MSU Extension Apples

In 2021, a light Honeycrisp crop was observed across much of Michigan as well as the Northeast and Upper Midwest. As a result, a heavy bloom is expected in many locations in 2022. Successful crop management this season is critical for producing a good crop this year and managing bienniality in future years.

The following elements are recommended:

• Start crop load management efforts early.
• Begin with pruning.
• Consider blossom thinning.
• Use chemical fruitlet thinning.
• Use return bloom applications.

Start crop load management efforts early

It is critical to begin managing crop load early. One of the best predictors of final crop load is the initial flower number on the tree. Terence Robinson, Cornell University, reported that in thinning trials over 18 years (from 2000-2017) for Gala/M.9, McIntosh/M.9 and Delicious/M.26 at Geneva, New York, the final number of fruits per tree was positively related to initial flower clusters per tree. This means that the more flower clusters on the tree, the greater the final number of fruits.

In any variety, earlier thinning will improve fruit size and quality, as well as return bloom. There are several reasons for this. First, resources such as carbohydrates are divided between the number of fruits on the tree, so if there are too many fruitlets, fruit size and quality will be low. In addition, the cell division period in fruits takes place primarily in the three to four weeks after full bloom; afterward, the fruits grow only by cell expansion. The potential final fruit size is determined by the number of cells in a fruit, so the cell division period immediately after bloom is critical.

Finally, floral bud induction and initiation for the next year takes place shortly after bloom. Fruitlets produce hormone messengers (gibberellins or GAs) that suppress this floral bud initiation. If there are many fruitlets on the tree, there are many GAs being produced which suppress floral bud initiation. Therefore, too many fruitlets on a tree results in fewer floral buds for the next year. A more thorough explanation of apple fruit growth can be found in the article, “Apple Fruit Growth,” in the Fruit Quarterly by Alan Lakso and Martin Goffinet.

Earlier thinning is especially important for Honeycrisp. Floral bud formation occurs much earlier in this variety than in other varieties. Based on work by Dr. Poliana Francescatto, Honeycrisp flower initiation occurs as early as 45-50 days after full bloom (DAFB), and induction may occur even earlier. In other varieties, floral initiation may be later: 65-80 DAFB for Fuji and 90-100 DAFB for Gala.

More information on early thinning can be found in this Michigan State University Extension article by Phil Schwallier and Amy Irish-Brown on “Predicting Apple Fruit Set Model.”

Begin with pruning

The first opportunity to manage crop load is with pruning. Having excessive buds on the tree results in resources (primarily nitrogen, carbon, and cytokinins) being divided between too many buds. The result is weak buds, low fruit set and small fruit. Removing the number of buds on the tree with pruning is one of the most economical ways to reduce the crop.

To begin managing crop load with pruning, it is best prune to a target crop load. Determining a target crop does not have to be challenging or complicated. Crop load should be determined based on the tree size and age. In a mature tree, we recommend roughly four to six fruit per cm² trunk cross sectional area (TCSA). There are many gauges and guidelines to assist in determining tree size and appropriate target crop, including the Equillifruit Disk developed by the National Institute for Agricultural Research (INRA) in France and these guidelines from the Good Fruit Grower. In addition, University of Wisconsin-Madison published this thorough article on calculating target crop load. In our experience, approximately 100 fruit per tree in a tall spindle system has been an appropriate target.

When pruning to target crop load for Honeycrisp, we recommend leaving approximately 1.8 to two times the number of buds compared to your target crop. This allows for some “insurance” buds in case of frost, other damage and natural thinning. In other varieties, such as Gala, the target bud load is less - approximately 1.5 times the number of buds compared to your target crop load.

Consider blossom thinning

We recommend beginning chemical thinning early with blossom thinning. While there is reluctance to begin so early, due to the risk of frost, thinning is relatively safe during bloom and will result in low to moderate effects. In addition, beginning early allows time to observe the effects and apply additional thinning applications as necessary. This approach of chipping away slowly at the crop, gives more control over the final outcome. There are several materials and approaches that can be used.

Caustic thinners. These materials act by burning parts of the reproductive tissues to prevent pollination and fertilization. Ammonium thiosulfate (ATS) and lime sulfur plus oil (LSO) are two materials that are effective for blossom thinning in apples. ATS burns the pistil of the flower only while LSO burns the pistil as well as the pollen tube. Both of these materials are recommended at a 1-2% rate (1-2 gallons per 100 gallon tank mix). ATS and LSO may not be labeled for blossom thinning in apples in all states. Be sure to check the label for appropriate uses.

