Author Archives: S. Sherif

Promoting return bloom in apple

The initiation of the next year flower buds occurs 2-6 weeks after bloom. Therefore, if the tree is exhausted in producing more fruits and/or vegetative tissues during this period, the initiation of flower buds will be negatively affected, and less return bloom will be obtained. Fruit thinning during this period, either chemically or by hand, helps the tree to direct some of their photoassimilates to the production of the next season’s flower buds. It has also been proven that treatments with some PGRs, particularly NAA and ethephon, after the thinning period ends (~ 6 weeks after bloom), can also promote bud initiation and subsequently return bloom.
Based on the results of many field trials, four bi-weekly sprays of NAA (5 ppm) and ethephon (150 ppm) beginning six weeks after full bloom promote return bloom in apple. These sprays can be added to the cover spray. However, if the tank is mixed with oil or a surfactant, the rates of NAA and ethephon should be reduced by 1/3. These rates should also be adjusted to the tree raw volume. Increasing water amount by 3-4X will enhance the positive effect of these treatments.

Updates on the carbohydrate thinning model- (May 16-21)

    The following is a note of caution from Alan Lakso, the developer of the apple carbohydrate-thinning model (May 11, 2017):

“• The apple carbohydrate model assumes that the trees are healthy, normal vigor, have no significant stress (frost, drought, nutrient) and no significant carry-over effects from previous years that might change the tree response to the weather. With the severe drought in 2016, there may be orchards that will have a weakened state coming into 2017 which would likely make the tree abnormally sensitive to thinners. So in those cases the model may suggest a stronger thinner concentration than is appropriate. Conversely, if irrigated last year, with the warm season and lots of sun, those trees may be in better than normal condition and harder to thin.
• With the current cold period, flowering, pollination, initial set and early fruit growth may not be very well modeled as we did not have much data under these very cold (or also excessively hot) conditions when the model was developed.
For all these reasons, you should have less confidence in the model this year. Use your experience with your unique situation.”

    Here is my take on this:

– I have not observed any unusual or extreme conditions in Virginia in the last few weeks that would seriously affect the accuracy of the model in predicting the timing and rate of chemical thinners.
– We have applied chemical thinners (e.g. 6-BA, NAA) to our Fuji and Gala trees in Winchester lab based on the model recommendations and we got fruit thinning enough to call it good.
– The fruit size in Winchester and Central Virginia is above 20 mm and rescue thinning using ethephon would be the only chemical thinning option at this time. For ethephon, the variety sensitivity and temperatures are more important factors to consider than the carbohydrate level.

    Therefore, I would recommend the following for this week’s thinning treatments:

– The weather forecast predicts temperatures close to 90 oF this week, which will increase the responsiveness to ethephon applications and might lead to over-thinning if the proper rates are not used.
– The carbohydrate model for Winchester and Central Virginia predicts carbohydrate deficit for the following six days (until May 22), which should also increase the responsiveness to ethephon. The model has already stopped providing any recommendations for the rates. However, if you will apply ethephon this week, I would recommend decreasing the rate by 30% at least and not to add oil or a surfactant to the ethephon-carbaryl mix.
– Rome Beauty and Golden Delicious are highly sensitive to ethephon and under the above-mentioned conditions; it might be risky to use ethephon with these varieties.

The following is the carbohydrate model for Winchester and Central Virginia.

Apple Carbohydrate Thinning Model-Updates-(May 08 – 12)

At such advanced stage of apple fruit development, 6-BA and NAA are no longer effective as chemical thinners and it is the time to start the “rescue thinning” applications. The fruit size for most varieties in Winchester is close to 20 mm and it would likely be above 23 mm in Central Virginia. Ethephon is the most effective thinner at this stage. Ethephon thins more effectively when the temperature is in the 70s to low 80s. Although the weather forecast predicts cool temperatures this week, increasing the rate of chemical thinners by 30% might mitigate this negative impact of low temperatures. Mixing carbaryl (1 pint/100 gal) and oil (1-2 qt/100 gal) with ethephon (0.5-1.5 pints/100 gal) enhances its effectiveness and it might show desirable effects under the weather conditions of this week. The addition of NAA (5 ppm) to ethephon/carbaryl mix might be an option if more aggressive thinning is still needed. However, this application should be avoided with Delicious and Fuji as it will lead to the development of pigmies. Adjusting the pH to 3-5 is recommended with ethephon applications. Non-ionic surfactants (e.g. Regulaid) can also increase the response to ethephon applications.
As shown in the graphs below, the apple carbohydrate-thinning model predicts carbohydrate surplus for the whole week and likely during the weekend in Winchester and Central Virginia. Under such conditions, the recommendations are to increase the rate of chemical thinners by 30%.

