinning have been observed for many years. Beginning in 2000, we began to study this variability by conducting annual spray timing trials in New York, which showed extreme variation in timing of response and thinning efficacy between years over the three-week period after bloom when chemical thinners are applied.
There are two major sources of this variability: spray chemical uptake and environmental effects on tree physiology. Variability in spray uptake includes the chemical thinner concentration, the environment at the time of application (temperature and humidity), application method and coverage, and drying conditions.

A second and more important source of variation is the sensitivity of the tree itself, which is related to the level of bloom, how many fruit are present at the time of application, leaf area, temperatures, sunlight, and tree vigor. Many of these factors are directly related to the balance of carbohydrate supply from tree photosynthesis in relation to the demand for carbohydrates from all of the competing organs of the tree (crop, shoots, roots, and woody structure).

Carbohydrates And Fruit Growth

Considerable research has examined the role of carbohydrates as pivotal to the fate of young developing apple fruit. Carbohydrates are stored as reserves in the dormant tree, but these reserves are depleted by bloom as trees use these to produce energy for pre-bloom growth and respiration.
After flower fertilization, young fruits require currently produced carbohydrates for continuous development, and the extent of this demand appears to be associated with the stage of fruit development and level of light. Immediately after petal fall, demand for carbohydrates by developing fruit is only moderate during the initial lag phase of an expolinear growth pattern. However, when fruit reach 8 to 10 millimeters in diameter (about one to two weeks after petal fall), rapid fruit growth results in an ever-increasingly large carbohydrate demand which may not be met by current photosynthesis.

Alan Lakso at Cornell University has developed a simplified mathematical model that mechanistically estimates apple tree photosynthesis, respiration and growth of fruits, leaves, roots, and woody structure. The model uses daily maximum and minimum temperatures and sunlight to calculate the production of carbohydrates each day and allocates the available carbohydrates to the organs of the tree. From these data the model calculates the daily balance of carbohydrates.

The value of the model in predicting chemical thinner efficacy has been studied since 2000 in both field and greenhouse thinning studies at Cornell University. In each year we identified periods during the thinning window where the model estimated either a carbohydrate surplus or a deficit and compared them to our observed thinning responses.

We have used the estimated supply-demand balance of the tree to predict or explain thinning response as follows: Carbohydrate surplus will support fruit growth giving less thinning, while carbohydrate deficits will limit fruit growth giving more thinning.

The carbohydrate model has potential to predict thinner responses prior to the application of thinners, thus allowing growers to adjust thinner treatment and timing to achieve an optimal amount of thinning. However, it imprecisely assesses the real effect of the chemical thinner after application.

A more precise method of early assessment of thinning efficacy after chemical application based on fruit growth rate has been developed by Duane Greene and others. The model is based on the observation that fruitlets which have slowed growth rates (less than 50% of the fastest growth rates) are usually destined to abscise. The model requires the measurement of the diameter of fruitlets on 75 spurs (375 fruitlets) at three and eight days after application of the chemical thinner to clearly differentiate abscising versus retained fruit. The growth rate of the fastest-growing fruitlets is used as reference to determine the percentage growth of fruitlets and what percent will abscise. Early estimates of thinning efficacy after application allow timely decisions about the need for a second chemical application if needed.

Precision Chemical Thinning

In the last three years we have developed an improved method of conducting chemical thinning that utilizes both the carbohydrate model and the fruit growth model. We have named the method “Precision Chemical Thinning.” This method uses the carbon balance model as a predictive tool for predicting response prior to application and the fruit growth rate model for early assessment of thinning response immediately following application.

The method begins with first calculating the final target fruit number needed per tree (based on desired yield) and secondly assessing the number of flower clusters on the trees (after pruning) by counting five representative trees. Once the number of flower clusters per tree is known (each cluster with five flowers) and the final fruit number needed for the desired yield, the percent of the initial flowers needed after thinning can be calculated. The optimum final fruit number per tree is different for each variety and depends on genetic fruit size of the variety (Gala is small genetically and Jonagold is large genetically) and the price in the market (large Galas have a much higher price than small Galas while Jonagolds that are too big have a lower market price) and the inherent bieniality of the variety (Honeycrisp are very biennial and must be managed at a lower crop load than Gala, which is not biennial).

With the variety-specific target of final fruit number per tree and the thinning task in mind, a precision thinning program is conducted by applying sequential thinning sprays followed by rapid assessment of the results in time to apply a subsequent thinning spray, and then an early re-assessment, followed by another spray if needed until the final target fruit number for each variety is achieved.

In practice, precision thinning begins with:
1. A bloom thinning spray at 60% to 80% full bloom.
2. The first spray is followed by a petal fall spray applied two to four days after petal fall (about one week after the bloom spray). Before the petal fall spray, the results of the carbohydrate model are used to guide the rate of chemical and the exact timing of the petal fall spray.
3. The first two sprays are followed by an assessment of the efficacy of those two sprays using the fruit growth rate model which indicates the percentage of thinning achieved with the first two sprays.
4. Then, if needed, a third spray is applied about one week after the petal fall spray. Before the petal fall spray, the results of the carbohydrate model are used to guide the rate of chemical and the exact timing of the third spray.
5. The third spray is followed by an assessment of the effectiveness of all previous sprays using the fruit growth rate model, which indicates the percentage of thinning achieved with all three previous sprays.
6. Lastly, if still more thinning is needed, a fourth spray is applied at 16 to 20 millimeters (about one week after the third spray) to achieve the target fruit number.

For many fruit growers, it may be impractical to use the fruit growth rate model on all varieties since more than 20 varieties are grown in New York. We suggest growers make the fruit diameter measurements on three varieties (two hard-to-thin varieties and an easy-to-thin variety) to guide the decisions for other varieties. We suggest growers measure fruit diameters with Gala, McIntosh, and Honeycrisp in the Northeast.

In 2013 more than 20 cooperating growers, consultants, and Extension staff implemented the precision thinning program on Gala and Honeycrisp in New York, Massachusetts, Vermont, and New Jersey. The results of fruit diameter measurements made after petal fall thinning sprays around May 19 or 20 show that the sprays provided significant thinning on Gala and Honeycrisp but that additional thinning was still needed. In general fruit set was reduced by about 70% from the bloom and petal fall sprays; however, the target was to reduce fruit set by 90%. Thus substantial thinning on Gala and Honeycrisp remained to be done. This suggested another spray in these block at the 10- to 12-millimeter (mm) fruit size stage. From this assessment we gave specific recommendations to each grower for another spray. A similar process was repeated after the 10-12 mm spray to determine if another final spray was needed at 18-20 mm fruit size stage.

Thinning Model Online

In 2013 the apple carbohydrate thinning model was placed on a web server at Cornell University, and is available at www.newa.cornell.edu. It is linked to on-farm weather stations in New York, Massachusetts, Vermont, New Jersey, and eastern Pennsylvania from which the model uses temperature and sunlight data beginning each year with the date of bud break in the spring to daily calculate tree carbohydrate balance. The web version of the carbohydrate model also uses weather forecasts for prediction of carbohydrate balance seven days into the future. The website allows apple growers or consultants to run the model and receive predictions in real time of carbohydrate balance and suggested chemical thinner doses.