Timing of the application can be determined in two ways. The first is using model guidance with the Pollen Tube Growth Model. One application is made after the target crop has been effectively pollinated and fertilized, burning the other open flowers. The second application is made to prevent further pollination of additional flowers that open after the first application. Thorough instructions for using the PTGM are outlined in this Fruit Quarterly article. Another, less precise way to time caustic thinning applications is by visual estimation. Again, two applications should be made, the first between 20% bloom and full bloom, and then again approximately 48 hours later. Applying earlier in bloom will result in greater thinning effect.

When using caustic materials, it is important to consider environmental conditions. There is a risk of negatively impacting fruit finish when materials are applied during cool, slow-drying conditions.

Plant growth regulators (PGR). These materials work by inducing some form of transient stress on the tree, resulting in the abscission of some fruitlets. Naphthalene Acetic Acid (NAA) has been one of the most commonly used PGR thinners in apples, both during bloom and post-bloom. We recommend one application of NAA at a rate of 10 ppm (4 oz) at full bloom.

Naphthalene acetamide (NAD) is another PGR material that may be used for chemical thinning during bloom. NAD is marketed as AmidThin by Valent and is less active than NAA. It is generally applied at 25-50 ppm, depending on the variety. There are several changes to the AmidThin label this year. First, the label clearly states that application is permitted during bloom for thinning apples. Second, the table for application rates has been adjusted to include new varieties, and varieties are grouped according to thinning sensitivity (Easy, Moderate, or Hard-to-thin). NAD should not be used on Delicious because this can result in pygmy fruit.

Application timing of NAA or NAD is highly dependent on environmental conditions. This is the case for most PGRs. Optimal timing is determined primarily by the temperature two days before and four days after the application. These materials are not active at low temperatures and should not be applied when temperatures are expected to be below 60 degrees Fahrenheit. Similarly, applications should not be made when daytime temperatures exceed 85 F, as overthinning may occur.

Many additional factors can influence the effects of chemical thinners. In particular, humidity, spray volume and drying time can play a role. Typically, the chemical is absorbed as droplets are drying. Therefore, longer drying time and humidity are likely to result in greater absorption. For adequate uptake, we recommend applying in no less than 100 gallons per acre.

Use chemical fruitlet thinning

Following bloom, the fruitlet thinning window begins. During this time, it will be important to use every opportunity available to manage the crop. To reiterate what was said above, chipping away at the crop allows us to observe the effects of thinning, apply additional thinning applications as necessary and have more control over the final outcome.

We recommend using applications of NAA at 7.5-10 ppm at petal fall and again at 8-10 mm. For more thinning, NAA can be used in combination with carbaryl at 1pt. This should be applied in a 100 gallons per acre spray volume, with nozzles adjusted to the top two-thirds of the canopy. If additional thinning is needed after the 8-10 mm stage, combinations of NAA (10 ppm) and carbaryl (1pt) should be applied to the tops of trees only.

To assist with thinning decisions, use the Carbohydrate Model to adjust rates according to conditions. This model calculates the carbohydrate balance based on fundamental tree physiology, including carbohydrate production by photosynthesis and carbohydrate demands of fruit and tree growth. For instance, when conditions are cloudy, less photosynthesis takes place and fewer carbohydrates are produced. This leads to a carbohydrate deficit, and thinning applications will have greater effects. More information about the carbohydrate model and how to use it is available from NEWA.

Want more precision? Using “precision thinning” can help guide decisions about whether to make additional thinning applications. Precision thinning involves measuring a sample of fruitlets at intervals after a thinning application to determine how much they are growing. The fastest growing fruitlets will persist on the tree. Fruitlets growing slower will abscise. This information can be used to decide if you are on track to set your target crop load or not. The fruitlet growth rate model is available on the Malusim app or website.

More instructions on precision crop load management are explained in this article by Phil Schwallier and Amy Irish-Brown. To learn more about precision crop load management, you can read this article in the Fruit Quarterly on Precision Crop Load Management. In addition, Jon Clements at UMass Amherst recently shared a ‘RECIPE’ for Predicting fruit set using the fruitlet growth rate model on his blog, which walks through the steps and tools available for precision fruitlet thinning.

Return bloom applications

Following the thinning window applications of PGRs can be used to promote floral bud formation for the next season. NAA is one very effective option. NAA should be applied at a rate of 10 ppm beginning after thinning applications, approximately five, seven and nine weeks after full bloom. Alternatively, summer ethephon applications can be used to enhance return bloom. Ethephon should be applied at a rate of 200 to 600 ppm at the same timing of five, seven and nine weeks after bloom.

Use caution when applying ethephon: This material will cause thinning in 1-inch fruitlets and is somewhat unpredictable. It is also very sensitive to temperature and may cause additional thinning if applied when temperatures are very warm.

See more recommendations and information on summer applications of NAA to enhance return bloom by Schwallier and Irish-Brown.

Video resources

These videos, recently published by Cornell Cooperative Extension, illustrate Pruning for Precision Crop Load Management and Bloom Thinning with the Pollen Growth Tube Model.