Apple Carbohydrate Thinning Model-Updates-(May 02 – 05)

Unlike last week, weather forecast predicts conditions that will likely lead to carbohydrate surplus and subsequently, less thinning efficiency this week (May 01-05). As shown in the images below, the carbohydrate model predicts accumulation of up to 35g/day carbohydrate this Friday. This, in turn, will necessitate a 30% increase in the rate of chemical thinners that will be applied tomorrow, Thursday or Friday. I believe we are still within the good thinning window in terms of fruit size, but this week might be the last chance for (6-BA + Carbaryl), or (NAA + Carbaryl) sprays to show an effect.
The following is the carbohydrate model for Winchester and Central Virginia.

Apple Carbohydrate Thinning Model-“Updates” for April 28 – May 1

The weather forecast predicts a hot and cloudy day for tomorrow (Saturday, April 29), which certainly will lead to more carbohydrate deficit. As you see from the graphs below, the carbohydrate balance will increase again by Tuesday and Wednesday, but the carbohydrate reserve will be below 20 g/day, which is not too high to influence thinning treatments (if needed) at this time.
For defruting you young apple trees (to fill the space and get a good tree structure), tomorrow might be a good day to do so. The temperature will be high (>85 oF) in Winchester and Central Virginia and the carbohydrate balance is very low (< -60 g/day) which together encourage for more fruit abscission. To defruit your trees, you can apply:
A) NAA (5-8 ppm) + Carbaryl (1-2 pt) + Superior oil (1qt)/100 gal; or
B) Ethephon (1-1.5 pt) + Carbaryl (1-2 pt) + Superior oil (1qt)/100 gal.
Although Ethephon should result in more aggressive thinning at such high temperatures; it might also cause tree growth reduction and might increase flowering the following year.
The following are the carbohydrate model outputs for both Winchester and Central Virginia.

Is it “Risky” to thin tomorrow?

Although the carbohydrate model still suggests thinning applications until Friday, I would consider any thinning application tomorrow and for the rest of this week as RISKY.
Today (April 26) is a perfect timing for thinning as you still have at least three days before the carbohydrate balance reaches to critical levels and it’s highly likely that fruitlets that will respond to today’s spray will be found on the ground in the weekend or early next week.
However, the weather forecast predicts conditions that will lead to more carbohydrate deficit this week and probably next week; and the carbohydrate model shows critical levels of carbohydrate balance (-60 to -80 g/day) this Saturday. At such low levels of carbohydrate reserve, the natural abscission of small fruitlets is highly likely. So, even if you do not apply any thinning treatments tomorrow (Thursday, April 27) or after tomorrow, some natural thinning will likely occur within and after the weekend.
The following are the recent updates of the model outputs, for Winchester and Central Virginia.

Note: I have noticed some delay between the time I post on the Horticulture blog and the time the updates can be seen by the public. I would advise that you keep this link ( in your bookmarks and go directly to the “Recent Posts” icon on the right-hand side of your screen to remain updated.

Apple Carbohydrate Thinning Model-“Updates” for April 23-30, 2017

As I mentioned in my last post, this week is a perfect timing for chemical thinning. However, I thought of sending daily updates as the weather forecast predicts more cloudy/ and or hot days this week that may lead to more carbohydrate deficit. Therefore, we should keep our eyes open for any change in the rates of chemical thinners that we intend to apply today (Wednesday, April 26) or tomorrow.

The following are the carbohydrate model outputs for Winchester and Central Virginia.

Apple Carbohydrate Thinning Model for April 23-30, 2017

This week of April 24-30 is the perfect timing for chemical thinning applications for both Winchester and Central Virginia. It’s as the books says about the optimal thinning conditions. Apples are in the size of 6-9 mm, compete aggressively with each other and consume a lot of the tree carbohydrate reserve; temperatures are above 80’s in most days and subsequently the transpiration rate will be high; some days are partially cloudy and will lead to fewer photoassimilates; and more importantly, the carbohydrate model predicts carbohydrate deficiency up to -60 g/day. The only thing that works against us is the high chance (90%) of rain tomorrow (Monday) and after tomorrow (Tuesday). However, we still have this Wednesday and Thursday as two ideal days to chemically thin our fruitlets.
If you were wondering about the best thinning applications to use at this stage, I would recommend using 6-BA for most varieties. Best results will be obtained when you mix 6-BA with carbaryl. NAA is another thinner you can use at this stage and also will give better results when mixed with carbaryl (i.e. Sevin). The mix of 6-BA and NAA might work well with most varieties and there is a good logic of having these two thinners mixed together at this time. However, this mix (6-BA + NAA) SHOULD NOT BE USED with Delicious and Fuji as it will likely result in the development of pygmies. With the coming hot weather this week, adding oil to the tank may result in overthinning. There are many variations among apple varieties and some are more sensitive to chemical thinners than others. The following table (Table 1) gives some indications about the sensitivity of different apple varieties to chemical thinners.

One final note, I run the model for Winchester and Central Virginia based on the information I retrieved today (April 23) from the weather stations in AHS Jr. AREC, Winchester and Batesville, respectively, and there is no guarantee that weather forecasting will remain unchanged the whole week. However, if I have detected any major change in the temperature and solar radiation values over the week, I will run the carbohydrate model again and keep you posted.

Apple Carbohydrate Thinning Model for April 17, 2017

The outputs of the carbohydrate model for Central Virginia area is based on the Batesville (Crown Orchard) weather station. The solar radiation and temperature values suggest carbohydrate deficiency through this week (until Friday, April 21). Since it’s rainy today in most parts, thinning applications (at the standard rate) tomorrow and till Friday should be efficient. Although it will be sunny and warm tomorrow and probably Saturday, leading to some carbohydrate accumulation (the blue line), interruptions with hot and/or cloudy days will reduce the 4-day average (red line) balance and promote fruit abscission.

For Winchester area, I ran the model based on the data retrieved from our weather station at the AHS Jr. AREC. As it appears in the figure below, the outputs look similar to that of Batesville and hence the timing for thinning applications will be similar as well. However, the predicated temperature and solar radiation values for Winchester don’t support any carbohydrate accumulation this week and hence it’s recommended to decrease the chemical thinner rate by 15%.

Crop load management of apples by chemical thinning

In modern apple production systems, chemical thinning of young fruitlets within the first four weeks after petal fall has become a key management element. Inadequate thinning of fruit during this period increases the need for hand thinning, which is generally more laborious and expensive. On the other side, if trees are left without thinning at all, the chances of having low quality fruits and less return bloom in the following year are very high. To achieve adequate thinning using chemical thinners, many factors should be taken into consideration. These include, but are not limited to, fruit size, carbohydrate reserve status of the tree, weather conditions and -to a lesser extent- variety sensitivity. As a general rule, sunny days and cold nights promote the accumulation of photoassimilates and consequently trees become less prone to shed any of their fruits and are less sensitive to thinning applications. Whereas, cloudy days and warm nights lead to carbohydrate deficit and therefore trees tend to retain only a limited number of fruits and become more sensitive to chemical thinning. This associating between weather conditions and the total carbohydrate reserve has encouraged scientists (Alan Lakso and Terance Robinson) at Cornell University to establish a model known as “The Cornell Apple Carbohydrate-Thinning Model” that provides instructions on the dose and timing of thinning applications based on the solar radiation, temperature and day length records acquired from a weather station in a grower’s orchard blocks. This model can be accessed online through the Network for Environmental and Weather Applications (NEWA)(
It is a good practice to visit the NEWA website and make yourself familiar with the inputs and outputs of this model, especially if you have a weather station in your orchard that is connected with NEWA server, or you know the one that is close to your operation and that you can depend on.
As we are moving toward a heavy crop load and, luckily, less frost damage for apples this year, we have to consider looking at the outputs of the carbohydrate model in order to decide the timing and the dose of the chemical thinner. Typically, the efficiency of thinning applications applied from petal fall until a fruit size of 20 mm correlates well with the carbohydrate reserve of the tree and these are the times where the model outputs will serve as useful guidelines.
This year, we, at AHS Jr. AREC, Winchester, will run the model weekly and provide the outputs of this model as graphs that show the carbohydrate reserve of apple trees, and the recommended applications. The following table (Table 1) describes the recommended dose of chemical thinners based on the carbohydrate balance of the tree.
Table (1): Decision rules for using the output of the carbohydrate model to adjust chemical thinning rate.

Fruit size is another major factor that determines the efficiency of chemical thinning. When fruits are young (~ 6 mm), their demand of carbohydrate is not much and hence thinning becomes relatively challenging. However, chemical thinners such as naphthaleneacetic acid (NAA) and Sevin (carbaryl) and, to some extent, naphthaleneacetamide (NAD; Amid-Thin) can be successfully used at this stage. As fruits grow to the size of 7-14 mm, their cells divide rapidly and their demand of carbohydrate becomes more than what vegetative tissues can supply, especially if weather conditions do not largely support photoassimilation. At this stage, fruits become more sensitive to chemical thinners such as 6-benzyladenine (6-BA), NAA and NAD. However, if thinning is not adequate at this stage due to unsuitable weather conditions, then another round of thinning application will be essential. As fruit develops to the size above 20 mm, the chemical thinning becomes a tangible challenge. At such an advanced stage of development, fruits become more tolerant to abscission as they have more carbohydrate reserve and the seeds produce auxin which interferes with ethylene action preventing abscission. Chemical thinning at this stage (known as delayed/ rescue thinning), relies mostly on the combined application of 2-chloroethylphosphonic acid (ethephon) and 1-naphthyl methylcarbamate (carbaryl). The following table (Table 2) shows the recommended thinning applications in each stage of flower/fruit development and the dose of each application.
Table (2): Materials, rates and timing of chemical thinning application (Source:2017 Spray Bulletin for commercial tree fruit growers).